[Advances in Food Research] Advances in Food Research Volume 1 Volume 1 || Deterioration of Processed Potatoes
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Deterioration of Processed Potatoes
BY A . FRANK ROSS1 Cornell University. Ithaca. New York
Page I . Introduction . . . . . . . . . . . . . . . . . . . 257
I1 . Common Types of Deteriorative Changes . . . . . . . . . . 259 1 . Dehydrated Potatoes . . . . . . . . . . . . . . . 259 2 . Other Products . . . . . . . . . . . . . . . . 259
259 1 . Dehydrated Potatoes: Browning . . . . . . . . . . . 259
280 . 261
266 f . Raw Material Factors Affecting Deterioration of the Dehydrated
Product in Storage . . . . . . . . . . . . . . 268 g . Packaging and Other Factors . . . . . . . . . . . 271
2 . Dehydrated Potatoes: Graying and Development of Off Flavors in Storage . . . . . . . . . . . . . . . . . . . 272
3 . Other Products . . . . . . . . . . . . . . . . 272 . . . . . 273
1 . Changes Associated with Browning . . . . . . . . . . . 273 2 . Sulfite Retention during Storage . . . . . . . . . . . 278
V . Control of Browning . . . . . . . . . . . . . . . . 279 1 . Methods of Suliiting . . . . . . . . . . . . . . . 279 2 . Attainment of Low Moisture . . . . . . . . . . . . 280 3 . Use of Potatoes Low in Reducing Sugars . . . . . . . . . 281
a . Availability . . . . . . . . . . . . . . . . 281 b . Factors Idluencing the Reducing Sugar Content of Potatoes . . 282 c . General Considerations . . . . . . . . . . . . . 283
4 . Combined Treatments . . . . . . . . . . . . . . 284 VI . Summary . . . . . . . . . . . . . . . . . . . . 285
References . . . . . . . . . . . . . . . . . . . 286 I . INTRODUCTION
111 . Factors Influencing Rate and Extent of Deterioration . . . . . . . a . Temperature and Moisture during Drying . . . . . . . 259 b . Inhibition of Browning during Processing by Suliiting . . . . d . Temperature and Moisture Content during Storage . . . . 263 e . The Effect of Suliiting on Browning in Storage . . . . . . c . Raw Material Factors in Relation to Browning during Drying
I V . Chemical Changes during Storage of Dehydrated Potatoes
About four hundred million bushels of potatoes are produced annually
Formerly Biochedt, Maine Agr . Expt . Sta . All of the authors unpublished data cited in this review are from a joint project of the Quartermaster Corps, U . 8 . Army, and the University of Maine . M . T . Hilborn, L . C . Jenness, and Emily M . Bartlett also participated in this project.
258 A. FRANK ROSS
in the United States. Of this amount, between 5 and 10% is utilized in the manufacture of potato food products. The potato industry is of com- parable importance in many other countries. The production of dehy- drated potatoes increased rapidly during the war years but this has declined to a much lower, peacetime level. On the other hand, the manu- facture of certain other products, such as potato chips and frozen French fried potatoes is rapidly increasing.
Although the production of processed potato products utilizes a com- paratively small proportion of the potato crop, it is nevertheless an im- portant stabilizing factor in the potato industry. Unless highly acceptable products are prepared and these products retain their quality until they reach the consumer, the potato processing industry will not prosper. Hence, the deterioration of potato products during and after processing is of interest not only to military authorities and to the consumer, but also to those concerned with the production, marketing and conversion of potatoes.
Because of the special requirements of the Armed Forces, emphasis dur- ing the past several years has been placed on dehydrated potatoes. A comparatively small number of published papers dealing with other types of processed potatoes indicate the existence of certain phenomena common to all. Hence, certain of the data obtained with the'dehydrated product will be applicable to other products more in demand in the civilian mar- ket.
While this review is concerned primarily with deterioration during stor- age of potato products, some attention will be given to changes during processing, or in other words, to deterioration during drying, canning, fry- ing, etc. It is becoming increasingly clear that certain deteriorative changes that occur during storage are identical in nature with undesirable changes that may occur during processing. In the first case, the changes are brought about by mild conditions over a long period of time, whereas in the second, identical changes are the result of extreme conditions for a short period. Thus the potential storage life of a given sample may be materially shortened by processing under adverse conditions.
Only those data on dehydrated potatoes obtained with a fully blanched product are reviewed here since it has been shown that blanching is essen- tial to the production of a good quality product (Cruess and Mrak, 1940, 1942a, 194213; Cruess and Joslyn, 1942; Davis et al., 1942; Beckley and Notley, 1941; Chace et al., 1941) but nothing is to be gained by a blanch longer than that required to inactivate enzymes (Cruess, Smith, and Balog, 1943; Campbell et al., 1945). Consequently, a discussion of enzymatic changes is not included.
DETERIORATION OF PROCESSED POTATOEB 259
11. COMMON TYPES OF DETERIORATIVE CHANGES
1. Dehvdrated Potatoes
One of the most important types of deterioration in storage of dried potatoes which have been properly blanched is the development of 8 reddish brown to dark brown discoloration, commonly referred to as browning. Parallel to the development of color there is formed a bitter scorchlike (caramel) taste and an equally undesirable odor. The badly discolored pieces do not reconstitute properly. In strips or dice, the color development is most intense in the center of the piece. Browning may occur also during the drying process. This so-called scorch or heat damage has been a frequent cause of trouble in dehydration plants. It now appears that heat damage and browning in storage are very similar if not identical in nature, differing chiefly in the conditions under which they occur.
