[Advances in Food Research] Advances in Food Research Volume 2 Volume 2 || Thermobacteriology as Applied to Food Processing
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Thermobacteriology As Applied to Food Processing
BY C . R . STUMBO Food Machbicry uicd Chstiiicul Corpordwir. Satr loss. Califorilia
I . Introduction . . . . . . . . . . . . . . . . . . . . 47 I1 . Thermal Process Evaluation . . . . . . . . . . . . . . . 49
1 . The General Method . . . . . . . . . . . . . . . . 49 2 . Mathematical Methods . . . . . . . . . . . . . . . 52
a . Slope of Thermal Death Time Curve . . . . . . . . . . 52 b . Heat Penetration Factors j, I and jh . . . . . . . . 53 c . Sterilizing Value of a Proceas . . . . . . . . . . . . 68
3 . Improvements in Methods of Process Evaluation . . . . . . . 01 I11 . Order of Death of Bacteria and Process Evaluation . . . . . . . . 01
1 . Concept of Bacterial Death on Which Methods of Procem Evaluation a r e b a w d . , . . . . . . . . . . . . . . . . . . 61
2 . Order of Death of Bacteria . . . . . . . . . . . . . . 62 66
a . Number of Cells . . . . . . . . . . . . . . . . 66 b . Nature of Medium in Which Bacteria Have Grown 66 c . Nature of Medium in Which Bacteria Are Suspended When Heated . 07
68 5 . Interpretation of Thermal Resistance Data for Process Calculations 70
a . Thermal Death Time Data . . . . . . . . . . . . . 70 b . Initial Concentration and End-Point of Destruction . . . . . 73
6 . Nature of Thermal Death Time Data Used in the Past to Establish Requirements of Commercial Processes . . . . . . . . . . 70
7 . Common Errors in Thermal Death Time Data . . . . . . . . 82 8 . Recent Improvements in Thermal Death Time Methods . . . . . 86
IV . Mechanism of Heat Transfer and Process Evaluation . . . . . . . 89 1 . Conduction-Heating Products . . . . . . . . . . . . . 90 2 . Location in Container Where Probability of Survival is Createat . . 91 3 . Convection-Heating Products . . . . . . . . . . . . . 95 4 . Influence of Resistance of Organism to be Destroyed 97 5 .Discussion . . . . . . . . . . . . . . . . . . . 100 0 . Theory and Practice . . . . . . . . . . . . . . . . 101 7 . Product Agitation During Process . . . . . . . . . . . . 103 8 . High-temperature Short-time Procews . . . . . . . . . . 104
3 . Factora Influencing Thermal Resistance of Bacteria in Foods . . .
. . . . .
4 . Methods of Measuring Resistance of Bacteria to Heat . . . . . .
. . . . . .
V . Summary and Discussion . . . . . . . . . . . . . . . . 104 References . . . . . . . . . . . . . . . . . . . . . 113
I . INTRODUCTION Most foods are very complex materials and the solving of virtually
any major research problem concerning them seldom involves the appli- cation of only a single science; usually. fundamental information from
48 C. R. BTUMBO
several sciences must be applied in the solution of a single problem. The past 50 years has been a period of rapid growth for the sciences, espe- cially for bacteriology, chemistry and physics. As the store of funda- mental information has grown through basic research in these and other sciences, more and more scientifically trained workers have become in- terested in the complex problems relating to food preservation. The correlative advancement of the food preservation industry has been grati- fying. Nowhere is the interrelationship of basic research, applied re- search and industry progress more striking than in the history of thermobacteriology as it has been applied to food processing. To one not familiar with the subject it might seem that the application of ther- mobacteriology to food processing involves only the science of bacteri- ology. Actually it involves bacteriology, chemistry, physics, mathematics and, to a lesser degree, other sciences.
