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Electromagnetic Radiation Fundamentals and Their Applications in Food Technology

BY BERNARD E . PROCTOR AND SAMUEL A . GOLDBLITH

Department of Food Technology. Massachusetts Ins t i tu te of Technology.

Cambridge. Massachusetts

CONTENTS Page

I . Introduction . . . . . . . . . . . . . . . . . . . . . . 120 I1 . Types of Radiation . . . . . . . . . . . . . . . . . . . 122

1 . Sonic and Ultrasonic Vibrations . . . . . . . . . . . . . 123 2 . Radio Waves . . . . . . . . . . . . . . . . . . . . 123

a . Dielectric and Induction Heating . . . . . . . . . . . 123 b . Radar Energy . . . . . . . . . . . . . . . . . 125

3 . Infrared Radiations . . . . . . . . . . . . . . . . . 126 4 . Ultraviolet Radiation . . . . . . . . . . . . . . . . . 127 5 . X Rays (Gamma Rays) . . . . . . . . . . . . . . . . 127 6 . Cathode Rays (Beta Rays) . . . . . . . . . . . . . . . 129

111 . Uses of Sonic a n d Ultrasonic Vibrations . . . . . . . . . . . . 130 I V . Uses of Radio-Frequency Heating . . . . . . . . . . . . . . 132

1 . Sterilization . . . . . . . . . . . . . . . . . . . . 132 a . Mode of Action of Radio-Frequency Sterilization . . . . . 132 b . Destruction of Hoof and Mouth Virus . . . . . . . . . 133 c . Pasteurization of Wine and Beer . . . . . . . . . . . 133 d . Sterilization of Packaged Bread . . . . . . . . . . . 134 e . Destruction of Insects in Grain . . . . . . . . . . . 134

2 . Blaiicliing of Foods . . . . . . . . . . . . . . . . . 136 3 . Defrosting of Frozen Foods . . . . . . . . . . . . . . 137 4 . Roasting of Coffee . . . . . . . . . . . . . . . . . . 138 5 . Baking and Cooking . . . . . . . . . . . . . . . . . 138 6 . Dehydration . . . . . . . . . . . . . . . . . . . . 142 7 . Moisture Determinations . . . . . . . . . . . . . . . 143 8 . MiscelIaneous Uses . . . . . . . . . . . . . . . . . . 143 9 . Economica of Radio-Frequency Heating . . . . . . . . . . 144

V . Uses of Infrared Radiation . . . . . . . . . . . . . . . . 146 1 . Dehydration . . . . . . . . . . . . . . . . . . . . 146 2 . Flour Milling . . . . . . . . . . . . . . . . . . . . 146 3 . Protection of Fruits and Vegetables from Frost Damage . . . . 146 4 . I n Poultry Rearing . . . . . . . . . . . . . . . . . 147 5 . In Chemical Analysis . . . . . . . . . . . . . . . . . 147

V I . Uses of Ultraviolet Light . . . . . . . . . . . . . . . . . 147 1 . Germicidal Action . . . . . . . . . . . . . . . . . . 147 2 . Aging of Meat . . . . . . . . . . . . . . . . . . . 152 3 . Activation of Provitamins D . . . . . . . . . . . . . . 153 4 . To Improve Coffee Flavor . . . . . . . . . . . . . . . 153

119

120 BERNARD E . PROCTOR AND SAMUEL A . OOLDBLITH

5 . As an Analytical Tool . . . . . . . . . . . . . . . . . 153 G . Detection of Infected or Washed Eggs . . . . . . . . . . . 154

V I I . Uses of Radioactive Isotopes . . . . . . . . . . . . . . . . 155 1 . Germicidal Action . . . . . . . . . . . . . . . . . . 155 2 . Applications in Agriculture . . . . . . . . . . . . . . . 156

V I I I . Uses of X Rays . . . . . . . . . . . . . . . . . . . . 156 1 . Germicidal Action . . . . . . . . . . . . . . . . . . 156 2 . Effect on Vitamins . . . . . . . . . . . . . . . . . . 157 3 . X-Ray Diffraction . . . . . . . . . . . . . . . . . . 158 4 . Fluoroscopy . . . . . . . . . . . . . . . . . . . . 158 5 . Destruction of Toxins . . . . . . . . . . . . . . . . 159 6 . Miscellaneous . . . . . . . . . . . . . . . . . . . . 159

I X . Uses of Cathode Rays . . . . . . . . . . . . . . . . . . 160 1 . Germicidal Action . . . . . . . . . . . . . . . . . . 163 2 . Effect on Vitamins . . . . . . . . . . . . . . . . . . 167 3 . Effect on Amino Acids . . . . . . . . . . . . . . . . 174

174 5 . Effect on Coffee and Its Components . . . . . . . . . . . 176

180

4 . Effect on Enzymes . . . . . . . . . . . . . . . . . . 6 . A s a Tool for Studying Some Chemical Reactions . . . . . . .

a . Nonenzymatic Browning . . . . . . . . . . . . . . 180 b . Meclianism of Spoilage of Fish . . . . . . . . . . . 184

7 . Effect on Packaging Materials . . . . . . . . . . . . . 187 8 . Effect o n Pharmaceuticals . . . . . . . . . . . . . . . 187 9 . Toxicity of Irradiated Materials . . . . . . . . . . . . . 187

X . Summary . . . . . . . . . . . . . . . . . . . . . . . 188 Acknowlrclginrnts . . . . . . . . . . . . . . . . . . . . 189 References . . . . . . . . . . . . . . . . . . . . . . 189

I . INTRODUCTION

I t is well known that conventional means of heat processing irrevers- ibly alter the flavor. color. and texture of many foods . Usually alterations are advantageous. but for some products it would be desirable to have them less extensive . The changing food habits of American con- sumers have made many of these alterations acceptable. but there are some foods (especially fruits and vegetables) whose flavors are preferred in the fresh state .

From the point of view of flavor alone. it is desirable to improve our present methods of processing foods . Some improvements have been made in this direction with a number of food products by use of high- temperature short-time processing . Much research has been conducted in recent years to ascertain whether it is possible to improve the color. flavor. and retention of nutrients in processed foods by means other than heat for processing . Among the alternative means that have been considered to attain this objective in food processing is the utilization of several of the radiations of the electromagnetic spectrum . With the hope of added improvement. research is being conducted a t the present time by

FIG. 1. Illustrative chart of the electromagnetic spectrum, showing the approximate locations o f various types of radiation. Roman numerals refer to main sections of report where uses of radiations are discussed; Arabic numbers refer to paragraphs of Section I1 describing the various types of radiation.

122 BERNARD E. PROCTOR AND SAMUEL A. GOLDBLITH

a number of laboratories in which electronic heating and “cold sterilization” by electromagnetic irradiations are being employed.

This review covers the known applications of electromagnetic energy in the field of food research within the past ten years, from sonic vibrations a t one end of the spectrum to gamma rays a t the other end. F o r orientation, some reference is made to previous applications. Although lack of space precludes complete citation of the earlier literature, this fortunately is adequately covered in the excellent treatise on the “Bio- logical Effects of Radiations” by Duggar (1936).

To orient the reader, a n illustrative chart of the electromagnetic spec- t rum is presented in Fig. 1. This chart shows the approximate locations, in the electromagnetic spectrum, of the various types of radiation discussed in this review. The Roman numerals refer to the sections of this paper where discussion of the uses of the various radiations may be found and the Arabic numbers to the paragraphs of Section I1 describing the various types of radiation.

I n an effort to make a logical presentation of the known applications of electromagnetic energy in the food field, this review has been divided into several sections. The first section contains a brief discussion of the various types of electromagnetic radiation. The subsequent sections are concerned with the applications of various types of radiation, each section being devoted to the uses and the potentialities of a specific type. The order in which the sections on radiations are presented is that in which the radiations occur in the spectrum, from long wavelengths to short wavelengths (Fig. 1) .

11. TYPES OF RADIATION

Infrared radiations, as well as the other types of electromagnetic radiations, are composed of packets of energy known as “quanta.” They consist of an electric and a magnetic vibration of high frequency, vibrations that travel in a straight line through space a t a velocity of 3 x 1O1O cm. per second. Electric and magnetic vibrations are always present together, each being associated with the other and perpendicular to the other.

The difference between the members of the electromagnetic radiations is chiefly a difference in the frequency of the waves or vibrations or a difference in the energy of the wave packets. For example, if the frequency were lo1* cycles per second (corresponding to a photon energy of 7 x ergs), this frequency would be typical for infrared radiation. If the frequency were lo8 cycles per second (corresponding to a photon energy of 7 x ergs), the frequency would be characteristic of a radio wave.

APPLICATIONS OF ELECTROMAQNETIC RADIATIONS 123

1. Xonic and Ultraso& Vibrations

These radiations occur in the low-frequency range of the electromagnetic spectrum. A frequency of 20,000 cycles per second (20 kilocycles) is the upper limit fo;- waves within the range audible to the average human being. Frequencies as great as 500 megacycles have been generated by methods recently developed. These radiations cover a tremendous range in both frequency and wavelength. For convenience, waves with frequencies up to 20 kilocycles per second are called sonic vibrations and those above, supersonic or ultrasonic vibrations. Sollner (1944) emphasized that there is no fundamental physical difference between ultrasonic and audible sound and that the difference in nomen- clature is based entirely on a physiological limitation in man’s hearing ability.

At the present time, a number of small ultrasonic generators are available on the market. The usual sonic or ultrasonic generator consists of some means of generating the radiations, such as a radio oscillator and a transducer, which today generally consists of a quartz crystal and accessory equipment. Various types of generators, trans- ducers, and resonating equipment are described by Carlin (1949).

2. Radio Waves

Since man first discovered fire, food has been heated by placing it near or in contact with another medium at a higher temperature. This medium may be air, metal, or some other material. Food is cooked by conventional methods by heating it from the outside in toward the interior, by conduction or convection of heat from the outer surface of the material where the heat is supplied. Such heating proceeds by conduction toward the center of the material, as, for example, in the case of rare roast beef, and the outer portions are always more completely cooked or heated as a result.

Radio-frequency heating, in contrast to conduction heating, is the heating of material a t an equal rate throughout the entire material by causing internal vibrations of the molecules that comprise its components. This means of heating is extremely rapid, and every part of the food or the material to be heated is heated uniformly.

I n the utilization of radio-frequency energy f o r heating, there are two basic methods, induction heating and dielectric heating.

Induction heating is the generation of heat in a substance that is an electrical conductor when that substance is placed in a varying magnetic field.

Dielectric heating is the generation of heat in a substance ordinarily

a. Dielectric and Induction Heating.

124 BERNARD E. PROCTOR AND SAMUEL A. QOLDBLITH

considered as an electrical insulator when that substance is placed in a varying electrostatic field. Radio-frequency heating may be illustrated in the manner shown in Fig. 2. Here the condenser plate A is positively charged, and the negative charges of the food molecules are attracted to it. Then the charge is reversed, and plate A becomes negatively charged. The positive charges of the food molecules are now attracted to plate A. When this happens, and it may happen several million times per second dependent on the frequency used, friction is set up between the molecules. Thus heating results equally throughout the entire food mass, not at the surface alone.

ELECTRONIC GENERATOR

+ + t + I t + + +

1 ‘=l-z? ELECTRONS STRAIN

s CLOSED

ELECTRONS STRAIN TOWARD B

I

FIG. 2. Radio frequency. A and B, condenser plates; (upper) positive charging (Kinn, 1947.) of condenser plate ; (lower) negative charging of condenser plate.

APPLICATIONS O F ELECTROMAGNETIC RADIATIONS 125

As the basic difference between induction and dielectric heating is dependent on whether the substance to be heated is an electrical conductor or insulator, the type of heating that should be used can be determined by ascertaining the resistivity of the material to be heated. The lower the resistivity, the greater is the electrical conductance of that material. According to Kinn (1947), those materials having a resistivity below 100-1000 ohms per centimeter can be heated by induction heating, and those having a resistivity above 1000 ohms can be heated by dielectric heating.

The radio-frequency generators most readily available operate a t from 20 to 150 megacycles. I n experiments with vegetables, &Toyer and Stotz (1947) found that arcing occurred when frequencies of from 7 to 10 megacycles were used. When oscillators operating at higher frequencies were used, the incidence of arcing was reduced materially with lowered voltage, and the same power output was obtained.

The frequencies of f rom 20 to 150 megacycles in commercial induction and dielectric heating equipment lie within a wavelength range of from 10.5 to 2 meters.

6. Radar Energy. Among the more recent developments of radar energy is one that represents a technical result of World War 11, namely, the cavity magnetron. This has been used for the production of ultra- high frequency radiations. Such tubes, having wavelengths of from 1050 to 3000 megacycles, may be used in equipment for the heating of foods (Morse and Revercomb, 1947 j Proctor and Goldblith, 1948a). With the use of such higher frequencies, arcing has been almost entirely eliminated.

Numerous difficulties had to be surmounted in the design of an ultra- high frequency equipment to defrost and cook frozen foods. For instance, peas have a dielectric constant of 9 a t 23" C. (73" F.) and of 2.5 when they are frozen. In order for an ultra-high frequency oven to operate efficiently, i t must be able to defrost foods and then heat them evenly to a temperature of approximately 60" C. (140" F.), despite the fact that each specific foodstuff has different dielectric constants a t different temperatures. The ultra-high frequency oven must also be able to heat, simultaneously, materials that have different dielectric constants, such as meat and bones.

The technical difficulties that had to be overcome may be better real- ized when one considers that the energy required to increase the temperature of food from -18" to 0" C., (0" to 32" F.) is about the same as that needed to raise the temperature of the thawed food from 0" to '7 C. (32' to 170" F.) (Morse and Revercomb, 1947). At least two

126 BERNARD E. PROCTOR AND SAMUEL A. COLDBLITEI

types of equipment employing the magnetron have been devised and are in present day use.

One difficulty observed with radar ovens is uneven heating. This has been ascribed to standing waves (the nodes of a sine wave at which

FIG. 3. Radar oven. (Courtesy of Raytheon Manufacturing Company, Waltham, Mass.)

there is no energy). Equipment a t present available (Fig. 3 ) has provided means for reducing the difficulties caused by tliese stancling waves, and further improvements are likely in the future.

3. Infrared Radiut ions

The infrared portion of the spectrum lies between the Hertzian (radio) waves and the visible region. Speed is the most important of all the advantages of radiant heat over other forms of energy transmis- sion. The speed of infrared heating is due t o the fact that in the transfer of heat from one medium to another, the rate of transfer is high when the temperature differential between the two objects is great. The rate of heat transfer decreases rapidly as the temperature of the object being heated approaches that of the source of the heat.

I n convection o r conduction heating, the temperature difference is from approximately 27" to 149" or 204" C. (80" F. to 300" or 400" F.). When infrared radiation is used, the temperature difference is

from 27" C. (SO" F.) to a lamp filament temperature as high as 2316" C. (4200" F.).

Although infrared radiations will penetrate thin paint films and trans- parent materials such as glass, they have a limited penetration into solids such as woods and metals. The rise in temperature inside such solids is due entirely to heat conducted from their surfaces.

Infrared lamps use tungsten wire filaments, just as do incandescent lamps. However, the primary function of infrared lamps is to produce

APPLICATIONS OF ELECTROMAGNETIC RADIATIONS 127

heat, with light as a byproduct. This is done by reducing the filament temperature from a Mazda lamp operating temperature of 2698.9’ C. (4890’ F.) to a drying lamp temperature of 2298.9’ C. (4170’ F.). The lower temperature results in less light, less glare, and a longer lamp life (over 5000 hours). The reduction in visible light results in an increase of energy converted into infrared radiation.

4. Ultraviolet Radiation

The ultraviolet region of the spectrum lies between the visible portion and the very soft x rays. D6rib@r6 (1947) has described some of the present day ultraviolet equipment and has compared it with earlier types. Formerly the active rays were produced by mercury vapor lamps at normal pressure in quartz tubes. The mercury vapor, however, absorbed part of the rays. Low-pressure lamps were then designed, which avoided this difficulty but required higher voltages. The recent development of fluorescent lamps is indicative of the new methods of attaining the desired effect.

5. X Rays (Gamma Rags)

X rays are among the oldest known of the artificially produced radiations. These radiations, also known as “gamma rays,’’ were discovered by Konrad Roentgen in 1895. Perhaps the simplest way of describing these radiations would be by the following analogy.

FIQ. 4. by bullets.

Resemblance betweexi generation of x rays and generation of sound waves (Sproull, 1946. Reprinted by permission of the copyright owners.)