Other types of deterioration have been described by workers in England. Tomkins el al. (1944) described a gray discoloration in potato strips stored at 15C. (59F.) or below, and Burton (1945b), the development of an off-flavor in low moisture potato powder stored at high temperatures.
2. Other Products
Canned potatoes have been described as having a long storage life (Rendle, 1945). However, certain deteriorative changes have been re- corded (Rogers, 1945; Rhodes and Davies, 1945). The former worker described the development of an amber-pink discoloration during heat processing and its intensification at a storage temperature of 373C. (100F.). The discolored potatoes had a burnt flavor. Rhodes and Davies described a breakdown or crumbling of canned potatoes during storage with a resultant poor texture.
Browning is also a problem in potato chip manufacture and in the prep- aration of French fried potatoes.
111. FACTORS INFLUENCING RATE AND EXTENT OF DETERIORATION
I . Dehydrated Potatoes: Browning
The early work of Mangels and Gore (1921) indicated that potatoes are rather sensitive to heat dam- age and that the extent of injury is a function of time, temperature, and humidity. More recent investigations have confirmed these results and have provided additional information on the conditions necessary to avoid this type of diacoloration. Nichols et al. (1925) recommended a finishing
a. Temperature and Mdisture during Drying.
260 A. FRANK ROW
temperature of 62.So-68.3"C. (145"-155"F.) for potatoes. In a summary of dehydration work in Canada, Davis et al. (1942) reported that one of the greatest difficulties in dehydrating potatoes is the danger of discoloration near the end of the drying period. He recommended a reduction in tem- perature to 62.8"-65.5"C. (145"-150F.) after an approximate moisture content of 10% has been reached. In their tabulated data, a greater sensitivity to heat damage of long stored potato tubers was recogniaed. A finishing temperature of 65.5"C. (150F.) was recommended for early season material and one of 62.8"C. (145F.) for that stored for a long period of time. Davis et al. pointed out that most material can be exposed to a high temperature a t the start of the operation while rapid evaporation is taking place and the temperature of the material itself is not much above that of the wet bulb, but that progressively lower dry bulb temperatures are required as drying takes place. Cruess and Friar (1943) observed dis- coloration when potatoes were dried at high humidities, held too long at 60C. (140"F.), and when they were dried at high finishing temperatures. They considered a 65.5"C. (150F.) finishing temperature as above the "scorch line" or dangerously near it. These observations establish a rela- tionship between heat reddening and moisture-temperature conditions dur- ing drying but data G Z ~ the specific effect of a given temperature as the material reaches different moisture levels are lacking.
It would appear that with a given temperature schedule during drying, browning can be minimized by a low relative humidity and a high rate of air flow. Reports are in disagreement however, on the effect of these fac- tors. Caldwell e.! al. (1945) have pointed out that if drying is too rapid, the rate of evaporation may exceed the rate of transport of water from the interior and drying out of a surface layer or case hardening occurs, result- ing in a rise of temperature in the interior of the piece.
b. Inhibition of Browning during Processing by Suljiting. Nichols and Gross (1921) compared blanching in 0.1% sodium bisulfite with other methods and in general rated the sulfited samples above the others. Cruess et at. (1944a,b) reported data showing that dipping blanched potatoes in a 0.5% bisuifite solution prevented reddening or browning during finishing at 7323C. (165F.). It was concluded that sulfiting would permit finish- ing temperatures 11.1"C. (20F.) above those in commercial use. Cald- well et at. (1945) found that sulfiting, either with SO2 gas or by dipping in sulfite solutions prior to blanching, effectively reduced or prevented heat damage. In none of the above reports were data given on the amount of sulfite retained in the dry product.
Tressler (1944) treated blanched potatoes in 0.1% bisulfite, dried at temperatures of 98.9"C. (210F.) and 85C. (185OF.) in the dehydrator and one of 623C. (145F.) in the finishing bin. The dry product contained
DETERIORATION OF PROCESSED POTATOES 261
153 p.p.m. SOa, was of good color, and was free of scorched pieces. The untreated product was of a dull color and contained scorched pieces. Friar and Van Holten (1945) found that about 300 p.p.m. SO1 prevented dis- coloration during dehydration at 73.8OC. (165OF.) whereas control samples darkened. The sulfited sample was only very slightly damaged even after two additional hours of drying at 73.8"C. (165F.) Green el al. (1946) made a study of the effect of different sulfiting practices on the color of the dried product. Since the samples were not examined until after 4% months' storage no distinction can be made between the discoloration which occurred during processing and that which occurred during storage. Samples sulfited after blanching, were of better color than those sulfited prior to blanching. Lots dipped in HSSOa and in "SO2 retained color equally well.
The reports cited above show conclusively that sulfiting is an aid in the prevention of heat damage, but most of the data are only qualitative in nature. Little consideration has been given to the variability of the raw material. The indiscriminate raising of the dehydration Wishing tem- peratures could cause trouble with some lots and might nullify the bene- ficial effect of sulfite. It should be kept in mind also that lots quite sen- sitive to heat damage deteriorate rapidly in storage and that merely preventing discoloration during drying is no assurance that the finished product will store satisfactorily.