The most widely used agent to accomplish food preservation today is heat. The primary object of thermal-processing foods is to free the foods of microorganisms which might cause deterioration of the foods or en- danger the health of persons who eat, the foods. However, if freeing foods of microorganisms were the only consideration involved in thermal- processing them, their preservation would be relatively a simple matter. Unfortunately many of the organoleptic and nutritive properties of foods are also affected by heat. For this reason it is imperative to the preser- vation of food quality that heat treatments given are very little more severe than just adequate to free the foods of undesirable microorganisms. Therefore thermal resistance of bacteria which may occur in foods is of primary concern. Information gained through basic research in thermo- bacteriology is essential to the establishment of scientific methods of food processing. Studies relating to factors which influence thermal resistance of bacteria in foods, relating to variations in thermal resistance among species of bacteria which are of concern in food preservation, and relating to mechanisms by which heat destroys bacteria yield information necessary to the formulation of applicable methods for evaluating the lethality of thermal processes for foods. That fundamental information of this type was very meager 30 years ago is believed to be the basic rea- son for so little real scientific progress in the art of thermal processing of foods prior to that time. Though presently available information of this type is far from complete, the past three decades of basic and ap- plied research has made some remarkable contributions-sufficient to per- mit of notable refinements in the thermal processing of foods.
With respect to evaluating thermal processes for foods from the stand- point of their capacity to destroy bacteria, fundamental information concerning the effects of heat on bacteria, tshough of primary concern, is
THERMOBACTERIOLOGY AS APPLIED To FOOD PROCEEISINQ 49
far from sufficient in itself. Of equal importance is basic information concerning rates of heating of the different foods during process. Mecha- nisms of heat transfer within the food itself during process must be considered also. Again it should be noted that presently available in- formation of this type is inadequate in many respects, though a great amount has accumulated during the past 30 years. Integration of lethal effects, determined from a consideration of bacteriological and physical data, involves the application of basic mathematical principles. Evalua- tion of heat processes with respect to their effects on nutritive qualities of foods involves &dies in biochemistry and nutrition.
Thermobacteriology as applied to food processing embraces a diversity of considerations, foremost among which is the evaluation of thermal processes with respect to their capacity to destroy bacteria in foods. Since most foods are hermet.ically sealed in containers (metal and glass chiefly) prior to their being heat-processed, chief concern has been evalu- ation of thermal processes for canned foods. The first scientific approach to this problem of applying bacteriological and physical data to evalua- tion of thermal processes for foods was the General Method described by Bigelow et al. (1920). Simpler and more versatile methods involving mathematical integration of heat effects were developed by Ball (1923; 1928). These methods have been used so extensively in the canning industry that no discussion of thermobacteriology as applied to food processing could be considered complete unless it included some descrip- tion of them. Prior to the development of the methods, time-temperature requirements of thermal processes for foods were determined almost en- tirely by trial and error. This in itself is sufficient to account for the slowness of progrew in refinement of the art of food processing prior to 1920.
It seems best to begin this discussion of thermobackriology as it is applied to food processing with a brief description of the methods of process evaluation, special attention being given to fundamental concepts on which development of the methods was based. It, is hoped that the following discussion will clearly indicate some of the many problems still existent with regard to further refinement in the art of thermal procewing of foods.
IT. THERMAL PROCESS EVALIJATION
1. The General Method This method described by Bigelow et al. (1920) is essentially a graph-
ical procedure for integrating the lethal effects of various time-tempera- ture relationships existent in a container of food during process. The he- temperature relationships for which the Iet.hal effects are integrated
50 C. B. BTUMBO
are those represented at the point of greatest temperature lag during heating and cooling of the product. (This point was found to be at or near the geometric center of the container.) Heating and cooling curves
Y IN YlNUIU
Fig. 1. Heating and cooling curves representing tempera- tures existent at cent.er of con- tainer of product (pureed spinach in &ounce glam con- tainer) during process (RT = 240F.).
are constructed to represent the tempera- tures existent during process (see Fig. 1). Each temperature represented by a point on the curves is considered to have a ster- ilizing, or lethal, value.
Thermal resistance of bacteria is repre- sented by thermal death time curves obtained by plotting time required to kill the spores of a given microorganism against temperature of heating (see Fig. 2). From time-temperature relationships represented by the thermal death time curve, it, is possible to determine a lethal rate value for each temperature repre- sented by a point on the curves describ- ing heating and cooling of a product during process. The lethal rate value assigned to each temperature represented
is equal to the reciprocal of the number of minutes required to destroy the organism in question at this temperature, destruction time corre- sponding to any given temperature being ascertained from the thermal
Fig. 2. Hypothetical thermal death time curve typical in form of curves obtained for spores of CZ. nporogenes and related organisms.