If a person were near the spot where the bullets from a machine gun were striking rapidly against the face of a large block of steel, he would be able to hear a plunking noise indicating the sound waves generated by the bullets as they struck the block. The generation of x rays is analogous to the generation of sound waves by bullets, if electrons are substituted for bullets (Fig. 4).


When bullets strike a steel block, heat is generated. Similarly when electrons or cathode rays from an x-ray tube strike the target, heat is generated so the target must be cooled. This cooling is usually accomplished by the circulation of a cooling medium, such as water or oil, around the target.

The electrons interact with orbital electrons of the target atoms and remove them from one orbit to another or completely remove them from the atoms. This is an unstable state, and the atom has a surplus of energy given to it when the electron was removed. The atom quickly attracts an electron to fill the vacant position and returns to its normal state. In so doing, it emits its surplus energy in the form of an x ray o r photon.

The atoms are then ionized.

1 O 4 0 8 12 16 20

PENETRATION (CM) IN MASONITE PHANTCM

Relative penetrations into water of soft, hard, and supervoltage x rays. FIG. 5. (Taken in part from Trump and Cloud, 1943.)

When electrons at 100 kv. or less are directed against a target such as gold or tungsten, the rays produced are referred to as “soft” x rays. When electrons a t 185 kv. or more are directed against the target, the x rays produced are referred to as “hard” x rays. Soft x rays have relatively slight and hard x rays have relatively great power to penetrate into matter. For example, the penetration of soft x rays into water is of an order of magnitude of millimeters, whereas that of hard x rays is of an order of approximately several centimeters. X rays produced at 1000 kv. and above are referred to as “supervoltage” x rays and have even more marked ability to penetrate into matter (Fig. 5).


6. Cathode Rays (Be ta Rays )

Artificially produced electrons or particles are called cathode rays. By irradiating with electrons directly without an intermediate target,

one realizes a greater portion of the electron efficiency than by irradiating with x rays, for the following reason. When the electrons are allowed to strike on the target, 95% of the electron energy is transferred into heat and is absorbed, and only about 5% goes into the production of x rays. Goldblith (1949a) has found that one can obtain the same biological effect with cathode rays in one five-hundredth of the time needed with x rays. But there is a limiting factor in the use of cathode rays, namely, their penetration into matter is much less than that of x rays of equivalent potential.

It has been found that the maximum ionization in a material being bombarded by cathode rays occurs beneath the surface. This is illustrated in Fig. 6 (Trump e t al., 1940).

0 I 2 3 4 5 6 7 8 9 1011 12 I3 I 4 1 5 Dipth of water in militmeter8

FIG. 6. Ifistrihutiori of ioriie:itioii with depth of penetration into water for cathode (Courtesy of Prof. J. G. Trump, M:tssacliusetts rays itwidetit with different energies.

Institute of Teclinology.)

The penetration (maximum range) of cathode rays into matter can be

(1)

determined by the following equation (Evans, 1947) :

R,,, = 0.54E - 0.15

where R,,, equals the maximum range expressed in grams per square


centimeter and E equals the voltage expressed in megavolts. This relationship is shown graphically in Fig. 7.

Within the past year, the authors (1950) have been able to demonstrate and evaluate the penetration of electrons into matter by

0 1 2 a novel means. This involves the use of dilute DEPTH N WATER (CM) concentrations of oxidation-reduction dye in-

pIa. 7. Pelletration (max- dicators such as methylene blue and resazurin imum range) of cathode in an agar gel in test tubes or glass equip- rays into water, a8 related ment. The maximum range can be graphi-

cally shown by the reduction in color. The to voltage. (Calculated from Evans, 1947.)

depth of the reduction can be measured fairly accurately. Data obtained in this fashion are presented in Table I.

g 2 %L 9 '

TABLE I

Depth of Reduction of Methylene Blue in Agar Gels by Cathode Rays at 1000, 2000, 3000, and 3200 Kv.*

Depth of reduction (cm.)

Methylene blue Theoretical

1000 0.40 2000 0.90 3000 1.35 3200 1.60

* Proctor and Goldblith ( 1 9 4 9 ~ ) .

0.39 0.93 1.47 1.58

Comparison of the data in Table I with results calculated according to equation (1) or derived from the curve in Fig. 7 shows excellent agreement in the values obtained.

111. USES OF SONIC AND ULTRASONIC VIBRATIONS

The bactericidal effect of high-frequency sound waves has been studied by a number of investigators, including Williams and Gaines (1930), who found it possible to kill cells of E . coli by exposure in liquid to high-frequency, audible sound waves. Chambers and Gaines (1932) reported that microorganisms were disrupted by intense sonic vibrations. Harvey and Loomis (1929) also observed the same effect. Le Galley and Patterson (1940) found that sonic vibrations destroyed cultures of S. aureus, E. coli, and B . subtilis.

Stanley (1934) discovered that supersonic vibrations destroyed purified tobacco mosaic virus. Similar results with tobacco mosaic virus were obtained by Takahashi and Christensen (1934).


Kvasnikov (1940) found that the bactericidal effect of ultrasonic waves was not specific and was confined to the heating effect resulting in the denaturation of the protein components of the cell. However, cell death was more rapid in a field of high-frequency waves than in the thermal control. This he considered to be due to the activation of proteolytic enzymes.

Von Euler and Skarzynski (1943) concluded from their observations that yeast is destroyed by supersonic vibrations at 800,000 cycles. En- zymes and viruses were found to be sensitive to ultrasonic waves.

Shropshire (1947a,b) found that bacteria were disrupted by sonic vibrations, and he described a means of measuring this effect.

A number of other uses for these vibrations have been studied, including the homogenization of milk (Chambers, 1938) and of ice cream mixes (Anderson, 1936).

Florsdorf and Chambers (1933) observed that solutions of egg albumin, of four times recrystallized egg albumin, and of synthetic plastein were almost instantly coagulated at 30" C. (86' F.) by audible sound (1000 to 15,000 cycles per second). Chambers and Florsdorf (1936) found that egg albumin is denatured by intense sound waves of similar frequency.

Doan (1938) reported that curd tension of milk may be lowered by sound waves, producing soft curd milk.

A number of other investigators have studied the disruptive effects of sonic and supersonic vibrations and the resultant homogenizing effect.

Proctor et al. (1946) investigated a number of potential uses of sonic vibrations produced at 360 cycles per second. They concluded that sonic oscillation of suspensions of active dry yeast, of fresh and old orange juice, of orange juice concentrate, and of eggs (whites, yolks, and whole eggs) would lower bacterial counts but is not a practical procedure for purposes of sterilization. Sonic vibration was found to decrease the viscosities of orange juice, apple juice, gelatin solutions, liquid egg, reconstituted skim milk powders, reconstituted whole milk powders, boiled cornstarch solutions, and boiled flour solutions. Other observations made following sonic oscillation dealt with the dehydration and rehydration characteristics of egg and milk products and cornstarch suspensions, and the canning pryperties of pasteurized and homogenized milks. These observations appeared to indicate no marked beneficial effects of sonic vibration in any of these fields of food research, other than a mechanical separation of agglomerated particles or the physical disintegration of very delicate tissues. For instance, chopped plant tissues o r animal tissues such as comminuted liver may be made into more uniform purees by sonic vibration. The stronger tissues, such as


the veins, segments in liver, and stem tissues of plants, are not changed or reduced in particle size by such treatment.

It is considered by the writers that any large-scale use of such vibrations for in-package food sterilization is contra-indicated because of numerous technical limitations, among which are the barrier effects of packaging materials.

IV. USES OF RADIO-FREQUENCY HEATING

1. Sterilization

a. Mode of Ac t ion of Radio-Frequency Sterilization. For a number of years there has been a great deal of discussion in the literature as to whether the lethal effect of radio-frequency action on microorganisms and insects is a specific effect of the radio-frequency energy per se or is an effect caused by the heat generated by the radiations.

According to Brown et al. (1947), i t is difficult to maintain suspensions at a low temperature because most solutions that carry bacteria have a high conductivity and hence heat rapidly when the electrical field is applied.

Fleming (1944) reported excellent destruction of bacteria in weak electrolytes with very slight energy. He found the maximum effect at a frequency of approximately 60 me. but also noted substantial effects a t any frequency between 10 and 300 mc.

Brown et al. (1947) attempted to repeat Fleming's work but found no evidence of bactericidal action when such low power concentrations were used. They boiled milk in a vacuum, with the energy supplied by a radio-frequency generator operating a t 27 mc. The temperature was maintained at 27" C. (SOo F.). Sterile water was fed into the chamber to keep the level of liquid constant. Treatment in this way resulted in no bacterial destruction.

Brown e t al. (1947) also carried out numerous experiments a t 185 and 400 mc with power densities of less than 10 watts per milliliter for periods of time short enough to insure no rise in temperature sufficient to kill bacteria. Again negative results were obtained. Space does not permit complete details regarding the many experiments conducted by Brown and his colleagues, but certain of their conclusions are given below :

"Specific effects due to the electric field without heating are possible with high field intensities. Since the fields must be extremely great, heating results. Unless the solutions have low conductivity, temperature effects will destroy microorganisms before sufficient time has elapsed for


the electric field to bring about destruction. Extended research is necessary in this field."

Gaden and McMahon (1947) studied the effects of radio-frequency energy produced at 16.5 megacycles on nutrient broth cultures of E. coli. Their data appeared to indicate that the high frequency electric field had a lethal effect over and above that which can be ascribed to the mass temperature rise. Examination of the results obtained strongly suggested to Gaden and McMahon that the lethal effects observed were due to preferential heating of the suspended bacteria. Thus heat sterilization of the cell was obtained without raising the temperature of the mass of the medium to a lethal value.

More recently Brown and Morrison (1948), using electrical methods, reported that the lethal effect of radio-frequency energy on microorganisms is due solely to the heat produced.

Proctor and Goldblith (1949c), using biological methods, came to the same conclusion. By exposing suspensions of E. coli in flasks containing sterile stones prechilled to -18" C. (0" F.), they could irradiate the suspensions for one minute without raising the temperature of the suspensions above 30" C. (86" F.) and found that the bacterial contents of the irradiated suspensions were not reduced. When a similar suspension was irradiated without the chilled stones for the same period of time, it became heated to 93" C. (200' F.) and bacterial destruction occurred. Hence i t would appear that the sterilizing effect of radio- frequency energy could be ascribed solely to heat. Burton, in 1950, found the same type of results.

Using the same technique, Proctor and Goldblith ( 1 9 4 9 ~ ) exposed a solution of the enzyme catalase to radar emanations at 3000 mc. for 1 minute. The temperature of the solution was kept below 30" C, (86' F.) by means of chilled stones. According to activity measurements and data on the ultra-violet absorption spectra of the solution, no destruction of the enzyme was observed under these conditions.

Zarotschenzeff (1944) experimented on the effect of dielectric heat on meat to be frozen, with the object of destroying the virus that causes hoof and mouth disease. He reported that this virus could be destroyed by dielectric heat without adverse effect on the texture and the flavor of the meat.

c. Pasteurization of Wine and Beer. Yang e t al. (1947) utilized radio-frequency energy a t 26-34 mc. for the pasteurization of wine. Table wines were successfully pasteurized in less than 4 seconds of treatment at a discharge temperature of 54" C. (130" F.). They attributed the shortness of the time required f o r pasteurization to the instant penetration of heat into the cells of the microorganisms when

b . Destructim of Hoof and Mouth Virus.

134 BERNARD E. PROCTOR AND SAMUEL A. OOLDBLITH

electronic heating was applied and to the possible breaking down of the cells by molecular dipole rotation. Improvement in flavor and better color of the wines were reported when the electronic method of pasteurization was used.

Titus (1946) also studied the practicability of radio-frequency radiations for the commercial sterilization of beer but obtained negative results. He claimed that, although beer might be sterilized by radio- frequency radiations in a laboratory demonstration, i t might not neces- sarily be successfully sterilized by this means in a commercial process. He found that when the beer was subjected to radio-frequency radiations at 15, 25, and 210 mc. for 5 or 10 seconds, with no rise in temperature, the beer was not pasteurized.

Sampson (1945) also discussed the possible use of electronic heating for the pasteurization of beer. He reported that, from the standpoint of flavor, beer pasteurized electronically was equal to that pasteurized by the usual process.

Hence it would appear from more recent work that the rapid sterilizing effect of radio-frequency waves is probably due to the rapid production of heat throughout the material exposed to the irradiation, although other possible causes have been suggested.

d. Sterilization of Packaged Bread. Cathcart, Parker, and Beattie (1947) successfully used radio-frequency heat generated at 14 to 17 mc. for destroying molds in packaged bread. Heating to 60" C. (140" F.) was found to be satisfactory. Bartholomew et al. (1948) utilized dielectric heat a t 26 mc. to sterilize suspensions of Penicillium and Aspergillus molds. Further experiments showed that both dielectric and induction heating could be used to sterilize Boston brown bread. Induction heating was found to be superior to dielectric heating, for the effect of the former was confined mostly to the surface of the bread where mold spores would normally occur.

e. Destruction of Insects in Grain. Webber e t al. (1946) have reviewed the literature on the effects of radio waves on insects and state that probably the first application of this form of energy to biological problems was made by d'Arsonva1 in 1893, who observed marked rises in the temperatures of laboratory animals that were exposed to high. frequency fields. These authors extended some of the researches and determined the lethal effects of high-frequency electric fields on insects in various media. They noted a correlation between time, temperature, field intensity, and mortality.

Studies made since 1927 indicate that Hertzian or radio waves can be utilized in eliminating insects (Headlee, 1931). Hadjinicolaou


(1931) found that radio waves were successful in killing adults, larvae, and pupae of insects feeding on st,ored products.

Mouromtseff (1933) reported that as early as 1927 the Baltimore and Ohio Railroad became particularly interested in the sterilization of millions of bushels of wheat stored in railroad elevators. J. H. Davis (Chief Engineer of electrical traction of that railroad), in conjunction with the Westinghouse Corporation, found that the rice weevil in wheat could be destroyed by radio-frequency energy a t from 40 to 50 mc. The temperature necessary to kill the weevil was reported to be 52" C. (125' F,). Lower final temperatures retarded propagation but did not completely destroy the insects. Exposed wheat grains were reported to sprout more readily and grow faster than untreated grains. The most favorable temperature for this was also 52" C. (125" F.), but even heating to 82" C. (180" F.) did not entirely inactivate the germinating property of the grain. Davis found that the safe minimum of energy for complete destruction of weevils was 443 watt-seconds per cubic inch. The lowest successful limit observed by him was 340 watt-seconds per cubic inch.

Tillson (1945) reported that it is possible to treat and completely sterilize badly infested grain by electronically produced heat without injury to the material. He claimed that complete sterilization without damage to the cereal grain is possible because the temperatures a t which insect life can be destroyed are well below the safe minimum temperatures for grain products. Tillson further stated that the entomologists of the United States Department of Agriculture have determined that a temperature of from 54" to 60" C. (130' to 140" F.) is necessary to destroy insect life, whereas the maximum safe drying temperature for wheat, for example, is 82' C. (180' F.) . This confirms the work of Davis that there is a margin of safety between the sterilization temperature and the inactivation temperature of cereal enzymes. Ac- cording to Davis (cited by Mouromtseff, 1933) when electronic heat is used, close temperature control is possible and heating takes place simultaneously on the outside and in the center of the kernel. As the infesting objects usually contain more moisture than the bulk of the material, selective heating often takes place and the weevils and eggs are destroyed without harm t o the product.

Proctor, J o s h , et al. (1948) investigated the effects of radar emanations at 2450 me. on flour beetles and on bread baked from this treated flour. Exposure of flour in a radar oven for from 9 to 12 seconds resulted in the destruction of adult beetles and larvae. Bread made with radar- treated flour resulted in loaves that did not differ appreciably from control loaves in color, odor, flavor, texture, and loaf volume.


Smith (1944) reviewed the application of radio-frequency heat for the sterilization of packaged finished products, such as flour and baking mixes, on conveyor belts. He found the process to be effective and economical.

2. Blanching of Poods

Moyer and Stotz (1945) showed that vegetables in Peters type cartons can be successfully blanched by radio-frequency energy at 150 mc. The packages were placed between condenser plates in an oven at 100" C. (212' F.), to prevent condensation of moisture on the electrodes and to counteract loss of heat from the carton by radiation. A high retention of vitamin C occurred when vegetables were blanched by radio-frequency heating under such conditions.