Sulfiting destroys most of the thiamine (Morgan, 1935; Davis et al., 1942; Tressler, 1944) but decreases the loss of ascorbic acid (Davis et al., 1942; Tressler, 1944).
c. Raw Material Fact#rs in Relation to Browning during Drying. Until recently, little attention was paid to the effect of variations in raw material on sensitivity to heat damage. Davis et al. (1942) recommended a lower finishing temperature for stored potatoes than for early season ones. Black (1943) reported that potatoes stored at 1.6'4.4"C. (35'40F.) were more sensitive to heat than were new potatoes but no explanation was given. Friar (1943) encountered some lots of new potatoes of the White Rose variety that discolored seriously during dehydration while Cruess and Friar (1943) observed that potatoes which were immature or that had been stored a t low temperatures often became yellow during drying. It seems probable that at least some of the discoloration they noted was heat dam- age, yet no distinction ww made between the yellow color due to the pres- ence of carotinoid pigments in such potatoes (Caldwell et al. 1943; 1945) and that due to heat damage. Caldwell et al. (1945) reported extreme sensitivity to heat damage with physiologically immature stock, with potatoes that had been in cold storage for long periods of time, and with tubers in which dormancy was "broken" and sprouting had begun. They
262 A. FRAN'K ROSS
suggested that the great sensitivity of such tubers was due to their high content of sugars and (or) amino acids. No analytical data, however, were reported in the foregoing papers.
The behavior of potatoes from cold storage and possibly of those form- ing sprouts makes it appear likely that the presence of sugars was an im- portant factor. The possible cause of the rapid discoloration of dehy- drated potatoes from immature tubers is more obscure. The data of Appleman and Miller (1926) show that the sugar content of developing tubers decreases as maturity is approached, yet the sugar content of the most immature lots examined was about 570 total sugars on a dry weight basis and about 1% reducing sugars. Experience has shown that these levels of sugar would not cause undue sensitivity to heat damage during dehydration and storage (Campbell and Kilpatrick, 1945). On the other hand, the amount of amino nitrogen tends to increase as the tubers ap- proach maturity.
Wright et al. (1945) found a definite correlation between the color of dehydrated potatoes and the storage temperature of the raw stock. The amount of color in the dried strips of various lots increased as the tempera- ture at which the raw stock was stored approached 0C. (32F.). Those lots stored at 10C. (50F.) and 16.4"C. (60F.) were lightest in color. These differences were correlated with changes in the total sugar content of the tubers, and the discoloration during processing was attributed to caramelisation of the sugar.
Experiments with Maine potatoes showed that lots high in reducing sugars were quite sensitive to heat damage (Ross et al., 1945). Several commercial lots of dehydrated potatoes, rejected because of heat damage, were found to be high in reducing sugars. When discolored pieces were separated from those having good color, the former were found to be con- siderably higher in reducing sugars than the latter. These data point to high reducing sugar content as a primary cause of sensitivity to heat dam- age, but, in the absence of data relating to other types of sugars, they can- not be considered conclusive.
Campbell and Kilpatrick (1945) obtained data correlating sensitivity to heat damage with the reducing sugar content rather than with that of total sugars. The raw stock (White Rose) was stored at 21.lo-23.9"C. (70"- 75"F.), 4.4"C. (40F.), and 0C. (32F.) immediately after digging. Anal- yses for total sugars and for reducing sugars were made at regular intervals; at the same time samples were dehydrated under standardized conditions. A t the end of 8 weeks, the potatoes held at the lower temperatures were shifted to storage at 21.lo-23.9"C. (70"-75"F.). Samples for analysis and for dehydrating were taken at the end of 1,2, and 4 weeks of the additional storage. Both two stage and single stage RYS~~IIIS of dehydration were
DETERIORATION OF PROCESSED POTATOES 263
used. The degree of heat damage was estimated by counting the dis- colored pieces and by measurement of the light absorbed by a clarified water extract of the dried potato. No heat damaged pieces were found in those prepared from the potatoes stored initially at 21.l0-23.1"C. (70"- 75OF.1, and, with the exception mentioned below, the extent of heat damage increased as the total and reducing sugars increased; i.e., as the storage temperature decreased. Subsequent storage of the potatoes high in sugar (first stored at low temperatures), at warm temperatures, reduced the ex- tent of heat damage but did not completely prevent it. The exception referred to above is of particular interest. Potatoes stored at 4.4"C. (40F.) for 4-6 weeks were more sensitive to damage by heat than those stored at 0C. (32F.) for the same length of time. The analytical data showed that there was a greater accumulation of reducing sugars in those samples stored at 4.4"C. (40F.) than in those at 0C. (32F.) but just the reverse was true for total sugars. There was a highly significant correlrlr tion (coefficient, +0.89) between the percentage of light absorbed by the water extracts of the dehydrated potatoes and the reducing sugar content in the raw material. Campbell and Kilpatrick advise against the dehydra- tion of potatoes containing over 2.5-3% reducing sugar (on the moisture- free basis).
Doty et al. (1946) stated that within a given variety of potato there ap- pears to be a rather close correlation between the degree of browning dur- ing dehydration and the amino nitrogen and reducing sugar contents. This relationship was not always observed when different varieties were compared.
d. Temperature and Moisture Content during Storage. It has been known since 1921 (Gore and Rutledge, 1921) that the browning of dehydrated potatoes in storage occurs at high temperatures and that the rate of brown- ing is influenced by the moisture level. Gore and Rutledge worked with slices and riced samples steamed 40 minutes before drying. When stored at 1.7"C. (35F.) the slices did not discolor in 700 days. Storage life at 23.9"C. (75F.) was dependent on the moisture content of the samples. Those containing 5.442% moisture discolored slightly in 700 days whereas those with 7.8-8.0% moisture turned brown in 222 days. When stored at 37.8"40.0"C. (100-105"F.) , all samples became brown in 222 days; those highest in moisture were the most severely discolored.