THERMOBACTERIOLOGY AS APPLIED TO FOOD PROCESSING 51
death time curve for the organism. For cxamplc if the thermal death death time curve indicated that 10 minutes were required to destroy the spores of a given organism a t 115C. (239F.), the lethal rate value assigned to this temperature would be 0.1. Lethality then is equal to the product of lethal rate and time, a process of unit lethality being that process which is just sufficient to sterilize n food.
According to concepts on which the method was based, i t may be said that each point on the curves describing heating and cooling of a con- tainer of food during process represents a time, a temperature and a lethal rate. By plotting the times represented against Corresponding lethal rates represented, a lethality curve representing the process is obtained. Fig- ure 3 shows such a curve on plain coordinate paper, lethal rate being represented in the direction of ordinates and time in the direction of abscissae. Since the product of lethal rate and time is equal to lethality, the area beneath the lethality curve may be expressed directly in units of lethality. To determine what in process time must be employed to give unit 3 lethality (sterility), the (cooling portion of the lethality curve is shifted so as t o give an area beneath the curve equal to I 1 y~ 1 I I u I I one. When the area is equal to 1, process O lo O TIME IN UINUTLS O ea time required to accomplish sterilization is Fig. 3. Lethality c,Irve based represented by t.he intersection of the cool- on values from cur,.es in ~ i ~ ~ , ing curve and the z-axis. This is a trial- 1 and 2. and-error procedure, and for this reason the method is sometimes referred to as the grapliical tl.inl-nncl-cl.i.oi, method.
Notable improvements in the General Method were made by Schultz and Olson (1940). A special coordinate paper for plotting lethality curves was described. Use of this paper considerably reduces the effort required for making calculations and reduces the chances of misplotting points. Formulac were introduced for converting heat-penetration data obtained for one condition of initial food temperature and retort tem- perature to corresponding data for different retort and initial tempera- tures. These improvements greatly increased the applicability of the General Method; however, the method is still laborious and is ordinarily used only for calculation of processes which are not, readily calculated by the simpler mathematical procedures devcloped by Ball (1923; 1928).
The basic concepts on which the General Method was developed are worthy of note. Time-temperature relationships which would account
52 C. R. STUMBO
for complete destruction of the spores of a given type of bacteria, were considered to exist. That is, it was believed that if the spores were exposed to a lethal temperature for some given length of time all of them would be destroyed. The thermal death time curve was considered to represent end-points of destruction. The influence of number of spores on severity of process required was accounted for only through the resist- ance values employed to construct thermal death time curves. No given number of spores was specified for obtaining these resistance values.
Another important concept concerns the integration of lethal effects produced, during process, a t a single point in the container of food-the point of greatest temperature lag. Since food a t all other points in the container was considered to receive more severe heat treatments, i t was assumed that the point of greatest temperature lag was the only point of concern with respect to calculating a process to accomplish sterilization.
The probable validity of these concepts will be discussed later. Suffice i t to say a t this point that the General Method has been abandoned, for the most part, because it is far more laborious to use than the simpler and more versatile mathematical methods. It should be pointed out, however, that the fundamental concepts on which the General Method was based also served as the basis for development of the mathematical methods.
8. Mat hematical Methods These methods, developed by Ball (1923; 1928) mathematically ac-
complish integration of the lethal effects produced by tirne-temperature relationships existent a t the point of greatest temperature lag in a con- tainer of food during process. The formulae developed for use are rela- tively simple and constitute a great improvement over the General Method for calculation of processes for most foods. I n the words of Olson and Stevens (1939), These formulas can be applied to any case wherein the major portion of the heating curve on semi-logarithmic paper approximates a straight line or two straight lines, and wherein the thermal death-time curve on semi-logarithmic paper is, or can be as- sumed to be, a straight line. Balls work not only greatly extended the scope of process calculations but simplified them as well. Less time was consumed in calculating processes, and provisi...