Further work by these authors was reported later (Moyer and Stotz, 1947), in which favorable results were obtained with respect to the retention of ascorbic acid and carotene. The authors point out, however, the importance of adequate cooling of the blanched material in the carton, if the advantages gained in nutrient retention by this means of processing are to be maintained. If adequate cooling of the blanched material in the carton cannot be made, the radio-frequency power might have to be supplied to the material on a moving belt and the heat removed in a refrigerated air stream immediately after the blanching.

Schade (1945a) studied the possibility of using radio-frequency heating to inactivate enzymes prior to the dicing of potatoes. Capacitative (dielectric) heating was not feasible because of the irregular shape and volume of the potatoes. Another experiment cited by Schade was the heating of potatoes conductively with radio-frequency heating. The conducting medium was a solution containing 0.1% of salt. I n 3 minutes 5 seconds the temperature of the liquid medium rose to 80" C. (176' F.), and that of the center of the potatoes rose to 95" C. (203" F.).

Moyer and Holgate (1947) found that €or the cooling of vegetables electronically blanched a t 150 me. a stream of refrigerated air gave better results than fluming with water.

Proctor and Goldblith (1948a) successfully blanched a number of vegetables extremely rapidly in thermoplastic bags by means of radar energy produced a t 3000 mc. The loss of vitamin C from the radar- blanched vegetables was negligible in every case. This is illustrated in Fig. 8. The major loss of ascorbic acid in water-blanched spinach was found to occur in the blanching water. This loss was minimized when the vegetables were blanched in sealed plastic containers.

Samuels and Wiegand (1948), using radio-frequency energy, successfully blanched cut corn and freestone peaches. I n the case of the corn,


.. 20

0 0

I8 c

0

l o - u a

:: 8 -

the loss of vitamin C was less when this vegetable waa blanched electronically than when it was blanched with boiling water. The loss was minimized even more by cooling the blanched corn in a blast of air a t a temperature of -29" C. (-20" I?.) rather than in running water.

D STEAM BLANCHED - -

4 FRESH VEOETAUX

0 RADAR BLANCHED C HOT WATER BLAHCHED

I L

CARROTS PEAS S P W A U I GRIP( BEANS

FIO. 8. Effects of different methods o f blanching on the ascorbic acid content of vegetables. (Proctor and Goldblith, 1948a.)

From these reports it would appear that blanching of vegetables by means of radio-frequency energy can be accomplished on a laboratory scale with a minimum loss of soluble nutrients. The process can be accomplished rapidly and can be done with the vegetable in the final container or in an inner sealed package, which may later be inserted in an outer package. Large-scale continuous equipment for the process has not yet been developed, and therefore the economics of such procedures is undetermined.

3. Defrosting of Prozen Poods

One of the successful uses of radio-frequency energy is in the defrosting of frozen foods. According to Lund (1945), radio-frequency energy has been adopted by a commercial company to speed the defrosting of frozen eggs for their bakeries. Cathcart and Parker (1946) reported that they successfully defrosted frozen foods such as eggs, fruits, vegetables, and fish in rectangular cardboard containers, by using radio-frequency energy a t 14 and 17 me. At present this procedure is being used extensively for rapid defrosting of frozen food packages by Cathcart (personal communication, 1949). Martin (1945) has also discussed similar experiments and has stated that the thawing time for a barrel of frozen berries intended for baking or processing may be reduced from seven days (thawing at room temperature) to 1 hour (dielectric heat).


Sherman (1946) has also indicated certain potentialities of electronic heat in this field.

Brown e t al. (1947) have discussed the theory of defrosting frozen foods by radio-frequency power.

4. Roasting of Coffee

The roasting of coffee by radio-frequency treatment has been attempted by Brown et al. (1947). They found that, although the roasting time could be cut a t least in half, the power requirements were high.

Proctor and Mezoff (1947) found that coffee could be roasted in a short time with radar energy a t 3000 mc., but an uneven roast resulted. Some beans were burned and some were under-roasted. This unevenness of roast was considered due to standing waves in the oven. If this interference could be eliminated, it would seem possible to roast coffee successfully with radar energy.

A simple method of determining dead spots in an oven in which food materials are heated by radar energy was found by Proctor (1947). A desk blotter pad, cut down to oven size, is soaked in water and a number of layers are exposed to radar energy in the oven. Mere inspection of the wet spots remaining after heating reveals the heating pattern.

5. Baking and Cooking

To the layman the use of electronic heat for cooking seems like magic. Much publicity has been given to the potentialities of electronic ovens in the home for the working housewife. It has been amply demonstrated that electronic ovens make rapid cooking possible, but they have certain limitations.

So far as taste and appearance are concerned, the use of radio-frequency heating is limited in .the case of food products prepared for the table. Because a food is cooked evenly throughout its mass by radio-frequency heat, no characteristic crust or carbonaceous exterior is formed. Hence, cakes baked by this method do not have crusts; meat thus cooked tastes like and otherwise resembles boiled or steamed meat, unless it is first seared to produce the external appearance and flavor of roasted meat. One commercial manufacturer of a radio-frequency oven provides an electrical searer with the oven so that the meat can first be rapidly seared and then rapidly cooked. Another approach to obviating this difficulty might be the use, on the exterior of the meat, of a meat sauce o r gravy that resembles seared meats in color and flavor.

Proctor and Goldblith (1948a) investigated the vitamin retention of foods baked and cooked by radar energy produced at 3000 mc. and compared the results with those obtained when the same foods were baked

APPLICATIONS OF ELECTROMAQNETIC RADfATIONS 139

and cooked by ordinary means. Additional results cannot be reported here because of space limitations, but it is sufficient to state that electronic cooking offers considerable promise SO far as nutrient retention is concerned.

Morrow (1945) has discussed the potential uses of electronics for cooking, dehydration, and processing of foods, especially baby foods.

Sussman (1947) has evaluated an oven employing radar magnetron emanations for the cooking of foods in a submarine. This was the same type of oven as employed by Proctor and Goldblith (1948a) in their work. As Sussman conducted an exhaustive study, his summary and conclusions are cited below.

(See Tables I1 and 111).

A. The (equipment) in its present stage of development is not practicable for

1. installation on submarines or elsewhere in the Naval establishment.

Supplementary equipment is required for: a. Baking b. Holding food warm preparatory to serving

TABLE I1

The Retention of Thiamine and Riboflavin in Hamburg, Haddock, and Frankfurters Cooked by Radar Emanations and by Gas *

Food and method Of

cooking

Hamburg As bought Radar cooked t

Solid component Juice component

Fried

Loss of moisture

in cooking %

Thiamine Riboflavin

Amount t Retention gamma/g. %

Amount t Retention gamma/g. %

- 5.0 - -

19.8

Haddock As bought ._

Solid component - Juice component -

Radar cooked t 6.4

Baked 19.2

As bought - Frankfurters

Radar cooked 2.9 Grilled 2.9

0.82 - 0.82 100.0 0.72 - 0.10 - 0.79 96.0

0.52 - 0.28 53.6 0.22 - 0.06 - 0.35 67.4

1.20 - 0.86 71.8 0.69 57.6

1.07 - 1.12 100.0 0.950 - 0.170 - 0.940 88.0

0.250 - 0.218 87.2 0.148 - 0.070 - 0.230 92.2

1.73 - 1.78 100.0 1.66 96.0

* Proctor and Qoldblith (19488). t Expressed on freeh moisture basis. Z Includes meat plus juice.

140 BERNARD E. PROCTOR AND SAMUEL A. QOLDBLITE

TABLE I11

The Retention of Thiamine and Riboflavin in Cake Mixes Baked in the Radar Range and in a Gas Oven *

Time Losof of moisture Thiamine Riboflavin

Product and type baking in baking Amount f Retention Amount t Retention of oven min. gamma/g. % gamma/g. %

Devil’s food “ A ” $ - - Unbaked

Radar 1 yti 7.1 Gas 25 13.1

Devil’s food “B”$ - - Unbaked

Radar 1% 6.7 Gas 25 14.4

Gingerbread “ B ” t Unbaked - -

Gas 25 10.8 Radar 156 8.0

White cake “ A ” (Cupcakes)

Unbaked - - Radar 1% 8.4 Gas 20 16.0

0.750 0.670 0.570

0.910 0.715 0.483

1.13 1.04 1.00

0.282 0.290 0.228

- 90.0 77.0

- 78.6 53.1

- 92.0 88.6

- 100.0

81.1

- - -

0.565 0.538 0.566

0.605 0.379 0.608

1.686 1.250 1.380

- - -

- 96.5

100.0

- 68.0

100.0

- 75.0 82.0

* Proctor and Goldblith (1948a). t Expressed on fresh moisture basis. t Cakes measured 8 (length) by 4% (width) by 1% (depth) in.

c. Browning foods d. Cooking effect is limited t o penetration of 3 inches. Because of small oven capacity, quantities of food must be cooked in small relays and as individual items-thus cancelling its reported electrical advantage over the electric range. The magnetron tube is short-lived. The average replacement time is ap- p r o ~ 17% hours during current tests totalling about 45 hours of cooking time. The timing device on the (equipment) operated defectively, frequently interrupting cooking prematurely.

For all cooking processes (pyrex or plastic cookware)

B. The manufacturer should be encouraged to develop further the device as a

1. 2. 3. 4.

practical appliance. There is essentially no smoke, fumes, or grease from cooking. There is very moderate heat radiation. Has excellent “short-order ” cooking qualities. I n conversion of electric energy to heat, tests indicated an efficiency 8u-

perior to the conventional electric range.


C. Foods in order of acceptability and adequacy of preparation: 1. Vegetables-retain natural flavor and color. 2. Eggs-little difference from conventional preparation. 3. Pies, cakes, cookies-little difference from conventional preparation. 4. Meats

a. b. c. Bread and other yeast products cannot be adequately prepared. a. b. Bottom remains soggy. c. d. Flavor is good.

Cooking is incomplete usually if more than 3 inches thick, No browning of meat surface, but this docs not affect palatability. Some meats requiring boiling remain tough.

Not of uniform consistency-doughy in spots.

Batter rises, has original color.

5.

D.

E.

F.

G.

H.

I.

Foods adequately prepared by (the equipment) found ready acceptability by test subjects aiid produced no adverse physical effects.

I t is suggested tha t a name not suggestive of wave propagation might avoid possible psychologioal aversions.

No adverse effects on the (equipment) operators were noted in 20 days of exposure to the device.

The operation of the (equipment) is simple and indoctrination in its usage is without difficulty.

With the tested unit, food can be prepared for 30 men by cooking small amounts repeatedly and employing a warming oven.

I n evaporating water to dryness the (equipment) utilizcs 56% less electrical eucrgy than the electric stove. I n preparing a meal for 10 men, no elcctrical advantage was noted because of tho limited oven capacity.

The sterilizing efficiency of microwaves on pathogenic bacteria which may con- taminate food was not determined. J.

Bollman e t aE. (1948) undertook a study a t the Quartermaster Food and Container Institute for the Armed Forces, Chicago, Illiuois, to evaluate the applicability and the practicability of the electronic range to large-scale feeding operations. The electronic range was evaluated from the standpoint of the speed of preparation of quantities of food sufficient to serve 100 men in a serving period of approximately 15 minutes. A number of foods were cooked electronically and compared with foods cooked by conventionai means. Bollman et al. found that, except for the baked items, a number of fresh and frozen products cooked electronically compared favorably in acceptability with those prepared by conventional means. The electronic cooking of large cuts of meat was found to be uneconomical because of excessive losses in weight during cooking.

These same investigators found, however, that although cooking with an electronic oven was rapid, the capacity of the oven was not great enough to justify its use in a short-order type of service in large army operations where the serving of 100 men should require only 15 minutes. With one oven, they found that 1 hour was needed to cook sufficient


beef patties electronically to feed 100 men one patty per man. They believe, however, that this type of equipment could be useful for special feeding operations such as between-meal service for a small number of men (an air-crew returning from its mission, guard duty personnel, etc.).

Thomas e t al. (1949) conducted a study to compare the retention of nutrients in meat and vegetables when cooked by the usual cooking methods with the retention in these same foods when cooked by electronic heat (radar energy). They found that the cooking time and the amount of water used were determining factors in the retention of nutrients in vegetables, The amount of water used had the greatest influence on the retention of water-soluble nutrients. The pressure cooking technique resulted in superior retention as compared with the retentions noted when boiling water and electronic range techniques were employed. The more water used, the greater was the leaching of nutrients that occurred. Destruction of ascorbic acid was also noted.

With meat patties, Thomas and her colleagues found that the thiamine retention was higher when the meat was cooked by high-frequency waves than when the meat was grilled. Niacin and riboflavin were equally stable with both methods of cooking. Beef roasts showed slightly better retention of thiamine, riboflavin, and niacin when they were prepared in a gas oven than when they were prepared in the electronic range.

One utilization of radio-frequency energy in the processing of foods that has recently come to the authors’ attention is the application of a radar oven for the bulk cooking of preserves. Preserves cooked in this manner are said to have better flavor and better retention of color than similar products cooked by usual commercial procedures.

6. Dehydration

Many drying processes are slow because of the difficulty of supplying heat to vaporize the moisture in a food substance. I n an ordinary oven the drying of food material progresses from the ouiside of the material in toward the center. Hence evaporation of the moisture on the inside requires energy from the outer portion. I n radio-frequency dehydration, energy is supplied to the center of the material as well as to the outside portion. Thus i t is possible to reduce the mvisture content of a material below 576, by radio-frequency dehydration, without obtaining case hardening which usually results when conventional means are used.

Markus (1944) indicated that electronic drying of ordinary powdered milk to a final moisture content of approximately 1% improved the storage life of this type of product.

Rushton (1945) experimented with the terminal dehydration of com-


pressed cabbage blocks and potato blocks. He estimated that to dry cabbage blocks from a moisture content of 9% to a content of 4% by high-frequency heating in open air would require from ll/$ to 2 hotm, and to dry potato blocks from a moisture content of 16% to a content of 6% by this method would require from 3 to 4 hours. The application of the method to large-scale drying would, therefore, present practical difficulties from the standpoint of careful regulation of the high-frequency power supply, to avoid heat damage to the blocks. He stated that better results and more rapid drying were obtained by drying in uacuo with high frequency.

Schade ( 1945a,b) has described the use of radio-frequency equipment for dehydration and has calculated the cost of capacitative dehydration.” From the standpoint of economics, the use of high-frequency capacitative heating is not recommended by Schade for the removal of large amounts of water, although he found that capacitative heating proved efficacious in removing the last percentage of moisture.

Brown et al. (1947) have reviewed extensively the development of radio-frequency energy for the drying of penicillin and have designed an automatic equipment, which is operated in conjunction with vacuum

7. Moisture Determinations

A method for measuring the moisture content of food materials by radio-frequency energy has been devised (Anon., 1945b). The material is placed between the two plates of a “capacitor,” an electrostatic field is produced, molecular stresses are set up which generate heat in the product, and the moisture is thus driven off by the heat generated in the sample itself. If the system is kept in a vacuum, evaporation is speeded and elimination of moisture from the surrounding atmosphere is achieved. The free moisture can then be eliminated in from 30 to 35 minutes without removal of any of the water of constitution.

Dunlap and Makower (1945) describe in some detail the radio-frequency dielectric properties of dehydrated carrots and discuss the application of radio-frequency energy to the determination of the moisture content of organic materials containing absorbed water.

A commercial model of an electronic moisture meter, which operates a t a frequency of 1 me., has been put on the market. It is calibrated directly to read the moisture content of cereals (Anon., 1949a).

8. Miscellaneous Uses Cerevitinov and Metlizkij (1935) studied the effect of a high-fre-

quency, low-powered generator on various fruit and vegetable crops, The economics of electronic heating is discussed in Section IV-9.


Cucumbers, apples, and pears, when subjected to radio waves f o r 1 second, were reported to keep better than control crops and lost less nutritive value and vitamin C. The irradiation of green tomatoes f o r 1 minute accelerated their maturing. The authors believe that high- frequency radio waves may be of importance to agriculture.

Lyman et a2. (1948) applied dielectric heating to cottonseed. The dielectric heating of cottonseed fo r 2 minutes a t a frequency of 14 me. per second and a power output of 1 kw. was sufficient to reduce markedly the moisture content of the cottonseed and seemed to retard materially the formation of free fatty acids in the seed during subsequent storage. The moisture contents were reduced from levels of 16-20% to 5-10% in less than 5 minutes of heating.