Tomkins et al. (1944), who worked with dried potato strips containing 50-100 p.p.m. SO, found that browning generally occurs at temperatures above 28C. (82.4'F.) and not at lower temperatures. At 37C. (98.7"F.) potatoes with 10% moisture browned much faster than those with 5%, which in turn discolored slightly faster than those containing 2.7% mois- ture.
264 A. FRANK ROSS
The workers at Continental Can Co. (1944) conducted storage tests on commercially dehydrated potatoes. In most instances, the samples were acceptable after 1 year at room temperature, but considerably darkened and inedible after 6 months at 36.7"C. (98'F.) and quite dark after 1 month at 54.4"C. (130F.).
The rate of darkening of a potato flour prepared from dried potato slices was found to be accelerated by an increase in moisture, particularly above 10.5%, and by high temperature (Burton, 1945a). Similar relationships were found with mashed potato powder (Burton, 1945b). The extent of discoloration was measured by means of a Lovibond tintometer and the limit of acceptability was stated to be between 1.0-1.5 Lovibond units. The storage life of samples held at 57C. (134.6"F.) varied with the mois- ture content aa follows:
Moisture content,% Storage life, days 4.9 9 6.5 3 7.1 2
Samples with still higher moisture content became unacceptable in less than a day. Power containing 12% moisture retained color for 10 months at 20C. (68'F.) but showed a slight discoloration a t 25C. (77F.). Fur- ther increase in storage temperature decreased the storage life as follows:
Storage temperature, "C. ("F.) Storage life, days 28 (82.4) 180 37 (98.6) 4 2 4 9 47 (116.6) 14 57 (134.6) 3
Howard (1945a, b) determined the effect of absolute moisture content on the storage life of diced potatoes containing about 300 p.p.m. of SO2 and packed in nitrogen. The relation of absolute moisture content2 to storage life, as determined organoleptically, is shown below:
Absolute moisture Storage life at 48.9'C. content, yo (120"F.), daya
4.5 56 6.7 26 8.0 18
* Determined by the method of Mdtower et d. (1946). This method cliffera from that described in the tentative apeoificationa of the Quarterrnester Corps (1946) in that the entire sample is ground to &mesh, and a portion of the ground sample is held in a vacuum oven at 70'C. (158'F.) for a longer period of time (40 hrs.).
DETERIORATION OF PROCESSED POTATOES 265
He also found that the higher the moisture content, the more rapid was the development of the brown color. Caldwell et al. (1945) noted that potato strips discolored more rapidly at high temperatures than at low ones and that samples stored in air with a high relative humidity discolored more rapidly than those stored in dry air. They claimed that browning prac- tically ceases when the moisture content of the material is reduced below 5%.
Legault et al. (1946) reported that the time required for a given color development varies exponentially with the absolute temperature if the moisture content is constant (between the limits of 4 4 % moisture), or with the moisture content at constant temperature. Thus storage life was doubled by a 3.4"C. (6.2"F.) drop in temperature or by a 2% (absolute) drop in moisture.
The present author obtained data on the effect of temperature on the storage life of dehydrated potatoes varying greatly in reducing sugar, sul- fite, and moisture content. Several varieties were studied, including samples dehydrated in an experimental drier and in commercial dehy- drators. Storage life was defined in terms of color development. The lat- ter was followed by determining the transmission a t 390 mp, of an aqueous alcohol extract. It was found that, in general, the storage life of a given sample was 8.5 times as great at 37.8OC. (100F.) as at 48.9OC. (120F.); 3.1 times as great at 48.9"C. (120F.) as at 54.4"C. (130F.) and 27 times as great at 373C. (100F.) as at 54.4"C. (130F.). Over the range studied, storage life was approximately tripled for each decrease in tem- perature of 5.6"C. (10F.) or approximately doubled by a decrease of 3.3"C. (6F.). The same relationship held regardless of the nature of the raw material used, the sulfite or moisture content, or the method of dehy- dration. These results appear to be in fair agreement with those of Legault et a2. (1946).
Data were obtained also on the relation of moisture content to the stor- age of dried potatoes. Moisture levels of approximately 1% (2.5oj, abso- lute) were obtained by packaging with CaO in accordance with the method described by Howard (1945a,b,c). Storage life was about doubled by a decrease in moisture of 2.5% (2.2% absolute). This is in close agreement with the data of Legault et al. (1946). It was found that in practically all cases a straight line was obtained when the data were plotted on semi-log coordinates, using percentage moisture as abscissa and storage life in days as ordinates. The slope of the line was nearly always the same, apparently being independent of variety, and of reducing sugar and sulfite content. An equation was derived for calculating the anticipated storage life (L2) of a sample at any moisture level (M2) provided the storage life (L1) at an- other moisture level (MI) is known. The storage temperature must, of
266 A. FRANK ROSE
course, be the same in both cmes. The equation formulated for the cal- culations of storage life is:
log L2 = m (MS - MI) + log d (1) In this equation L1 and Lz represent change in storage life in days, MI and Mz moisture content in yo, and m is a constant, the value of which depends only upon the method used for determining the moisture content. Where the method described in the tentative specifications of the Quartermaster Corps (1945) is used, m = -0,119, whereas when the method of Makower et al. (1946) for absolute moisture is used, m = -0.131. It is not known whether or not these relationships hold outside of the moisture range studied (1.2% to 7.5%).
e. The E$ect of Suljiting on Browning in Storage. The chronology of commercial sulfiting in England, Canada and the United States has re- cently been reviewed by Green et al. (1946) and need not be repeated here.s The beneficial effect of sulfiting on the storage life of dehydrated potatoes has been recognized by several investigators (Davis et al., 1942; Cruess and Mackinney, 1943; U. S. Dept. Agr., 1944; Tomkins et al., 1944; Mackinney, 1945a,b; Caldwell et al., 1945). In most cases, no data are given relative to the actual increase in storage life that could be attributed to sulfite. Statements range from sulfite helps slightly to preserve quality during storage a t 37C. (Tomkins et al., 1944) to sulfite affords a means of very considerably prolonging storage life (Caldwell et al., 1945). It is probable that this difference is due in part to variations in the amount of SOZ in the product.
Howard (1945a) compared unsulfited and sulfited samples at the same moisture level and found that the former became unacceptable in 10 days at 48.9C. (120F.) while the latter, containing 300 p.p.m. SO2 was ac- ceptable for 25 days. When the samples were packed in containers with CaO, the storage life of the unsulfited sample was 60 days and that of the sulfited one over 80 days.
Unpublished data of the present author show that storage life is in- creased considerably by increasing the sulfite content of the finished prod- uct. The beneficial effect of sulfite, however, is not so apparent when the sugar content of the dried product is high (Table I).
In the paper by Green et al. (1946) reference is made to a report by English investi- gators (Barker et al., 1943b) on the effect of a number of antioxidants on browning, none of which waa aa effective or as satisfactory aa sul6te. A second report is cited (Barker et aZ., 1943a) in which data are given on the beneficial effect of sulfite on ascorbic acid retention and on retardation of deterioration in respect to color and flavor. Un- fortuaatslJr these reports are not available to the author.
DETERIORATION OF PROCESSED POTATOES 267
Katahdin potatoes containing 7% moisture
Irish Cobbler potatoes containing about 625
p.p.m. 501 + 7% moisture Reducing Increased swam, % p.p.m.
Reducing sugars, %
1.9 1.9 1.9 6.6 6.6 6.6
Increased storage lifea
1200 323 813
1.7 2.2 5.2 1.9 2.1 3.1
* Ratio of storage life of suKted sample to that of the unsulfited sample.
These data show that progressively increasing amounts of sulfite generally cause progressively greater increments in storage life. It also appears that the effect of a given amount of SO2 depends in part upon the reducing sugar content of the sample; a given amount of sulfite being more effective at lower reducing sugar levels than at higher levels. All samples, how- ever, did not behave in this manner. When the Green Mountain variety was sulfited, the reducing sugar content of the product had no consistent effect on the effectiveness of the sulfite added (up to 928 p.p.m.). It wa& concluded that sulfite in concentrations ranging from 200 to 500 p.p.m. would result in a 50-100% increase in the storage life of dried potatoes relatively high in reducing sugars. In cases when the reducing sugar contents are lower much greater increases in storage life can be expected. Legault et al. (1946) stated that the magnitude of the sulfite effect is a function of concentration and that it persists in accord with the factors that govern the rate of disappearance of sulfite (principally moisture and tem- perature).
The combined effect of low moisture and suEte is greater than either alone (Howard, 1945a; Mackinney, 1945a,b). In the present authors laboratory it was found that if a given amount of sulfite doubled the stor- age life of a sample at high moisture, the same amount of sulfite also would double storage life at a low moisture content.
The amount of SO2 that can be used is limited by the quantity tolerable to the consumer. As losses during storage and during rehydration cannot
268 A. FRANK ROSS
be predicted, tolerances are established for the dry product at time of packaging. In this country the maximum acceptable limit is considered to be 500 p.p.m. (U. S. Dept. Agr., 1944; Tressler, 1944) while in England the limit is 300 p.p.m. (Lovern, 1945). Barker and Burton (1944) state that more than 150 p.p.m. in mashed potato powder causes the product to have an unpleasant taste. The fact that larger amounts cannot be toler- ated in this type of product probably is due to smaller losses during re- hydration. In a recent paper (Green et al., 1946), it was reported that the pH of the sulfiting solution affects not only the rate of loss of SO2 from dehydrated potatoes in storage, but also the amount lost during reconsti- tution. Potatoes dipped in an SO2 solution (with or without orthophos- phoric acid) lost more SOs during storage and rehydration than those dipped in a sodium sulfite solution. A given amount of sulfite, however, in the acid dipped potatoes was much more objectionable to taste than was the same amount of SOa in potatoes dipped in the alkaline sodium bisulfite solution. These data help to explain some differences of opinion on the tolerable limits for SO,. They also offer the possibility of develop- ing methods whereby a high SO2 content could be maintained in storage, without impairing the taste of the product. There is a belief in the potato drying industry, that sulfite imparted to the product by combustion fumes in direct-oil-fired tunnel dehydration is less objectionable (from the stand- point of taste), than that imparted to the product by use of a dip.