Lion and Could (1949) found that exposure of Saccharomyces cere- i / is iac to high-frequency radio waves a t 25 and 56 mc. caused increased rate of yeast growth, in the order of 30-4070 when exposure times of 10-20 seconds were used. With exposure beyond 150 seconds there was il decrease in the rates. Similar results were obtained by them when Penicillium sp. was exposed to these emanations. A field strength of 200 volts per em. was used. Treatment up to 30 seconds increased and longer treatment inhibited production.

Burton (1949a, b [see p. 196 for references]) has reviewed the literature on bactericidal effects of radio-frequency waves and other electromagnetic radiations, chiefly in respect to sterilization of milk.

9. Economics of Radio-Frequency Heating

Perhaps one of the greatest deterrents to the use of radio-frequency power is the cost of equipment for large-scale operations. Most of the equipment thus f a r on the market is relatively small and not designed for commercial operation.

Bock (1948) states that i f radio-frequency heating is t o be econom- ically justifiable, it should pay for itself by redusing processing costs and increasing the quality of the product.

The thermal requirements in terms of kilowatts of radio-frequency power may be calculated, according to Bock, by Lhe two following equations :

lb. per hr. X specific heat X O F . rise 3413

KW =

If water is to be evaporated, then

Ib. per hr. of water evap. x 970 3413

KW =


The sum of the values calculated by these equations plus an additional 10% of the sum for thermal radiation is considered by Bock a reasonable estimate for the required generator rating.

Smith (1944), in discussing t,he prevention of insect contamination in packaged foods such as flour, baking mixes, and cereals, states that a 3-kw. megatherm unit developing 10,000 B.T.U. 's per hour can sterilize, that is, kill the insects in approximately 480 one-pound packages per hour. The operating cost is 5 cents per hour, based on a rate of one cent per kilowatthour. Hence sterilization or de-infestation of insects can be obtained for 0.01 cent per package. If the amortization costs of the original unit are totaled, the cost of replacement parts and the power costs over a 10-year operating period would amount, on the average, to 17 cents per hour of operation. These data have been reaffirmed in another article (Anon., 1945a).

Kinn (1948) has presented a number of charts and tables for determining the economic feasibility of radio-frequency heating.

Sherman (1946) discusses the maintenance and operating costs of high-frequency generators. Service cost records, based on several hun- dred high-frequency generators ranging in output from 3 to 75 kw. and located throughout the East and Middle West, have indicated that the average annual maintenance cost is approximately 5% of the purchase price of the generator. According t o Sherman, the total of all the elements of operating cost rarely exceeds 10 cents per hour per kilowatt of output. A 3-kw. output unit can be operated for about 30 cents per hour, whereas a 25-kw. output unit can be operated for only $1.30 per hour. These figures take into account an overall electrical efficiency of 50%, a conservative tube life of 2000 hours, an equipment life of ten 2400-hour work-years (300 days X 8 hours), and an annual maintenance cost equal to 5% of the purchase price.

The initial cost of the equipment f o r use with foods, according to Sherman, varies with the size. For a 13-30 mc. machine, small units vary in cost from $750 t o $1000 and larger units from $400 to $500 per kilowatt of output.

Schade (1945a) has calculated the cost of dehydration by high- frequency heating. He states that the capital investment for high-frequency equipment that will vaporize 1 lb. of water per hour, a t an initial temperature of 25" C. (77" F.), is $500. This means, roughly, one million dollars per ton of water vaporized per hour. He calculated the operating costs to be $40 per ton and the cost of electricity alone to be from $6 to $8 per ton.


V. USES OF INFRARED RADIATION

1. Dehydration

Infrared lamps have been used for some time in industries other than the food industry, for example, for the “drying” of paint on metal surfaces in the automobile industry.

Sluder et al. (1947) utilized infrared lamps in a vacuum-belt drier for the dehydration of orange juice. Banks of infrared lamps supplied the source of heat and were controlled easily and accurately by means of rheostats. When this type of equipment was used on a pilot-plant basis, a satisfactory dried powdered orange juice was obtained. This equipment has been utilized since for drying numerous other food products a t relatively low temperatures, including gluten, whole egg, egg white, and pharmaceuticals, with excellent results.

2. .Flour Milling

Fournier and Digaud (1944) found that infrared radiation had little destructive action on the thiamine and riboflavin contents of wheat germs.

Prat t (1945, 1948) has described an infrared toasting unit to treat wheat germs in a flour milling system. Using this unit was said to make possible the inactivation of the fat-splitting enzyme and to change the germ to a friable condition without substantially altering the vitamins or mineral components. The dried germs were then returned to the grain material passing through the milling system. Prat t states that by this means a sterile product of improved palatability and keeping quality can be obtained.

3. Protection of Prwits and Vegetables from Frost Damage

Experiments have been conducted at Michigan State College to develop a practical method of protecting fruits and vegetable crops from frost damage by the use of infrared radiation (Anon., 1947). An infrared heat unit has been developed, which is reputed to show considerable merit in preventing frost damage. This unit is still in the experimental stage, and an improved model is planned which will maintain vegetation temperatures from 6 to 8 degrees warmer than the air temperatures over an entire acre of ground.

Further work has gone on a t this institution and on January 1, 1948, the College turned over to industry, for manufacture, complete drawings of an infrared, radiant type of frost control machine (Hansen and Farrall, 1949). During an extensive series of tests a t the College Farm


in the fall of 1948, tomatoes under field conditions were protected by one of these units from a radiation type of frost, when the temperature a t 1 in. above the ground was -4' C. (26' F.) outside the protected area. A number of similar installations are now under test in the citrus groves of California.

4 . I n P o d t r y Rearing

Another use of infrared radiation has been described by Hemker (1947). Infrared radiation is applied to brooders as a heat source. Two lamps are mounted on opposite sides of the brooder, and zones of heat are thus provided. The chicks serve as their own thermostats by moving to a cooler area if one zone is t,oo warm for their comfort. Infrared lamps are in common use for brooding chicks, turkey poults, and baby pigs wherever electricity is available a t a reasonable cost.

5. I n Chemical Analysis

The use of infrared for chemical analysis is becoming of extreme importance. The infrared absorption spectra of a number of compounds have been determined and the results published (Barnes et al., 1948).

The application of this technique to foods has been retarded for several reasons: (1) Pure chemical compounds of many types must first be analyzed and their infrared absorption spectra determined. Then it will perhaps be possible to apply this technique to foods. (2) A great many nutrients are water soluble and cannot be analyzed by this technique. Such compounds must be analyzed by this technique in solvents other than water, preferably carbon bisulfide. However, a large number of chemical compounds in foods are oil soluble, and for such compounds this technique may be applicable. (3) Most foods contain many complex chemical compounds, and analysis and interpretation of the infrared spectra of these would be very difficult.

Proctor and Kenyon (1949) have applied infrared absorption technique to follow deteriorative changes in citrus oils and have been able to correlate changes in odor and oxygen absorption with chemical changes as determined by the infrared absorption spectrum.

Downing et al. (1946) found that infrared spectroscopy is a valuable technique for the examination of technical DDT, to detect isomeric distribution and impurities in the DDT.

VI. USES OF ULTRAVIOLET LIGHT

1. Germicidal Action

The bactericidal effects of ultraviolet light were observed as early as 1903 (Rentschler et al., 1941)) but they were not studied with any degree


of quantitative accuracy until more recent years. Gates (1929a,b), in his excellent researches, showed that the wavelength for maximum germicidal action was 2660 A. His and other important researches have been thoroughly discussed by Rentschler et al. (1941) and by Ewell (1948).

Rentschler et al. (1941) have made significant contributions to the quantitative aspects of the bactericidal effects of ultraviolet radiation. They found that for E . coli the destruction was the same when the seeded plates were irradiated at 5' C. (41" F.) as at 37" C. (98.6" F.). They showed, further, that for this organism there were widely different resistances to ultraviolet radiation, depending on the different stages in its life cycle. A sublethal dosage of radiation was shown to retard the rate a t which colonies developed after irradiation.

Ewe11 (1948) reported that air motion between light source and bacteria reduces the killing time for microorganisms being irradiated. The greater the distance of the surface being irradiated from the ultraviolet lamp, the longer is the time required for killing a certain percentage of microorganisms. This author stated that Mucor (one of the common meat moulds) is about six times more resistant to the bactericidal action of ultraviolet radiation than is E . coli. Pewicillia are from 5 to 15 times as resistant as bacteria, and yeasts have the same resistance as or are slightly more resistant than bacteria.

Ewell (1946) stated that, in application of ultraviolet lamps to food preservation and food processing, ozone had a bactericidal effect and that commercial lamps producing a maximum amount of irradiation at 2537 A (which is close to the peak bactericidal wavelength of 2660 A found by Gates, 1929a,b) also produced and transmitted some irradiation a t 1850 A. This wavelength produced some ozone by reacting with the oxygen of the air, thus increasing the bactericidal effect of the 2537 B irradiation.

DQribBrB (1947) has reviewed the use of ultraviolet light for germicidal action. IIe found that at low temperatures the intensity of the active rays decreases considerably, the lamps giving 60% of normal production a t 0" C. (32" F.) and 30% at -18" C. (0" F.). Under these circumstances, a longer time is necessary for bactericidal irradiation than at ordinary temperatures [9" to 38" C. (48" to 100" F.)].

I n the United States, 30-watt ultraviolet lamps are commonly used for sterilization. The capacity of these lamps, with a suitable reflector, is 1000 cubic feet of air per minute (DBribBr6, 1947). The active ultraviolet 'rays were first produced by mercury vapor lamps a t normal pressure in quartz tubes. The mercury vapor, however, absorbed part of the rays. Low-pressure lamps avoided this drawback. These, however,

TABLE I V

Irradiation Time for Destruction of Bacteria, Moulds, and Yeasts by Ultraviolet Light

Clicks t on Standard Tantalum Cell Meter

Substance 80-90% kill 90-9570 kill 95.100% kil l Bacteria

Streptococcus hemolyticus (alpha type) Streptococcus hemolyticus (beta type) Streptococcus lactis Staphylococcus aureus Staphylococcus albus Neisseria catarrhalis Micrococcus piltonensis Micrococcus sphaeroides Sarcina lutea Gorynebacterium diphtheriae Shigella paradysenteriae Eberthella typhosa Pseudomonas Jluoresccns Escherichia coli Proteus vulgaris Serratia marcescens Phytomonas tumefaoiena Bacillus anthracis Bacillus fusiformis Bacillus mbt i l i s Bacillus mbt i l i s spores Rhodospirillum rubrum

Saccharomyces ellipsoideus (wine yeast) Saccharomyces ellipsoideus

(wild yeast from grapes) Saccharomyces sp. (from orange juice) Saccharomyces cerevisiae

Brewers’ yeast Bakers’ yeast Common yeast cake

Pcnicilliuna roqucforti Penicillium expansum Penic ill ium digitatum Aspergillus glaucus Aspergillus f l a w s Aspergillus niger Oospora lactis Y u c o r racemosus Rhizopus nigricans

*Anon. (1941).

Yeasts

(Molasses distillery yeast)

Molds

15-18 15-23 25-30 15-20 18-22 15-20 35-40 50-60 80-95 20-30 15-20 15-20 10-20 15-20 10-15 15-20 15-20 20-30 10-15 25-35 40-50 25-30

20-30 40-50

25-30 25-35

12-15 10-20 20-30

18-22 23-28 30-35 20-24 22-25 20-25 40-50 60-65 95-110 30-40 20-25 20-30 20-25 20-25 15-20

20-25 20-25

30-50 15-20 35-45 50-65 30-35

30-40 50-60

30-35 35-40

15-20 20-25 30-40

22-25 28-35 35-40 24-30 25-30 25-50 50-60 65-75

110-130 40-50 25-30 30-35 25-32 25-30 20-30 25-30 25-45 50-65 20-40 45-55 65-100 35-40

40-60 60-80

35-50 40-60

20-30 25-40 40-60

150 200 500 350 500

1500 80

180 1500

, I

t A “click” is the amount of bactericidal ultraviolet energy produced by a Sterilamp in 1 second at a distance of 6 in.


required higher voltages. More recently, the development of fluorescent lamps has provided new ways of attaining the desired effect.

An exeellent chart has been published (Anon., 1941) on the ultraviolet irradiation time for bacteria, moulds, and yeasts when a commercial lamp is used that produces 84% of the total energy radiated between 2500-6000 B at the 2537 A wavelength (Table IV) .

Mallmann and Churchill (1946) have presented an excellent review of the use of ultraviolet light in the control of microorganisms in food storage rooms. They also discuss laboratory tests on the uses of carbon dioxide, glycols, hypochlorites, and quaternary ammonium chloride compounds for the same purpose.

The use of ultraviolet light in dairies and in food plants has been reviewed by Dennington (1945). Sanitization of the air in dairy barns has been achieved with the use of these radiations. Dennington states that the milk room where the fresh milk is strained and cooled is another place where ultraviolet rays can be effectively utilized. These ultraviolet lamps are also used to irradiate washed milk cans, coolers, and storage tanks (Kennedy, 1945).

Porter (1940) has reviewed a number of applications of ultraviolet light in the food industries, including meat, baking, and dairy industries. Hasson (1945) has discussed the use of ultraviolet light in sterilizing dairy equipment.

Another use of ultraviolet light in the dairy industry has been to sterilize air in conjunction with air filtration for producing heat-sterilized bottled cream (Havighorst, 1945). Such sterilized cream has been produced and filled aseptically into presterilized bottles which, with their caps, are steam-treated. It is claimed that the product will stay satisfactory from a flavor standpoint for one year and retain its natural consistency for that time at room temperature.

Nicholson (1947) has described equipment for pasteurizing milk by exposing it to ultraviolet light in thin films on the surfaces of revolving cylinders. Such equipment may also have application in the pasteurization of fruit juices.

It is reported in trade publications that manufacturers of milk bottle caps have irradiated the hood type caps from the moment the paper is unrolled until the end of the manufacturing process, thus helping to eliminate the possibility of bacterial contamination (Anon., 1939).

Koller (1939) found that low-pressure mercury-vapor lamps, when used under proper conditions, are powerful bactericidal agents if the bacteria are directly exposed to the radiation, but that the effectiveness is greatly reduced by even such slight shielding as is offered by a film of grease.

APPLICATIONS O F ELECTROMAONETIC RADIATIONS 151

Yesair and Williams (1942) found ultraviolet light ineffective for the sterilization of spices.

Smith and Perry (1941) used ultraviolet rays t o lower the bacterial content of vegetable hydrocoolers. The light was able to penetrate 13 cm. of distilled water without apparent diminution of energy. Turbid waters of even slight depth markedly decreased the effectiveness of the light. Submerging the lamps in water resulted in a noticeable decrease in the bacterial content of the water.

A similar use of ultraviolet light has been known in the fish industry since 1938. Immersed lamps are used in water tanks for reducing the bacterial load and have been found efficacious (personal communication, J. T. R. Nickerson, 1949).

The use of ultraviolet light in air sanitation has also been applied in the syrup field, as syrup makers install ultraviolet lamps over syrup vats and storage tanks to eliminate surface mould (Kennedy, 1945).

Many hospitals are wing ultraviolet light to decrease the number of organisms in the air (Salle, 1943). The quota of energy needed f o r destroying the spores of different species of moulds by ultraviolet radiations are given by Nagy (1947).

Luckiesh and Knowles (1948) showed that irradiated cultures of E . Cali developed greater resistivity t o sublethal dosages of ultraviolet energy (A 2537 A) than did unirradiated cultures. The greatest increase observed in the resistivity of the irradiated strains of E . coli was approximately 100%.

Hall and Keane (1939) showed that the spores of B. stearothermo- philus-Donk are killed in dry white sugar irradiated by ultraviolet light in the region of 2537 A. The lethal action was enhanced by turbulence of the sugar during irradiation. On the average, 47.8% of the spores were killed by irradiating eight successive strikes of sugar with 24 lamps in a sugar granulator. The results obtained in this experiment indicate the possibiilty of sterilizing white sugar and other such food ingredients by ultraviolet light.

Lang (1945) also worked on this problem and found that ultraviolet light may be used to destroy thermophiles in sugar. The need for sugar free from thermophiles, particularly for the canning of nonacid vegetables, is important, and the use of ultraviolet light offers interesting potentialities.

Cathcart et at. (1942) found that ultraviolet rays (2000 to 2950 A) effectively reduced the counts of S. aureus and 8. enteritidis in the air and on smooth surfaces but did not penetrate the surface of custard products. Irradiation reduced the degree of contamination of these


products from the atmosphere. However, an off aroma was present in custard products exposed to these lamps for 2 hours or more.