f. Raw Material Factors Aflecting Deterioration of the Dehydrated Product in Storage. Although it has been known for many years that potatoes stored under different conditions vary greatly in sugar content (Miiller- Thurgau, 1882; Appleman, 1912), and that this factor is an important one influencing the browning of potato chips (Sweetmm, 1930, 1931; Rogers et al., 1937; Wright et al., 1936; Denny and Thornton, 1940, 1941b, 1942a,b), no report prior to 1945 has been found relating reducing sugars to the browning of dehydrated potatoes in storage. Several reports ap- peared in 1945 showing that the sugar content of dehydrated potatoes is definitely related to this type of deterioration and that it is reducing sugar, not sucrose, that is primarily concerned. Ross et al. (1946) reported a positive correlation between the amount of reducing sugars in the raw ma- terial and the relative amounts of color developed by the dehydrated prod- uct when stored at 54.4"C. (130'F.). At a constant moisture level, almost a straighbline relationship was obtained between these two factors. The data presented cannot be translated into terms of storage life; however, they show comparative effects of moisture and of reducing sugars. In another experiment by the author, samples varying in moisture and sugar content were compared after 14 days at 54.4%. (130'F.). One sample contained 8.2% moisture and was prepared from tubers containing 7 mg.
DETERIORATION OF PROCESSED POTATOES 269
0.15 0.22 0.21 0.30 0.86 0.56 1.64 3.67 0.79 1.62 0.83 4.08
reducing sugars per ml. juice. The figures for a second sample were 6% and 15.5 mg., respectively. The samples were identical in color after storage and were superior to a third containing less moisture and consider- ably more sugar (25.5 mg./ml. juice).
Caldwell et al. (1945) state that the dehydrated potatoes produced from immature tubers, from sprouting tubers, and from tubers held in cold stor- age for long periods of time do not keep well in storage. They attributed this to high levels of sugars and amino acids but gave no data in support of this contention. Burton (1945b) found a good correlation between the
0.17 0.24 0.25 0.41 0.65 0.71 2.44 4.12 0.61 1.91 1.14 3.00
Effect of Initial Sugar Content upon the Degree of Browning of Mashed Potato Powder (13% Moisture) after 4 Days at 67OC. (13.4.6F.)4
0.92 0.96 1.06 1.16 2.22 5.64 7.80
14.39 15.72 17.87 19.95 20.76
0.00 0.50 0.00 0.45 0.71 4.37 3.72 6.00
14.32 14.34 17.98 13.68
Total reducing su8&w
0.32 0.46 0.46 0.71 1.51 1.27 4.08 7.79 1.40 3.53 1.97 7.08
Color after storage,
1.8 1.6 1.7 2.8 2.9 3.8 8.1
12.4 5.5 6.9 4.8
Compiled from data of Burton (194513).
initial reducing sugar content of mashed potato powder and the degree of browning after 4 days a t 57C. (134.6F.) (Table 11). Samples at a given reducing sugar level browned a little more if their sucrose content was high than if it was low. Burton concluded from these and other data on sugar changes during storage, that the rate of browning in the early stages is determined by the initial reducing sugar (hexose) content but that the in- tensity of color developed reaches a maximum which is determined by the total amount of sugar present.
Data relative to the effect of different levels of reducing sugar on the storage life of diced potatoes have been obtained in the authors laboratory. The methods used differed from those of Burton in that diced potatoes
270 A. FRANK ROSS
with a lower moisture content (7%) and the lower storage temperature of 48.9"C. (120'F.) were used. Furthermore, samples were judged by the time required to reach a certain color level, instead of measuring the color after a fixed time interval. The storage life of dehydrated potatoes was found to be roughly inversely proportional to the reducing sugar content. The graphical presentation of the data (Fig. 1) shows considerable scatter,
A KATAHDIN ASEBAGO 0 IRISH GOBBLER
Fig. 1. Storage life at 48.9"C. (120F.) of dehydrated potatma containing 7% mois- ture and various amounts of reducing sugars. (Ross, 1947.)
but the line drawn on log-log coordinates appears to be a fairly good ex- pression of the relationship between these two variables. The scatter could not be accounted for by the potato variety used, or by the relative amounts of sucrose, glucose and fructose.
The line drawn in Fig. 1 can be expressed by the equation
log y = 1.38 - log 2 (2)
DETERIORATION OF PROCESSED POTATOES 271
where y = storage life (in days) a t 48.9"C. (12OOF.) of samples at 7% mois- ture (determined by the method described in the tentative specifications of the Quartermaster Corps, 1945) and 2 = reducing sugar content in %. Equation 2 was generalized into the form
(3) log ( y / a ) = b - log z
where a is a function of the temperature and b a function of the moisture content. Data previously discussed were used for deriving the following empirical equations for calculating a and b:
log a = - log 3 (4)
b = 1.38 + (7-M) log 1.315 (5 ) where T is the temperat'ure under observation in O F . and M is the moisture content in %. Equation 3 can be expected to give no more than a fair approximation but may prove useful in further study of these and other variables.