I n the ice cream industry, ultraviolet light has been used when the mix has been cooled on a surface type of cooler. Under such conditions the bacterial count may be reduced by more than SO%, according to Dennington (1945).

Ultraviolet light is used in locker plants to reduce the bacterial count on the surfaces of meats previous to the freezing of the meats (Denning- ton, 1945).

Direct exposure of vegetables to ultraviolet light may result in the formation of brown spots on the green leaves. Hence ultraviolet rays should not be permitted to strike vegetables directly. They are used, instead, to sterilize the air in rooms where vegetables are stored and thus extend the time before slime and spoilage develop (Dennington, 1945).

Vaillancourt (1945) has reviewed the use of ultraviolet light in the poultry industry. He stated that ultraviolet light speeds growth in chiclrs and reduces mortality in brooder houses.

Veloz (1945) has also reviewed the agricultural application of ultraviolet light.

Bender (1942) has shown in actual brewery tests that airborne infec- tions can be effectively checked by the use of ultraviolet lights over the fermenting vats.

2. Aging of Meat

The "Tenderay" process of aging meat is another application of ultraviolet radiation for surface sterilization. By this process, which uses a temperature of approximately 15" C. (SO" F.), air currents, and ultraviolet light to reduce surface mould, meat is aged more rapidly than by the conventional means (Tressler and Evers, 1947).

Fredholm (1946) studied the germicidal and chemical action of ultraviolet light on meat, pork, sausages, animal fats, and eggs. Irradiation of the air surrounding the foods as well as the food itself decreased the speed of bacterial decomposition so that a longer storage time (up to 30% longer) was obtainable for meat, pork, and sausages. The rancidity of fa t was increased by direct irradiation, whereas the storing of fat in an atmosphere of irradiated air was said to cause no increase in rancidity.

Volz et al. (1949) found that pork fa t which had been exposed to ultraviolet radiation became more susceptible to development of rancidity during subsequent storage in the frozen state. The studies indicated that pork might have an appreciably shorter storage life a t -18"


or -12" C. (0" or 10" F.) when the meat cooler was equipped with ultraviolet lamps.

3. Activation of Provitamins D

One of the most important uses of ultraviolet light is for the activation of the provitamins D, which was initiated by Steenbock (1924, 1928, 1935) of the University of Wisconsin. Cows irradiated with artificial ultraviolet light or exposed to sunlight secrete milk of increased vitamin D potency. The most common procedure is to irradiate the milk itself in a thin film. This procedure is one requiring careful control, because excessive irradiation may produce an unpleasant odor and taste. A number of process modifications have been patented (Rosenberg, 1945).

A similar use of iiltraviolet light has been made in poultry husbandry. Here ultraviolet light serves two purposes. First of all, poultry mortality has been reduced as much as 68% by the germicidal activity of ultraviolet light in poultry houses (Kennedy, 1945). In addition, the growth of the young birds is said to be accelerated because of the activation of provitamins D.

4. To Improve Coffee Blavw

A patent was issued (Kennedy, 1944) for the ultraviolet irradiation of green coffee. It is stated by the holder of the patent (Kennedy) that when the chlorogenic acid content of green coffee berries is above 776, the flavor of the roasted coffee is harsh or otherwise objectionable. When the green coffee berries of such composition are irradiated with ultraviolet light for as short a time as 15 seconds, the treated berries, when roasted, are said to have a markedly improved flavor, provided they were irradiated on all sides.

5. As art Analytical Tool

Ultraviolet radiations have proved of tremendous value as an analytical tool. Numerous compounds have been identified by means of their characteristic ultraviolet absorption spectra.

Further use of ultraviolet light in this respect has been made for the estimation of thiamine and riboflavin in foods. Here ultraviolet light is used as incident energy for producing fluorescence of thiamine and riboffavin. Other vitamins, as vitamins A and D, are measured quanti- tatively by determining their extinction at certain wavelengths in the ultraviolet spectrum, after certain preliminary procedures.


6. Detection of Infected or Washed Eggs

According to Lorenz (1950) ultraviolet light has recently been found to have two important uses in the egg industry : (1) to detect egg spoilage due to fluorescent bacteria and (2) for presumptive determination of whether or not a lot of eggs has been washed. Earlier Urbain (1945) developed the scheme for detecting high count eggs using ultraviolet fluorescence.

I n spoilage determination a source of long-wave or near ultraviolet of moderately high intensity is necessary. A suitable lamp is the 100-watt General Electric C-H4 with a suitable violet filter. Mercury-vapor lamps intended for therapeutic use are less satisfactory because of the preponderance of far ultraviolet in their emission spectrum. The lamp is most suitable when shielded with a small aperture and used like a candling light, but may also be used with somewhat less precision without this feature.

The development of Pseudomoms organisms produces a fluorescent pigment in the albumen. Sufficient ultraviolet penetrates the cell to excite this fluorescence which may be seen as a bright greenish glow when the eggs are illuminated with the above lamp. When used in egg grading, fluorescence is a very sensitive test, and the mere beginnings of colony growth may be detected as a greenish spot floating in the albumen. With further development of spoilage, the entire egg appears brilliantly green. After spoilage has progressed still further and yolk and albumen start to mix, the egg becomes nearly opaque to ultraviolet and looks dark before the light. At this latter stage, such eggs can ordinarily be detected before an ordinary candle. Detection with an ordinary candle, however, is not possible a t the earlier stages of this type of spoilage.

The normal egg shell has a red or red violet fluorescence which is bleached by a t least some types of soiling. Subsequent removal of the soil does not affect this bleaching. Consequently soiled eggs that have been washed clean have a mottled appearance when illuminated with ultraviolet light. This determination is not absolute, however, because bleached streaks and blotches appear on unwashed apparently clean eggs (the mechanism and causes of bleaching the shell fluorescence are not clearly understood.) Nevertheless when several dozen washed eggs are examined at one t h e , the incidence of bleached spots and blotches is sufficiently characteristic to distinguish these from unwashed clean eggs. This technique has been useful in packing plants to detect washing prac- tices, although it is not useful in picking out individual washed eggs from a pack. The advantage of the method is that it permits very


rapid checking of large numbers of eggs. The requirements for the ultraviolet light are less critical for this purpose than they are for the detection of albumen fluorescence. Almost any sort of ultraviolet lamp is satisfactory.

VII. USES OF RADIOACTIVE ISOTOPES

The uses of isotopes, both stable and radioactive, have solved many of the heretofore baffling problems in nutrition, biochemistry, metallurgy, and other fields. These uses have been adequately covered by Hevesy (1948) and others. Potential applications of radioactivity to agriculture and foods will be reviewed in this section.

1. Germicidal Action.

soon after the discovery of radium and x rays, investigators in the field of bacteriology were interested in the effect of radiations on microorganisms (Pacinotti and Porcelli, 1898 ; Prescott, 1904). I n fact, the first investigations at the Massachusetts Institute of Technology on effects of electromagnetic radiations on bacteria were conducted by Prescott (head of the Institute’s Department of Biology and Public Health for twenty years, Dean of Science for ten years), for whom the Samuel Cate Prescott Laboratories of Food Technology a t the Institute are named.

More recently Schmidt (1948) studied the effects of radioactive phosphorus (P32) on E . coli. He found that the beta radiations of P32 had a lethal effect on E. coli and that, in general, this effect was related to the initial concentration of the P32. Complete sterilization of the suspension was not obtained, even in the highest concentration used. With 1000 microcuries per milliliter, approximately 1% or less of the cells were able to survive the irradiation.

From these data it would appear that complete sterilization of food materials containing bacteria that are not spore formers cannot be obtained by application of radioactive elements. Moreover, according to Schmidt, a t the prices for radioactive isotopes listed at the time his experiments were conducted, the use of 1000 microcuries for a No. 2 food can would cost $3660.

I n addition to the factor of cost, still another must be considered, namely, that radioactive isotopes may prove harmful to health. A number of radioactive isotopes, such as sodium, have a short half-life, but many more, such as carbon14, have a half-life of the order of 3000 years. Hence one would be unwilling to release, for human consump- tion, food products irradiated with such isotopes (even if t.he food


products were sterile) until the radioactivity of the products was ex- hausted.

Pearson et al. (1948) studied the effects of radioactive iodine, radioactive phosphorus, and high voltage x rays on forty-three types of fungi. The fungi were exposed to 20 microcuries of radioactive phosphorus or iodine or were exposed to an equivalent dosage of x rays (1100 roentgens). Most of the fungi survived the exposure to the beta and gamma radiations a t the levels cited without observed alteration in their morphology or pathogenicity.

From the above reports it would appear that radioactive substances would have slight usefulness in the food industry, in so far as the sterilization of microorganisms is concerned.

2. Applications in Agriculture

One application of isotopes in agriculture is the incorporation of radioactive minerals into fertilizer for the purpose of studying the mineral requirements of plants. Considerable work along these lines has been done by the United States Department of Agriculture and the agricultural experiment stations of the country. The results have been reported by Hendricks and Dean (1947) and reviewed by Hendricks (1949). They found that superphosphate and calcium metaphosphate are the most efficient phosphatic fertilizers. The localization of the specific activity of the phosphorus was accomplished by the use of P32, Geiger counters, and radioautographic technique.

The applications of isotopes in agriculture are almost unlimited. Iso- topes can be used to determine the nutritive requirements of plants in different types of soil, t o ascertain the proper location and type of fertilizer, to establish the proper timing of the application of the fertilizer, and possibly to determine the metabolic changes in plant tissues.

VIII. USES OF X RAYS

1. Germicidal Action

X rays have been of interest to the chemist, the bacteriologist, and the food technologist almost since their discovery by Roentgen in 1895. Some of the literature on microbiological investigations with x rays has been reviewed by Dunn et d. (1948).

Dunn e t aE. (1948) also determined the effects of supervoltage x rays on a number of bacteria, yeasts, and moulds. They found that these microorganisms could be destroyed by x rays and that the dosage necessary for destruction varied with different types of microorganisms. For instance, E. coli was completely destroyed by 150,000 roentgen units ( r ) ,

APPLICATIONS OF ELECTROMAUNETIC RADIATIONS 157

A. aerogenes by 250,000 r, and putrefactive anaerobic spore formers by 1,500,000 to 2,000,000 r.

The mechanism of x ray destruction of bacteria is explained according to the “direct hit” theory, also called the “target” or “Treffer” theory. This theory, which is thoroughly discussed by Lea (1947) in his excellent book on “Actions of Radiations on Living Cells,” states that bacteria are destroyed by direct hits of the photons on the cells. The radiosensitivity of a particular species of bacteria is measured by determining the dosage of x rays necessary to destroy 63% of the bacteria or, conversely, the dosage for 37% survival. This is known as the “mean lethal dose” or the “37% dose.”

On the assumption that the destruction of bacteria is brought about by direct hits, the survival curve would be exponential and therefore a straight line when plotted on semi-logarithmic paper as a first order reaction. According to Lea (1947), the destruction of the bacteria by photons is based on the mathematical probability of a direct hit taking place.

Proctor, Van de Graaff, and Fram (1943) found that ground meat could be sterilized by irradiation with supervoltage x rays. As x rays produced a t high voltage are able to sterilize food materials without raising the temperature to an appreciable extent, this might appear to be an efficient method of “cold” sterilization. However, approximately 95% of the electron energy goes into heat in the x ray equipment when it strikes the target interposed between the electron source and the object irradiated and only 5% into the production of x rays. Hence, from a power standpoint, the x-ray process is extremely inefficient as a means for sterilization of food.

As an example, with electrostatic accelerators operating a t 3 million volts, approximately 8 minutes of operation a t maximum efficiency would be required to produce sufficient radiation to destroy completely bacterial spores in one can (No. 2, approx. 532 ml.) of meat product. Although this would be possible, the time required and the low eficiency from a power standpoint would appear to make such a method one of questionable application for the food industry, where high speed operations and economy are most essential.

The use of electrons, or cathode rays, presents a somewhat different and more favorable situation, which will be discussed in Section IX.

2. Effect on Vitamins

The effects of x rays on vitamins have been studied a t the Massa- chusetts Institute of Technology (Goldblith and Proctor, 1949 ; Proctor and Goldblith, 1948b, 1949b). Niacin was found to be the most radio-


resistant of the three vitamins compared, i.e., niacin, riboflavin, and ascorbic acid. Ascorbic acid was the most radiosensitive. When a mixture of several vitamins was irradiated, the relative effect on any specific vitamin was changed. This may be illustrated in the following manner. Although, chemically and radiobiologically, ascorbic acid was far more radiosensitive to x rays (both hard and soft) than niacin, when thk two were mixed and irradiated, the niacin was destroyed to a greater and the ascorbic acid to a lesser extent than when the two were irradiated separately. Hence in the mixture of the two vitamins the niacin protected the ascorbic acid from the radiations.

Proctor and Gtoldblith (194913) found that x rays produced at 3000 kv. were from seven to eight times more destructive to vitamins than x rays produced at 50 kv. A possible explanation is that photons accelerated by 3 mev are sufficient to disrupt the nuclei of atoms, whereas 50-kv. x rays may effect only a rearrangement of the crystal lattice structure of molecules and atoms.

3. X-Ray Diffraction

An important analytical use of x rays is that of x-ray diffraction, for identification of compounds and for determination of structural changes in compounds. When the possibility of x-ray diffraction by crystals first occurred to Laue (1912), x rays were generally regarded as being electromagnetic waves of short wavelength. After the discovery of x-ray diffraction by Laue, a number of physicists turned their attention to this phenomenon. One of the foremost was W. H. Bragg, who has done some classical work in this field. For a more extensive treatment of this subject, the reader is referred to Sproull (1946).

4. ~ l U ~ 0 S C O p y

Certain chemicals fluoresce when exposed to x rays. For example, when a substance to be examined is placed between an x-ray source and a cardboard coated with barium pIatinocyanide, the variations in density of the substance are outlined on the coated fluorescent screen by varia- lions in the degree of fluorescence.

For inspection purposes, fluoroscopy or the use of a fluorescent screen has four disadvantages when compared with radiography. A fluoroscopic screen lacks the integrating power of a photographic film or the contrast achieved by an x-ray film and does not have the resolving power. Lastly, it is only temporary. I n spite of these disadvantages, there are instances where fluoroscopy is useful in the food industry.

As a means of determining metallic contaminants in foods, fluoroscopy has proved useful in equipment whereby foods or foods in packages may


be conducted past a fluoroscopic screen and viewed by an operator. One disadvantage in this use, however, is the eyestrain which has been reported to occur with some operators.

Kennerly (1945) mentioned the application of fluoroscopy for evalu- ating fruit. I n 1936-1937, some of the orange growing areas in Cali- fornia were subjected to a hard frost. According to the standard flotation test used as a criterion for the rejection of damaged fruit, almost half the crop in these areas would have been discarded. The more accurate x-ray fluoroscopic method showed that 50% of the fruit rejected by the flotation test met edible standards. I n that season, 100 fluoroscopy units costing $300,000 were reported to have saved fruit valued a t approximately $7,500,000. This equipment was used to advantage in early 1949, following another occurrence of unusual weather or hazardously low temperatures.

I n spite of the four disadvantages mentioned previously, fluoroscopy has its advantages in the evaluation of fruit. First of all, it is a speedy method. It is easy to examine one orange per second fluoroscopically and reject the bad ones, but it would be difficult and extremely expensive to radiograph them. Secondly, the equipment is not expensive compared with the value of the fruit saved thereby. Fluoroscopy is also adapted to inspection of moving objects, such as those on a conveyor belt. Radi- ography is not equally adaptable t o such situations.

5. Destruction. of Toxins

Ephrati (1948) has found that x rays have a destructive effect on bacterial toxins. He states that his experiments support the theory that the effect of the radiations is indirect. Purification of tetanus toxin greatly increased its sensitivity to x rays. However, the dosage needed to destroy the toxin is so great and the time required for its production is so long that the practicability of using x rays for toxin destruction seems remote. Purified tetanus toxin was destroyed by 7 X lo6 r, whereas at this dosage crude toxin was only 20% inactivated.

6. Miscellaneous

At the Balsgard Frui t Breeding Institute in Sweden, Granhall (Anon., 1949b) exposed two pear shoots to x-ray irradiation and grafted them to a mature tree. The two treated graftings brought forth excep- tionally large, abnormally pointed fruit which ripened from three weeks to a month after other pears in the region. Other investigators in Sweden, a t Svalof Institute, found that x-ray radiation techniques produced a hardy, early ripening barley ideally suited to the Swedish climate. The fruit breeders hope to use the x-ray grafting technique to


develop new varieties of fruits that can mature in and survive through Swedish winters.

Recently the Grindrod Corporation of Wisconsin (Haeuser, 1948) has announced a method for the rapid aging of cheese with the use of x rays.

Beadle (1946) has described experiments in which mutations were produced in Neurospora by the use of x rays. By this means Neuro- spora mutations resulted, with changed requirements for various vitamins or amino acids.

Prescott and Dunn (1949) cite data obtained by Demerec and asso- ciates, who obtained increased yields of penicillin by inducing mutations of P . chrysogenum X-1612 by the use of x rays.

IX. USES OF CATHODE RAYS

In recent years several types of apparatus for producing high-voltage cathode rays have been developed. One of these is the Van de Graaff Electrostatic Generator (Trump and Van de Graaff, 1948) and another is the Capacitron developed by Braseh and IIuber (1947).

The Van de Graaff generator offers a means for production of electrons by carrying charges from ground to terminal on a rapidly moving belt and accelerating the electrons down an evacuated x-ray tube at constant potential. With existing equipment, electrons are produced at from 3 to 4 mev. A generator capable of producing 12 mev. is under construc- tion. X rays are produced by allowing the electrons to strike a gold target. If the target is removed, the electrons o r cathode rays can be used. A 3-million volt Van de Graaff accelerator, now in use at the Massachusetts Institute of Technology, is illustrated in Fig. 9.

The Capacitron of Brasch and Huber is an electric impulse generator, for the production of electrons. This apparatus produces high voltages and great electron intensities during ultra-short periods of time (of the order of microseconds).

The majority of the experiments cited in this section were conducted with these two types of equipment.

I n addition to the Capacitron and the Van de Graaff accelerator cited above, a number of other sources of ionizing radiations are available. These include the betatron, the linear accelerator, the synchrotron, the proton-synchrotron, and the synchro-cyclotron. Although it is not the purpose of this paper to discuss the physical aspects of these various particle accelerators, an effort has been made to summarize, in Table V, the characteristics of each.

The betatron, the synchrotron, the proton-synchrotron, the cyclotron, and the synchro-cyclotron seem to present fewer potentialities for the sterilization of food materials, mainly because of their relatively small


beam current. The linear accelerator, however, may offer some potentialities as a source of ionizing radiations for the sterilization of food and drug materials. A linear accelerator has been built recently a t the Massachusetts Institute of Technology which is capable of 21 mev. with an average output current of from 30 t o 40 microamperes. Such a power output is sufficient to sterilize biological materials.

FIG. 9. Van de Graaff Electrostatic Generator, with steel pressure tank removed. (Courtesy of Prof. J. G. Trump, Massncliusetts Institute of Technology.)

Accelerator

Voltage Multiplier

Van de Graaff

Resonant Transformer

Betatron

Linear Accelerator

Synchrotron

Bevatron

Cyclotron

Surge Generator

Cascade Generator

Particle

Proton

Electron @ ion

Electron @ ion

Electron

Electron @ ion

Electron

@ ion

@ ion deuteron a particle

Electron or @ ion

Electron

* Theoretically unlimited. t Deuterons.

TABLE V

Characteristics of Some Particle Accelerators

Maximum energy (mev.)

1.5

5-12

2

-110

(50)

350

3000

20 t

10 0

5

4

output current

15 ma

ca. 4 ma

10 ma ( f )

<0.1 pa

0.001 pa

0.001 pa ( S )

800 f l t

200 F n 50,000 amp. (max. burst)

Wave form

Continuous

Continuous or pulsed

Continuous

Pulsed

Pulsed

Pulsed

Pulsed

Modulated

Pulsed 12-20

per min.

Pulsed

$ Internal target. 9 Protons.

Present application X rays for industrial use; sterilization of f oads and biologicals

Nuclear research ; deep tissue therapy; industrial radiology; sterilization of foods and biologicals

Deep tissue therapy; industrial radiology; sterilization of foods and drugs

Deep tissue therapy; industrial radiology; sterilization of foods and biologicals ( S )

Nuclear research; deep tissue therapy ( S ) ; industrial radiology ; possibly sterilization of foods and biologicals

Physical research

Deep tum'or ; physical research

Nuclear research ; artificial radioactivity

Sterilization of foods and drugs; tumor treatment

Sterilization of foods and drugs; tumor treatment

7 External target.


Besides the Van de Graaff accelerator, other sources of power for the production of x rays are in use a t the present time. One is the resonant transformer, manufactured by the General Electric Company (Charlton c t al., 1940), which is capable of producing x rays-and possibly of utiliz- ing the electrons themselves a t a voltage above 2 mev. Another source for the production of high voltages, which has been used for supervoltage x-ray therapy, is the transformer rectifier equipment. This equipment has been utilized in England.

I. Cermicidal Action.

The literature prior to 1932 regarding the effects of cathode rays on bacteria has been reviewed by Coolidge and Moore (1932). As early as 1926, they reported that cultures of S. aurcus, B . coli, B. subtilk, and B . prodigiosus were killed by short exposure to cathode rays. Wyckoff and Rivers (1930) reported that the absorption of a single 155-kv. electron was sufficient to cause the death of B. coli and B. aertryke and that all, or nearly all, the electrons absorbed were lethal to the bacteria.

Dunn et al, (1948) showed that electrons are capable of destroying bacteria in a relatively short period of time ( 3 to 29 seconds). These investigators used a Van de Graaff electrostatic generator operating at 3000 kv. With the equipment existing at the time, 1,000,000 roentgen- equivalents-physical (rep) of beta (cathode) rays were produced in a period as short as two-fifths of a second.

These same investigators also found that spore formers could be destroyed by from 1.5 to 2.0 x lo8 rep of cathode rays. Many types of microorganisms were destroyed rapidly by these radiations. Several miscellaneous types of products were also exposed to cathode rays, among these surgical sutures. The sutures were found to be sterilized by cathode rays without loss in tensile strength. Sutures sterilized by conventional means are said to lose an appreciable amount of their tensile strength. Proctor, Goldblith, and Fram (1950) found supervoltage cathode rays efficacious in sterilizing a number of spices containing aerobic and anaerobic spore-forming organisms.

Proctor, Nickerson, and Goldblith (1948) found that sheep casings, both washed and salted (such as are used as containers for comminuted meat products like frankfurters and sausages) could be sterilized by 1.5 X lo6 rep of cathode rays without loss of tensile strength. Although their experiments were made on a laboratory scale, it appears possible that fresh casings can be sterilized by cathode rays, which would avoid the extra handling required in salting and subsequently soaking them in water before use.

Huber (1948) sterilized a number of food products and pharmaceu-

164 BERNARD E. PROCTOR AND SAMUEL A. QOLDBLITE

ticals by cathode rays produced in a Capacitron. The dosage required for sterilization he measured in terms of the number of electron impulses, each impulse being of the order of second. B. s u b t i l k was apparently destroyed by four impulses. The capacitor bank has a charging frequency of 50 to 100 times per minute, with a peak output voltage of 3 million volts.

I n further work, Brasch and Huber (1948) found that when electron impulses from the Capacitron of relatively great intensity and extremely short duration were used, the inactivation dosages required for spores and particles of small size (such as phages and viruses) differed from the published data by factors ranging from 5 to 80. I n the case of B. m e s e n t e r i c u s spores, they stated that they were able to produce inactivation with 11,000 rep. They compare this dosage with the average dosage of 120,000 rep required with conventional sources of beta, gamma, or x radiation reported in the literature (Lea, 1947).

Brasch et al. (1949) determined the sterilization dosages for a number of bacteria with high intensity electron bursts from the Capacitron. Concentrations of from lo4 to 1010 organisms per milliliter were irradiated. A number of the dosages found necessary to sterilize different organisms are given below.

Organism Sterilization Dosage

(rep) E. coli 100,000 S. aureus 200,000 A. aerogenes 150,000 B. subtilis 250,000 C1. sporogenes 400,000 B. botulinus 400,000

The results of the work by Brasch and Huber (1948) and by Brasch e t al. (1949) indicate that the dosage of electrons necessary for 10070 kill of these species of organisms was much lower than that reported by Dunn e t al. (1948) and others as necessary in the case of x rays. Ac- cording to Brasch and Huber (1948), these results seem to contradict the “target theory” of inactivation of microorganisms by radiation, which postulates that the effect of a given dose of radiation is independent of whether the radiation is administered at high intensity €or a short time or at low intensity for a prolonged time. Brasch and Huber claim that the lower 100% kill dose of ultra-short bursts of electrons may call f o r a re-examination of the target theory.

All theories are subject to constant re-examination, and the target theory is no exception. Before this theory is revised, however, it may be that consideration should be given to some of the quantitative aspects


of the measurement of irradiation dosage of equipment that produces electron impulses. Among these are ( a ) the calorimetric calibration of dosages and the possibility that in the intervals between bursts there may have been a loss of heat by radiation, ( b ) variation in reported amperage from 30,000 to 50,000, and (c) irradiation intervals of second which may be somewhat difficult t o control o r to measure in length.

It has been determined recently (Proctor and Goldblith, 1949d) that the uniformity of the electron beam of the continuous electrostatic accelerator can be improved for biological purposes by scattering the electrons with aluminum foil. As a result, some of the lethal dosages reported in the literature may be higher than are actually required a t present.

It is obvious that with either of the sources of electrons cited above, regardless of immaterial discrepancies that may exist in the actual required dosages, it is possible to destroy microorganisms of all kinds with great rapidity, even spore-forming bacteria, in any medium and in containers of glass, fiber, or metal, with temperature rises of not over a few degrees F. Perhaps more important is the effect of the same irradiations on the foodstuff containing the microorganisms.

Stern e t d. (1949) found that electrons produced by the Capacitron destroyed bacteria by destruction of desoxyribonucleic acid. Taylor e t al. (1947) reported on the effects of x rays on thymus nucleic acid. They felt that the occurrence of nucleic acid, especially the desoxyribose type, in the chromosomes of all known cells warranted an investigation of the effects of x rays on this substance. They found that irradiation with 56,000 roentgens caused a decrease in the viscosity of thymus nucleic acid, which was independent of temperature. There was no splitting off of purine- o r pyrimidine-containing fragments small enough to penetrate a cellophane membrane.

Dunn e t al. (1948) reported that milk could be pasteurized by supervoltage cathode rays in a short period of time. The bacterial count was reduced from 37 million bacteria per milliliter in raw milk to 2 bacteria per milliliter in irradiated milk. Cottage cheeses were prepared from the raw milk and from the milk treated with cathode rays, lactic acid being used as starters. There appeared to be no organoleptic difference between the cheeses produced from raw and irradiated milks.

Proctor and O'Meara (1949) have done further work on the pasteurization of milk by cathode rays produced at 3 X loo rep. They found that irradiation of milk at a temperature of from 10" to 16" C. (50" to 60" F.) in dosages needed for pasteurization produced an off flavor and an off odor. These off conditions decreased on storage of the milk and


could be minimized by irradiation of the milk in the frozen state. These investigators found a difference, however, in the amount of destruction of bacteria in raw milks irradiated in frozen and unfrozen states (Table VI) . The data show that sterilization of raw milk by beta rays requires more energy when the milk is in the frozen state than when it is in the liquid state at 10" to 16" C. (50" to 60" F.).

TABLE VI

Effect of Freezing and Irradiation by Supervoltage Cathode Rays on the Survival of Microorganisms in Raw Milk *

Dosage (rep 1

100,000 200,000

500,000

1,000,000

Control

300,000

750,000

Unfrozen milk

Bacteria Survival

pcr ml. ( % I 275,000 -

9,000 3.3 700 0.25 130 0.05

5 0.00 0 0 0 0

Frozen milk

Bncteria Survival per rnl. (%) 300,000 -

30,000 10.0 9,000 3.0 2,700 0.9

34 0.01 4 0 0 0

Proctor and O'Meara (1949) .

Similar results were reported by Brasch and Huber (1948), who found that changes in taste and odor of numerous food products brought about by irradiation with cathode rays at room temperature could be obviated by irradiation of the products in the.frozen state after partial evacuation of the irradiation chamber.

Proctor and O'Meara (1949) have observed that fresh orange juice can be sterilized by supervoltage cathode rays. The microbiological data for this experiment are presented in Table VII.

TABLE YII

Effect of Supervoltage Cathode Rays on Yeasts Present in Fresh Orange Juice *

Dosage (rep)

Control 100,000 200,000 300,000 400,000 500,000

1,000,000

Yeasts per ml.

5,500 600 250

45 20 4 0

Survival (%I

9.0 4.55 0.82 0.36 0.07 0.0

-

* Proctor and O'Mears (1949). Plated on Sabaraud's medium.


The irradiation of orange juice also resulted in off odor and off flavor somewhat similar to those of stored, heat-processed orange juice. The addition of 0.5% of ascorbic acid to that already present in the juice resulted in no noticeable difference in flavor after irradiation in comparison with the flavor of the irradiated control containing no additional ascorbic acid. This off flavor was variously described by a taste panel as “cooked,71 “sweet,” and “tart.” The titratable acidity and the p H of the irradiated samples were found to be the same as those of the control sample. As in the case of milk, irradiation of orange juice in the frozen state obviated off flavor and off odor development.

From these experiments it appears that pasteurization of milk and of orange juice can be readily accomplished by electron bombardment. However, undesirable changes in flavor occur, which appear to be obviated by irradiation of the material in the frozen state. Greater dosages of radiation fo r these products in the frozen state appear to be necessary, however, to accomplish the same degree of microbial destruction as noted with nonfrozen irradiated products.

Proctor and Goldblith (1949~) found that hamburger could be completely sterilized by 1,500,000 rep of supervoltage cathode rays. A slight increase in the peroxide number of the fat occurred upon irradiation. Flavor changes in these samples of meat also occurred on irradiation, which were discernible by 94% of a panel of 60 individuals. When the same patties were later broiled, the off flavors disappeared and there was no detectable difference in flavor or odor between the control and the irradiated patties, as shown by statistical analysis of the judgments of 60 individuals. Therefore, it may be concluded that irradiation of hamburger patties by supervoltage cathode rays sufficient to cause sterility, from the standpoint of microorganisms, produced an off odor detectable by some persons but one that is volatile and can be dissipated by the normal cooking procedures for such a product.

Irradiation of canned chopped meat products, such as chopped beef, lamb, veal, and pork, by supervoltage cathode rays resulted in a tendency toward discoloration of the meat (Proctor and Goldblith, 19494. An off flavor developed in some of the products. The extent of discoloration and of off flavor appeared to be related, a t least in part, to the presence of curing agents in the product.

2. Eflect on Vitamins

Among the factors to be considered in the evaluation of any sterilizing process are the effects of the process on food nutrients such as vitamins and amino acids.

Huber (1948) studied the effects of electrons on vitamins. The elec-

168 BERNARD E. PROCTOR AND SAMUEL A. GOLDBLlTE

.32

.28

.24

.20-

3 8

2 - .Ib- .-

.I2 b

.08

.04

trons were generated by a Capacitron, and dosages were presented as the number of impulses, each of a duration of 10-o second. The electrons were accelerated at 3,000,000 volts. Under the conditions of the experiment, Huber found no loss in potency of riboflavin, pyridoxine, pantothenic acid, and niacin in concentrations of 0.07, 0.04, 0.10, and 0.35 mg. per gram, respectively. With thiamine, there was a loss of 0.2 mg. per milliliter in a concentration of 4.6 mg. per milliliter.

Proctor and Goldblith (1948b) investigated the effects of supervoltage cathode rays produced by a pressure-insulated Van de Graaff generator on solutions of pure niacin in absolute ethanol, diluted when necessary with distilled water. Cathode rays produced at 3 megavolts and 10 microamperes had a destructive effect on niacin in a concentration of 100 micrograms per milliliter in as short a period as 15 seconds. In- creased time of exposure resulted in increased destruction of the niacin. There was greater retention of niacin when it was irradiated by cathode rays in the presence of methionine than when it was irradiated alone. The reverse held true when niacin was irradiated with cysteine and

-

-

-

-

-

-

0 t I I

210 230 2% 270 290 310 3.30 Wwch.nalh (mu)

FIO. 10. Effect of high-voltage cathode rays on ultraviolet absorption spectra of niacin in solution (100 mcg. per ml.) irradiated for 45 and 60 seconds at 10 miero- amperes. (Proctor and Goldblitli, 1948b.)


cystine. More niacin was destroyed in dilute than in concentrated solutions. These results may be interpreted as indicating that the action of cathode rays on niacin is indirect, an interpretation that conforms with the “indirect action” theory. This work confirms the results obtained with enzymes irradiated by x rays by Forssberg (1946) and by Tytell and Kersten (1941), and with acetylcholine by Dale (1943).