Further data showing the marked effect of reducing sugars on browning were obtained by Wiegand et al. (1946). Color development was followed by measuring the amount of light absorbed by an alcohol-water extract by means of a colorimeter. In a typical experiment, samples prepared from Netted Gems and containing 0.79, 0.99, 1.53 and 6.20% reducing sugars, respectively, gave readings of 98, 128, 167 and 888, respectively, after 15 days at 54.4"C. (130OF.). They also reported that samples prepared from Netted Gems from one area developed less color in proportion to their re- ducing sugar content than did samples prepared from the same variety grown in other areas. They did not report moisture contents, however, and the differences noted may have been due in part, a t least, to variations in moisture. Doty et al. (1946) reported browning in low-sugar potatoes during dehydration, the extent of which was correlated with the amino acid content of the raw potatoes.
g . Packaging and Other Factors. According to most investigators, pack- aging in inert atmospheres, such as nitrogen or carbon dioxide, has little or no effect on the rate or extent of the development of browning in dehy- drated potatoes (Tomkins et al., 1944; Continental Can Co., 1944; Burton, 1945b). This does not necessarily mean that oxygen has no effect on the browning reaction, for small amounts of oxygen are usually present in gas packed cans. Consequently, the report of Legault et al. (1946) that the influence of oxygen on the rate of browning is positive, though relatively small, is not necessarily in disagreement with the earlier reports.
Balog and Cruess (1946) noted that debydrated potatoes compressed
272 A. FRANK ROSS
and then sealed in cans were of much better color and flavor after storage for 2% years at 28.3O-29.4"C. (83"-85"F.) than were loosely packed sam- ples. They attributed the superior keeping qualities of the compressed pack to a 95% reduction in the volume of air in the cans. In view of the previously discussed reports on gas packing, where the oxygen content was reduced to about 1%, the basis for this explanation is not apparent. It seems unlikely that the ratio of oxygen to product in air-packed com- pressed potatoes would be less than that in nitrogen-packed mashed potato powder (Burton, 1945b).
Treatment of the potatoes with calcium chloride or sodium chloride solu- tions before or during blanching has been reported to decrease sensitivity to heat damage (Campbell and Kilpatrick, 1945). The same authors, as well as others (Legault et al., 1946), also report that extensive leaching re- sults in decreased browning.
The browning reaction in mashed potato powder is accelerated by an increase in pH, at least over the range pH 5.6 to pH 10 (Burton, 1945b). When the powder was subjected to pH 1.2, however, hydrolysis of sucrose occurred and the rate of browning was increased. In the same paper, data were given to show that the browning reaction is inhibited by hydroxyl- amine and by cyanide.
3. Dehydrated Potatoes: Graying and Development of '(O$" F ~ U O T S in Storage
Tomkins et a2. (1944) found that sulfiting (50-100 p.p.m. SO,) appre- ciably delays the tendency of potato strips to turn gray at 15C. (59F.) or less. They observed that this type of deterioration was much the same in all samples, whether they varied in moisture content or were packed in air, nitrogen, or carbon dioxide. Their taste panel data, however, show pro- gressively lower color and flavor ratings for low moisture samples (2.7%) as the storage period increased. This tendency became less apparent as the moisture content of the sample was increased. The estimated aver- age storage life of samples stored at 15C. (59F.) was more than 15 months when judged on culinary quality. Individual lots varied from 7 to over 17 months. These samples contained 7y0 moisture and presumably were sulfited.
The development of an "off" flavor in mashed potato powder at low moisture levels has been reported by Burton (1945b). The reaction is considered to be retarded by high moisture contents.
8. Other ~ T O d U C t S
Ruschmann (1932) has reported tBat silage made from potatoes high in sugar was brownish in color and inferior in quality. Rogers (1945) found
DETERIORATION OF PROCESSED POTATOES 273
that the reducing sugar content of potatoes used for canning affected flavor and color. Those high in such sugars, when canned by a variety of ac- cepted methods, developed a pinkish-amber color at the centers of the tubers and were of poor flavor. At high temperature storage 37.8"C. (100'F.) there was further deterioration in both color and flavor.
Several investigators have related the browning of potato chips to the sugar content of the potatoes used for their manufacture (Sweetman, 1930, 1931; Rogers el al., 1937; Wright el al., 1936). More recently, Denny and Thornton (1940, 1941b, 1942a) showed that this browning is correlated with the reducing sugar content rather than with that of sucrose. They recommended the use of potatoes for chip making containing not over 3 mg. of reducing sugars/ml. of juice.
The color intensity of French fried potatoes also appears to be a function of the sugar content of the tubers used (Wright et al., 1936) although no attempt has been made to distinguish between the effect of reducing sugars and that of sucrose.
Iv. CHEMICAL CHANGES DURING STORAGE OF DEHYDRATED POTATOES
1. Changes Associated with Browning
Several reports have been made showing that decreases in sugars and in amino acids accompany the development of the brown color. Analysis of a browned sample of potato flour showed a small decrease in sucrose, hexose sugars, and amino nitrogen (Burton, 1945a). In another report on the browning of potato powder induced by storage at high temperatures, Burton (1945b) gave the results of several analyses of browned samples. The figures given in Table I11 were calculated from the data of Burton to illustrate the relationship between the degree of browning and the decrease in total sugars. With few exceptions the degree of browning correlated well with the decrease in sugars but not with the decrease in amino nitro- gen. Other of the data showed that in any particular lot of potatoes low in sugar a correlation existed between the degree of browning during the early stages of discoloration and the loss of hexose sugars. A loss of su- crose occurred in many but not all samples. A decrease in glucose content occuyed in all samples and this loss most nearly paralleled the increase in brown color. Analyses of potatoes containing considerably more sucrose than hexoae sugars indicated marked losses of the former and smaller losses of hexose sugars. Burton assumed that hydrolysis of sucrose had occurred in such samples at the high temperature. From these and other of the data showing a marked correlation between the initial hexose content and browning, he concluded that in the early stages of the reaction the rate of
274 A. FRANK ROSS
Color of water extracta, Lovibond units
1.7 1.8 2.7 4.8 6.5 6.9
11.6 13.3 14.1 28 (approx.)