Ultraviolet absorption spectra of the irradiated niacin solutions were measured by Proctor and Goldblith (1948b), to obtain information as to what occurs when niacin is irradiated by cathode rays. The ultraviolet absorption spectra of niacin, both the control and the samples irradiated by cathode rays produced at 3 megavolts and 10 microamperes, are presented in Fig. 10. Irradiation resulted in a slight displacement of the absorption maximum and minimum, a definite decrease in the extinction of the absorption maximum, and an increase in extinction in wavelengths greater than the absorption maximum.

Further investigation by Goldblith e t al. (1949), who used niacin labeled with C14 in the carboxyl group, showed that the niacin was decarboxylated by relatively low dosages of supervoltage cathode rays. Splitting of the pyridine ring, however, did not occur until greater dosages of cathode rays had been applied. (See Fig. 11 and Table VIII.) The splitting of the pyridine ring was shown colorirnetrically by the

COUNTS IN GAS Q

COUNTS IN LIQUID

0 I 2 3 4 5 c DOSAGE (rep 1 lo6 I

FIG. 11. Decarboxylation of radio-niacin by high-voltage cathode rays. (Gold- blith et at., 1949.)


TABLE VIII

Effect of Cathode Rays Produced at 3000 kv. on Niacin Labeled with 0‘ in Carboxyl Group as Determined by Radioactivity Measurements of Both Gaseous

and Liquid Phases * Per cent

Total count8 Total counts of counts Sample Dosage per see. per sec. left in

No. (rep) of co, in solution solution

1 0.17 x loa 1145 11,000 72.4 2 0.33 x lo8 2100 11,350 74.6 3 0.66 x loe 2140 7,950 52.2 4 1.32 x loe 4970 6,450 42.4 5 1.98 x loa 6410 4,920 32.4 7 5.28 x loa 8400 3,780 24.9 8 Control 104 15,200 99.3

* Goldblith et RZ. (1949).

cyanogen-bromide reaction. Cyanogen bromide plus an aromatic amine forms an addition product with niacin. An intact ring structure is necessary (Waisman and Elvehjem, 1941).

The effects of supervoltage cathode rays on riboflavin and carotene were also studied by Goldblith and Proctor (1949). The results (Tables IX and X) obtained with riboflavin were similar to those noted when this substance was irradiated with both hard and soft x rays. Carotene was radiosensitive to cathode rays, and its destruction was greater in the more dilute solutions.

From the experiments cited above, it would appear that as the com- plexity of the medium grows and the concentration of the solute increases, the net effect of cathode rays on any one compound is lessened.

Brasch et al. (1949) have irradiated a polyvitamin preparation with electrons produced by the Capacitron, in order to destroy mould spores. They found no destruction of the B-complex vitamins and of vitamin C.

Observations by Proctor and O’Meara (1949) on the effect of cathode rays on orange juice showed that ascorbic acid in orange juice is radiosensitive. When pure ascorbic acid (initial concentration 8 mg.% ) was placed in a solution of 0.25% oxalic acid and subjected to a dosage of 1.0 x loe rep of cathode rays, 92% of the ascorbic acid was oxidized. When this same solution of ascorbic acid and oxalic acid was frozen and then irradiated with the same dosage, there was a loss of only 9% of the ascorbic acid. Upon repeating these experiments with orange juice, Proctor and O’Meara found an even higher retention of the ascorbic acid in the juice following irradiation. This greater retention may be accounted for by the presence of a heterogeneous group of substances


Vitamin and

sample t Riboflavin

Control Y G F E I

TABLE IX

Effect of High-Voltage Cathode Rays on Stock Solutions of Riboflavin and Carotene *

Time of irradiation (see.)

- 15 15 30 45 60

Target current

(pa.)

- 5 10 10 10 10

Carotene Control - -

Y 15 10 I 30 10 E 45 10 F 60 10

+ Qoldhlith and Proctor (1949) . t Concentration before irradiation 100 pg./ml.

Volume irradiated . . . . . . 2.5-3 ml. Voltage . . . . . . . . . 3 megavolts

Vitamin and

Bample t Ribo,Ravin

Y I E F G

Dosage (rep x 109

- 0.33 0.66 1.32 1.98 2.64

- 0.66 1.32 1.98 2.64

Vitamin retention

a f te r irradiation

(%I

100 47.8 20.9 16.9 5.1 5.1

100 50 20 6 1

TABLE X

Effect of High-Voltage Cathode Raps on Solutions of Riboflavin and Carotene of Varying Concentrations *

Concentration of vitamin

Time of irradiation (see.)

15 15 15 15 15

Target current

(pa.) (

Before After Dosage irrad. irrad.

:rep x 109 (pg./ml.) (ag./ml.)

0.33 100 47.8 0.33 75 33.0 0.33 50 9.58 0.33 25 0.978 0.33 10 0.181

Reten- tion of vitamin

(%)

47.8 45.5 19.2 3.91 1.81

Carotene B 15 10 0.66 50 19 38 N 30 10 1.32 50 4 8 D 15 10 0.66 33.3 10 30 A 30 10 1.32 33.3 1.8 5.4

Ooldblith and Proctor (1949) . t Volume irradiated 2.5-3 ml.

Voltage . . . . 3 megavolts


in orange juice, which probably act as protecting substances. This finding is explained by the “indirect action” theory of radiations (Lea, 1947). Moreover, the concentration of ascorbic acid in the oxalic acid solution was more dilute (8 mg. %) than that in the orange juice (29.5 mg. %). The explanation for the greater retention of the vitamin and

TABLE XI

Effect of Supervoltage Cathode Rays on the Essential Amino Acids in Haddock *

Amino acid Phenylalanine

Loss Gain

Tryptophan Loss Gain

Methionine Loss Gain

Cystine Loss Gain

Valine Loss Gain

Leucine Loss Gain

Histidine Loss Gain

Arginine Loss Gain

Lysine Loss Gain

Threonine Loss Gain

Per cent loss or gain in amino acid at dosages of

rep rep rep

9 x 106 2.7 x 10’ 5.7 x 106

0.0 2.33 6.10 2.75 - -

3.77 -

0.70 -

2.94 -

9.80 -

5.61 -

6.93 -

6.92 -

4.68 -

0.00 -

- 1.27 - 2.03 - 6.36

1.90 -

0.50 -

2.32 -

6.40 -

- 0.91

6.06 -

8.41 -

1.78 -

0.00 -

- 3.87

- 2.74

- 8.11

- 4.12

4.23 -

5.95 -

* Proctor and Bhatia (1950).

APPLICATIONS OF ELECTROMAGNETIC RADIATIONS 17 3

the better flavor when the orange juice was irradiated in the frozen state has not yet been determined.

I n studying the mechanism of the destruction of ascorbic acid in pure solutions, Proctor and O’Meara found that the ascorbic acid is oxidized to dehydroascorbic acid and then to breakdown products. No evidence of 2:3 diketogulonic acid was found. These conclusions were drawn from the resuIts of chemical and spectrophotometric studies. SimiIar

TABLE XI1

Effect of Cathode Rays on Amino Acids in Solutions of Varying Concentrations *

Amino acid and

dosage

Tryptophan ( d l ) Control 100,000 rep 250,000 rep 500,000 rep

1,000,000 rep

Leucine ( 1 ) Control 100,000 rep 250,000 rep 500,000 rep

1,000,000 rep

Histidine.HC1-(1) Control 100,000 rep 250,000 rep 500,000 rep

1,000,000 rep

Phenylalanine ( d l ) Control 100,000 rep 250,000 rep 500,000 rep

1,000,000 rep

Cystine ( I ) Control 100,000 rep 250,000 rep 500,000 rep

1,000,000 rep

Per cent retention of amino acid in solution with concentration o f :

100 ag./ml. 250 pg./ml. 500 pg./rnl. 1 mg./ml.

100.0 100.0 100.0 100.0 81.0 92.2 94.8 91.2 68.6 91.2 97.0 90.4 58.8 77.6 90.0 88.6 36.6 62.2 79.4 74.6

100.0 100.0 100.0 100.0 85.5 97.8 97.9 98.6 62.3 91.6 98.0 97.9 34.5 82.8 94.2 96.4 6.9 51.0 78.0 94.9

100.0 100.0 100.0 100.0 44.5 52.4 66.8 80.7 17.5 35.9 61.3 62.7 6.0 29.2 53.2 64.2 0.0 12.4 28.2 43.1

100.0 100.0 100.0 100.0 56.5 76.8 88.1 91.6 23.6 57.4 75.6 86.4 3.1 33.1 57.9 79.6 0.0 9.6 35.3 70.8

100.0 100.0 100.0 100.0 45.8 72.2 92.0 97.7 38.2 66.6 79.9 95.0 22.7 53.7 70.0 90.2 0.0 35.7 61.3 82.0

* Proctor and Bbstia (lQ4Q).


results were obtained when fresh orange juice was irradiated; 2:3 diketogulonic acid, as measured chemically, was not found.

3. Effect on Amino Acids

Proctor and Goldblith (1949~) irradiated ground beef patties and protein hydrolyzates with cathode rays at 1,500,000 rep and found that the losses in essential amino acids were small.

Proctor and Bhatia (1950) irradiated haddock fillets with dosages of 900,000, 2,700,000, and 5,700,000 rep. At the two higher dosages a salt- fish odor was observed in the haddock, and the flesh became crumbly and bleached in appearance. Using the microbiological assay method, they likewise studied the effects of cathode rays on the essential amino acids of haddock. Some of the results of this study are presented in Table XI.

The lowest dosage used, namely, 9 X lo5 rep, was sufficient to destroy the microorganisms in the fish. The two higher dosages were used to determine the possible extent of breakdown of amino acids. The data in Table XI demonstrate that cathode ray irradiation does not cause any significant destruction of the essential amino acids in fish muscle, as determined by microbiological assay.

Proctor and Bhatia (1949) irradiated pure solutions of amino acids, with the results tabulated in Table XII. The data show the effect of dilution in increasing the radiosensitivity of the amino acids. They explain, furthermore, why little or no destruction of these amino acids occurred in the hamburger patties and the haddock cited in this review, because these food materials contain a number of compounds (vitamins, amino acids, and enzymes), that are capable of reacting with the products of the ionizing radiations.

The relative radiosensitivities of the five amino acids considered in Table XI1 are compared in Table XII I , where the effects of 250,000 and 500,000 rep of cathode rays on solutions containing 100 micrograms of theamino acids per milliliter are presented. On the basis of increasing percentage destruction of the amino acids, that is, of increasing radiosensitivity, these compounds may be listed in the following order : tryptophan, leucine, cystine, phenylalanine, and histidine.

4. Effect on Enzymes

Working with enzymes, Huber (1948) reported that hyaluronidase retained 97% of its activity, clarase 87%, and trypsin loo%, when irradiated with cathode rays. No data were presented concerning the purity of the enzymes before irradiation.

Brasch et at. (1949) reported changes in enzyme activity as a result of bombardment with cathode rays produced in the Capacitron (see


TABLE XI11

Relative Radiosensitivities of Some Amino Acids

Per cent destruction at dosage of:

Amino acid t 250,000 rep 500,000 rep

Histidine.HC1- ( I ) 82.5 94.0 Phenylalanine ( d l ) 76.4 96.9 Cystine ( 1 ) 61.8 77.3 Leucine ( I ) 37.7 65.5 Tryptophan ( d l ) 31.4 41.2

* Proctor and Bhatia ( 1 9 4 9 ) . t In solution a t concentration of 100 pg./ml. ( S e e Table X I I ) .

Table XIV). It would appear from their data that enzymes in the crude state are relatively radioresistant to electrons produced in the Capaci- tron, for the dosages shown in Table XIV are far in excess of those reported by Brasch e t al. as necessary t o destroy spore-forming bacteria. (See Section IX-1.)

TABLE XIV

Effect of Cathode Ray Irradiation on Enzyme Activity *

Compound

I Amylase Protease Hyaluronidase Urease Diastase Trypsin Pepsin Papain

* Brasch et al. (1949).

Medium

Commercial clarase

Soy flour Barley

. . . . . .

......

...... Meat

Dosage (rep) 0.8 x loa 0.8 x lo6 0.5 x loo

2 x 1 0 s 0.9 x 100

3 x 106 3 x 10' 3 x 106

Loss in activity

(70) 15 15 13

8 14

None None None

Proctor and O'Meara (1949) found that when raw milk was irradiated by supervoltage cathode rays, a dosage of 200,000 rep was necessary to destroy the peroxidase whereas a dosage of approximately 15,000,000 rep was necessary to destroy the phosphatase activity. When the raw milk was frozen before irradiation, the peroxidase was not inactivated by 1,000,000 rep and the phosphatase was not inactivated even by 20,000,000 rep. To obtain a greater peroxidase content in the raw milk, the milk was allowed to sour. As peroxidase k derived mainly from microorganisms, i t increased in the sour milk. Under these conditions of a fairly large concentration of this enzyme, between 1,000,000 and

176 BERNARD E. PROCTOR A N D S A M U E L A. OOLDBLITH

3,500,000 rep were needed for inactivation of the peroxidase in the milk in the unfrozen state but 6,600,000 rep did not inactivate the enzyme completely when the milk was in the frozen state.

In apple juice, the above investigators found strong phosphatase activity after the juice was irradiated with 1,000,000 rep of cathode rays.

Although these resu1t.s are only qualitative in nature, they do indicate that inactivation of enzymes in foodstuffs requires much greater radiation dosages than those required for the destruction of the most resistant microorganisms. This was also demonstrated by Nickerson et at. (1950), who sterilized mackerel tissue by 1.5 x lo6 rep of cathode rays but still found strong proteolytic activity as shown by amino-nitrogen determinations. Moreover, from the unpublished observations of Proctor and O'Meara it is evident that greater dosages of cathode rays are necessary to destroy the enzymes in frozen materials than in the same foodstuffs in the unfrozen state.

Further research on the effects of sterilizing dosages of cathode rays on various types of plant and animal materials is in progress a t the Massachusetts Institute of Technology.

5. Effect on Coffee and Its Cmponents

Proctor and Goldblith (1949~) found that temporary bleaching of a roasted coffee infusion occurred when the infusion was irradiated by supervoltage cathode rays. When the irradiated infusion was allowed to stand a t room temperature for 18 hours, the color of the infusion par- tially returned. The color measurements were made by integration of visible absorption spectra and are presented in Table XV. The rate of return in color was retarded by storage at temperatures lower than room temperature (Table XVI 1. Such irradiated coffee infusions were found to have lost their typical flavor and aroma.

TABLE XV

Effect of Cathode Rays Produced at 2500 kv. on the Color of Coffee Infusions Irradiated by 6.6 x 10" rep

Area Length of under Per cent of storage t . curve original

Sample (hours) (mm.a) color Control - 3190 100 Irradiated 0 1830 57.4 Irradiated 18 2760 86.9

* Proctor and Goldblith (1949~). t Storage at room temperature (appros. 300 0. or 860 F.).


> t- ijj 1.2- B n

kO.0- 3 0

0.4

TABLE XVI

Effect of Temperature of Storage on Rate of Color Reversal of a Coffee Infusion Bleached by Cathode Rays Produced at

2500 kv. for 6.6 x loe rep

-

Length of

storage Sample (hours)

Control - Irradiated 0 Irradiated 2

Irradiated 4% Irradiated 24

Area (mm.')

2645 1282 1193 1342 1590

Temperature of storage

9" c. 30" C.

Per cent of Per ccnt of original Area original

color (mm?) color 100 2645 100 48.6 1275 48.2 44.9 1448 54.8 50.8 1740 65.9 60.1 2128 80.4

* Proctor and Goldblith (1949~).

The ultraviolet absorption spectra of these infusions were determined, to ascertain any chemical changes that might have occurred. The results are presented in Fig. 12. Even though there were such great changes in the color of the solutions after storage, essentially no further changes in the ultraviolet absorption spectra were observed after storage. The irradiation resulted in a partial loss of caffeine, as indicated by the decrease in the extinction at 272 mp, and an even greater loss of chlorogenic acid, as shown by the decrease in extinction at 323 mp. Similar

L I I I I 200 240 280 320 360

WAVELENGTH (mu)

FIG. 12. Ultraviolet absorption spectra of infusions of roasted coffee irradiated by high-voltage cathode rays and examined immediately and 18 hours afterwards. (Proctor and Goldblith, 1949c.)

178 BERNARD E. PROCTOR A N D SAMUEL A. QOLDBLITH

CONTROL

-

I I I

WAVELENGTH (mu) 0 2 60 300 340

Fm. 13. Ultraviolet absorption spectra of infusions made from irradiated and Cathode rays were produced at nonirradiated decaffeinated ground coffee beans.

2500 kv. for a dosage of 6.6 x l o e rep. (Proctor and Goldblith, 1949c.)

results were obtained when an infusion of coffee made from decaffeinated ground beans was irradiated (Fig. 13) . Irradiation of an infusion made from green coffee by supervoltage cathode rays resulted in the ultraviolet absorption spectra shown in Fig. 14 (Goldhlith, 1949a).

It was observed that irradiation of infusions made from regular coffee, decaffeinated coffee, and green coffee resulted in the same absorp-

I .6 I

220 260 300 340 380 WAVELENGTH (mu)

3%. 14. Ultraviolet absorption spectra of infusions made from green coffee and irradiated by high-voltage cathode rays; also the spectrum of an infusion passed through aluminum oxide. (Goldblith, 1949a.)


0.5 -

0.4 -

t

E d 5 -

E0.3-

0.2

0.1 -

1. CONTROL 2. 670,000 rBp. 3. 1,330,000 4. 2,660,000 '

6. 33,300,000' 5. g660,OOo

I I I 200 240 280 320 360

WPVELENGTH )rup

FIQ. 15. Ultraviolet absorption spectra of pure solutions of caffeine (100 mcg. per (Goldblith, 1949a.) rnl.) diluted 1 :10 and irratlizted by high-voltage cathode rays.

6 I . Control

3. 10.4~10 rep. 19 Hrs. .6 4. 10.4 x106 rep.'O'Hr.

2. 20 x 10 p p .

e a C e P

s 0

.-

3 -4 e a

.2

I I 1 t I

Wavelength (mu) 220240 260 280 300 320

FIQ. 16. Ultraviolet absorption spectra of prune juice irradiated by cathode rays (Proctor and Goldblith, and allowed t o stand at room temperature for 19 hours.

1949a.)


tion spectra. The ultraviolet absorption spectrum of a coffee infusion can be determined chemically by passing the infusion through a column of aluminum oxide. Both the irradiated infusion and the infusion passed through the aluminum oxide column had not only the same absorption characteristics but also were devoid of the normal coffee odor and flavor. In further observations on coffee, Proctor and Goldblith found that

when ground coffee beans were irradiated by cathode rays in relatively heavy dosages, there was no change in the ultraviolet absorption spectrum. This would again illustrate the “indirect action” theory of radiation, because extensive changes in the ultraviolet absorption spectra of the infusions were observed, whereas no changes at all were observed in the spectra of the ground dried beans.

Pure solutions of caffeine were irradiated by supervoItage cathode rays, and the destruction of caffeine was determined by measurement of the ultraviolet absorption spectra (Fig. 15).

6. As a Tool for Studying Some Chemical Reactions

It has been demonstrated by the present authors that ionizing radiations may be useful as a tool in the study of certain chemical reactions.

a. Noltenzymatic Browning. The mechanism of nonenzymatic browning of dehydrated fruits has been adequately reviewed by Stadtman (1948). The compound 5- (hydroxymethyl) furfuraldehyde (HMF) has been shown to be associated with the nonenzymatic browning of de-

.?

.6 1. Control = W e ._ s .5

2.4 8

0 P -

.3

.2

. I

Wovrlsn~th (mu)

330. 17. Ultraviolet absorption spectra of browned apricot extracts irradiated by cathode rays for dosages up to 20.1 x lo8 rep. (Proctor and Goldbith, 1949a.)


hydrated fruit products. This compound has a minor absorption maximum in the ultraviolet a t 225 mp and a major peak a t 285 mp. The observations of Proctor and Goldblith (1949a) demonstrated that prune juice has an absorption maximum a t 285 mp in the ultraviolet (Fig. 16) . It was found that irradiation of prune juice by supervoltage cathode rays resulted in the formation of a new peak a t 265 mp, provided sufficient dosage was used. lipon irradiation, the color of the prune juice was found to be bleached, a phenoinenon previously noted with coffee. After the irradiated prune juice had been permitted to stand for 20 hours, some of its color returned, as measured by integration of the visible absorption spectra. However, no change was found in the ultraviolet absorption spectrum after this period (Fig. 16) . This appears to indicate that, although HMF was destroyed by the irradiation, there

FIG. diated

18. by high-voltage cathode rays.

Ultraviolet absorption spectra of pure solutions of furfuryl (Proctor and Goldblith, 1949c.)

alcohol irra-


was no regeneration of HMF even when a portion of the color of the juice returned.

Similar results were obtained by Proctor and Goldblith (1949a) when an extract of dried, browned apricots was irradiated, namely, the formation of a new peak at 265 mp (Fig. 17) in place of the previous peak at 285 mp.

- 20

ia .%

5 1.6 5

a k 1.4

12

I .o

0 8

0.6

0 4

0 2

0 2w zzo 240 260 zeo JOO 320

WAVELENGTH lnrumu)

FIQ. 19. Ultraviolet absorption spectra of pure solutions of furfural irradiated by high-voltage cathode rays. (Proctor and Goldblith, 1949c.)

Levulinic acid has an ultraviolet absorption maximum at 265 mp (Singh et d., 1948). The formation of levulinic acid from HMF is chemically possible (Pigman and Goepp, 1948). The amount of HMF present in the extracts of dried fruits could not account for all the compound formed, which would have an ultraviolet absorption maximum a t 265 mp if it were levulinic acid, for HMF has a high and levulinic acid a low molecular extinction coefficient. It is possible that some of the sugars present were converted to levulinic acid by the radiations.

Proctor and Goldblith (1949a) also showed that pure solutions of furfuryl alcohol, furfural, and furoic acid, homologues of HMF, could not be converted by cathode rays into a compound absorbing maximally


at 265 mp (Figs. 18 to 20). An increasing loss in extinction was found to occur when these compounds were irradiated, but there was no conversion to compounds having ultraviolet absorption maxima a t 265 mp, for there was no shift in the absorption maximum in any case.

Boiling of solutions of sucrose, fructose, and dextrose in N / 1 sulfuric acid resulted in the formation of HMF. Irradiation by supervoltage

FIG. 20. Ultraviolet absorption spectra of pure solutions of furoic acid irradiated by high-voltage cathode rays. (Proctor and Goldblith, 1949c.)

cathode rays resulted in a shift of the absorption maximum from 285 to 265 mp (Figs. 21-23). Here again the quantity of HMF present was not sufficient to account for all the product formed (absorbing a t 265 mp), if that product was levulinic acid.

Irradiation of powdered sucrose when dissolved in water was also found to give an ultraviolet absorption maximum at 265 mp (Goldblith, 1949a).

The use of cathode rays as a tool for studying nonenzymatic browning has amply demonstrated that HMF is by no means the sole compound


responsible for the nonenzymatic browning of dried fruits, because only a temporary bleaching action occurs when an extract of dried fruits is irradiated, whereas the HMF present is permanently converted into a compound absorbing maximally a t 265 mp.

WAVELENGTH mnu) FIG. 21. Ultraviolet absorption spectra of a 1% solution of sucrose in N-sulfuric

acid, boiled 4 minutes, cooled, and irradiated with high-voltage cathode rays. (Proc- tor and Goldblith, 1949c.)

b . Mechanism of Spoilage of Fish. It has not been well established whether fish deteriorate mainly through bacterial action or enzymatic action, or by a combination of both. Up to the present time there has been no satisfactory method of inactivating enzymes or destroying bacteria without destroying or inactivating the other. As microorganisms are destroyed by “direct hits” of ionizing radiations and enzymes are inactivated by the “indirect” action, there is a great margin in the dosages of radiation necessary to destroy bacteria and enzymes. Many types of bacteria, perhaps all, are destroyed by 2 )( lo6 r of ionizing radiations such as cathode o r x rays, whereas most enzymes in foods require 107 rep for inactivation.

Because of this difference in sensitivity of microorganisms and enzymes, Nickerson e t al. (1950) were able to study the mechanism of


spoilage of mackerel. Fillets were wrapped in a film of polythylene tubing and irradiated by 1.5 x lo6 rep of cathode rays, a dosage sufficient to destroy the bacterial Aora without destroying the enzymes. By comparison of the results of chemical tests, it was possible to study the

> I-

v) z W 0

-

a 0

I. CONTROL 2500,000 rep. 3.1,000,000 rep. 4.3,O 0 0,O 0 0 rep. 5.6,70 0,000 r e p. 6.19,400,000 reR

111111 220 260 300

YWVEL6NGTH (mu)

FIQ. 22, Ultraviolet ahsorption spectra of 0.5% solutioii of fructose in N-sulfuric (Proctor acid, boiled 4 min., cooled, niid irradiated by high-voltage cathode rays.

and Goldblith, 1949c.)

mechanism of spoilage of fish fillets that were stored in crushed ice over a period of days. Half of these fillets contained enzymes and viable bacteria and half had only enzymes present. An interesting observation was that when an irradiated mackerel fillet was left a t room temperature fo r approximately 30 days, the fillet, although rancid, had not autolyzed. Hence it seems possible that autolysis in this fish is due to bacterial action.

It was found that there were only slight increases in the trimethylamine content of irradiated (sterile) mackerel fillets held in ice. The trimethylamine content of unirradiated (not sterile) fillets remained virtually constant until bacterial increases were noted. The proteolytic action of enzymes in mackerel flesh was nearly as potent in irradiated as


in unirradiated tissue. Regardless of this fact, sterile (irradiated) mackerel tissue did not change in physical appearance during storage for 30 days at room temperature, except that the oil became rancid, which caused rusting. The volatile acids content of the unirradiated (non- sterile) fillets was found to be appreciably higher than that of the

WAVELENGTH (mu)

FIQ. 23. Ultraviolet absorption spectra of 0.5% solution of dextrose in N-sulfuric (Proctor acid, boiled 4 minutes, cooled, and irradiated hy high-voltage cathode rays.

and Goldblith, 1949c.)

irradiated fillets held under storage in ice. This shows that volatile acids were produced mainly by bacterial action.

Proctor et d. (1950) also utilized supervoltage cathode rays as a tool to study the mechanism of spoilage of haddock. By irradiation of paired haddock fillets with sterilizing dosages of electrons, they showed that bacteria were the primary cause of spoilage of the haddock fillets and that enzymatic processes were of little or no importance. Further con- firmation was also obtained that trimethylamine is an indication of spoilage of haddock fillets and that this compound is produced solely by bacterial action.

With the use of similar techniques, further research on other fish


species is in progress in the Food Technology Laboratories a t Massa- chusetts Institute of Technology.

7 . Effect on Packaging Materials

I n experiments conducted at the Massachusetts Institute of Technology with the Van de Graaff electrostatic accelerator (Proctor and Landrock, 1949), it has been found possible to sterilize empty containers rapidly. Cans containing spore suspensions of putrefactive anaerobes were completely sterilized by 1.5 X lo6 rep of cathode rays. Empty fiber milk containers were also sterilized by this technique, Cellophane film was treated by sterilizing dosages of cathode rays, with no effect on the water-vapor permeability as measured a t - 18" C. (0" F.) .

These experiments, together with numerous others, indicate the possibility of sterilizing most, if not all, of the fibers and plastic materials ordinarily used for the packaging of food materials, if within the thick- ness limitations of the equipment used for irradiation.

The same holds for metal and glass containers, although it has been ubserved that clear glass containers attain a coloration on irradiation which is roughly proportional to the dosage used.

8. Effect on Pharmaceuticals

It has been possible to sterilize a number of pharmaceuticals by means of supervoltage cathode rays. The potency of some pharmaceuticals is unaffected by this type of radiation, but the potency of others is seri- ously affected. Penicillin and streptomycin, for example, were reported by Huber (1948) to be sterilized without loss in potency. It was found in preliminary experiments by Goldblith (1949b) that streptomycin and penicillin were not inactivated by 3,000,000 rep of supervoltage cathode rays but that insulin and zinc insulin were almost completely destroyed.

Brasch e t al. (1949) found no loss in potency of sodium-penicillin and streptomycin-sulfate when these were exposed to a dosage of electrons, produced in the Capacitron, greater than 5 X lo6 rep.

9. Toxicity of Irradiated Materials

At the Massachusetts Institute of Technology, a number of rats were fed for a period of two years on a diet consisting mainly of food treated by sterilizing dosages of supervoltage cathode rays. No toxic effects of this diet were observed. Although the number of rats used in this study was limited, the animals fed for two years on a diet solely of irradiated foods grew as well and had as long a life span as those fed on a control diet. Further experiments of the same nature with rats in successive generations are in progress.


X. SUMMARY

For over half a century, many phases of the use of electricity and various segments of the electromagnetic spectrum have been investigated for widely varying types of food processing. The number of investigators and the intensity and scope of such endeavors have been markedly increased in the last decade, with the development of new equipment by physicists and electrical engineers.

The application of such information and equipment to actual food processing is still in its infancy, although encouraging results have been attained. One may expect much in this field in the future, and perhaps in the not distant future, but many questions are yet to be solved before attainment of economical, efficient means of processing large quantities of food at the rapid rates of production required in large food-manufacturing operations.

With the exception of ultraviolet light, x rays, and cathode rays, which do not produce heat, the various other electrical methods seem to be without significant bactericidal effects other than those resulting from the production of heat. Some of the latter methods are capable of rapid production of heat, which is advantageous from the standpoint of food processing.

Large-scale use of sonic or ultrasonic vibrations for sterilization of packaged foods is contra-indicated because of technical limitations, including barrier effects of packaging materials.

The use of radioactive isotopes to sterilize foods presents certain health hazards, concerning which too little is known at present. In addition, the degree of inactivation of microorganisms necessary in the food industries is not likely to be attained by such means.

Cathode rays present interesting possibilities, although the installation and maintenance costs involved may limit the extent of such usage. Another limiting factor may’ be the color and flavor changes which vary greatly, dependent on the products so treated. Means of minimizing certain of these undesirable changes have been found.

The fact that so much interest is currently being given to the investigation of such applications in the food field indicates their potential importance to the food-processing industry if applications in line operations should prove feasible. However, the first applications are likely to occur in the pharmaceutical field or in other fields where the units costs of the material are higher and the economic factors are somewhat less limiting than in food-processing operations.

Irradiation with cathode rays and with other segments of the electromagnetic spectrum has served as the origin for new research tools which


are proving most helpful to many investigators in the food and other fields. An example of this is some work now under way at M. I. T. which indicates the possibility of sterilizing segments of aorta for storage in a n “aorta bank” similar to blood banks, for subsequent surgical transplants.

It is hoped that this review will serve to indicate to other investigators the progress in the application of elretromagnetic radiations in food processing thus far made along so many dcvious paths. Vast, uncharted fields for the application of such radiations are daily being opened by the development of new physical and electrical equipment, capable of use in the food field. Realization of the possible uses of such equipment should conatitute a challenge to food technologists to participate and collaborate in thr activities of their colleagues in physics and engineering, who will usually be found to have an avid interest in foods.

ACK NOWLEDGXIENTS

We are indebted to Dr. Robeit J. Van de Graaff, Associate Professor of Physics a t the Massachusetts Institute of Tcclinofogy and originator of the electrostatic ac eelerator, for his assistance in our initial experiments on the effects of x rays and for his encouragemelit in the exploration of the effects of cathode rays.

We also wish t o expiess our clecp appreciation of the splendid cooperation of Dr. John G. Trump, Associate I’rofessoi of Electrical Engineering at the Massachusetts Institute of Trclinology, for his \;ilurtf couiisel in tlie field of high-voltage radiations and for placing equipment a t our dispos:il, without which our work reported here on cathode rays would have been inipossihle.

To Kenneth A. Wright of tlie IIigli-Voltage I,:iborntories, Department of Electrical Eugineering, Massachusetts Institute of Technology, we :we indebted for his constant willingness to devote t line and thoriglit t o the surmounting o f technical difficulties in radiation techiiiqucs in our c:ithotle ray investigations.

The authors wish to express their appreei:ition to Miss Elsie A. Wilson for her valued editorial assistance in the asscnibly and compilation of reference materials and the preImratioti of this manuscript.

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