browning is determined by the initial hexose content but that the maxi- mum amount of color developed may be determined by the total amount of sugar present. His surmise would appear to be justified in part but there is nothing in the results to show that sucrose itself does not partici- pate in the later stages of the reaction. Unpublished data of the present author concerning changes in dried, diced potatoes during storage at 48.9"C. (120F.) showed a consistent reduction in fructose, but the values of glucose were quite variable, sometimes showing increases of such magni- tude that the total figure for reducing sugars in the browning product was
Total sugar Low Amino-Nitrogen Loss" g./1@J g. g./100 g.
0.26 0.03 0.31 0.04 1.00 0.01 2.19 0.04 0.62 0.05 2.81 0.10 4.64 0.07 2.57 0.03 2.06 0.12 6.60 0.22
The Relation oj Color I n t m * t g to Loss of Total Sugars and of Amino Nitrogen in Mashed Potato Powder Samples Containing 18% Moisture and Stored at
67C. (134.6"F.) for VariozcS Periods of Timea
larger than in the control sample. It was concluded that intermediate compounds were formed which reacted as reducing sugar during the quan- titative determination for glucose by reduction of dinitrophenol, or that the increase in glucose may have come about as a result of a hydrolysis of sucrose (analyses for sucrose were not made). A loss of amino nitrogen also occurred during browning and for any given variety the reduction was greatest in the most severely discolored samples. In this respect, however, there was considerable variation between varieties. Some sam- ples became severely discolored with only negligible losses in amino nitro- gen while others lost up to half of t h t originally present in reaching the m e stage of discoloration. While it seems probable that losses in re-
DETERIORATION OF PROCESSED POTATOES 275
ml./lOO g. 1.2 1.6 6.2 8.0 13.7 20.7 25.7 25.9
ducing sugars are associated directly with the development of the brown color, the relation of amino acid changes to browning is not as clear cut. The possibility of these being independent reactions has not been excluded.
Absorption of oxygen and an evolution of carbon dioxide during storage of dried potatoes have been observed. Tomkins et al. (1944) noted these gaseous changes in sealed cans of potato strips and reported that the changes were greater at 37C. (98.7"F.) than at 28C. (82.4"F.) and were less at 15C. (57F.) than at 37C. (98.7"F.). There was a greater change when samples were high in moisture than where they were low. Data ob-
Lovibond units 0.1 0.6 1.6 4.3 7.1 11.3 13.2 13.8
Changes in Gas Composition Occurring during Browning of Mashed Potato Powder Containing 1.2% Moisture and Stored at 67OC. ( 1 34.6'F.)
Lovibond units 0.3 0.4 1.0 6.1 7.5 11.8 12.9 13.9
Length of atorage,
m1./100 g. 1.0 2.0 7.1 12.5 13.5 13.5 13.5 13.5
1 2 6 10 16 30 41 62
I Nitrogen pack ~ ~~
ml./l00 g. 0.9 1.2d 2.3 4.2 8.0 14.8 17.4 17.4
0 Compiled from data of Burton (1945b). b Atmosphere contained 13.5 ml. 01 per 100 g. powder. 8 Normal temperature and pressure. d These results are presumably high because of traces of oxygen.
tained at the Continental Can Co. (1944) also indicate similar changes and agree with data referred to the above with respect to the effect of tempera- ture. Burton (1945b) found that when mashed potato powder of low mois- ture content was stored at a high temperature there was an absorption of oxygen accompanied by the development of an "off" flavor, but very little browning Qr COZ evolution occurred. With powder of high moisture con- tent there was a more rapid intake of oxygen, evolution of C02, and de- velopment of brown color (Table IV). After the disappearance of all of the oxygen, evolution of carbon dioxide continued at a slower rate. This latter rate w1t9 about the same as that of samples packed in nitrogen. In
276 A. FRANK Roaa
both cases the rate of browning was the same, the color developed to the same maximum intensity, and the rate of browning fell as this was ap- proached. Parallel with this decrease in rate of browning was a decrease in the rate of carbon dioxide evolution. These data were taken as an indication of the occurrence of two reactions in samples at a high moisture content :
(1) Anaerobic development of a brown color accompanied by CO, evolu-
(2) Absorption of oxygen accompanied by evolution of COI. tion.
A third possible reaction, occurring at low moisture levels, was stated to be an absorption of oxygen unaccompanied by evolution of carbon dioxide but resulting in the development of an off flavor. This type of flavor deterio- ration is reduced when the moisture content of the product is relatively high. It is quite possible that in potatoes relatively high in moisture, the substance causing off fiavor changes chemically, rather than accumulates as it forms. Reactions 1 and 2 are substantiated by data and their exist- ence in dehydrated potato strips and dice appear to be consistent with the available data. There are fewer data relative to the third reaction and very little or no data are available on this type of change in potato strips, dice, or shreds. The data of Tomkins et al. (1944) show greater deteriora- tion of flavor in diced potato strips containing 2.7% moisture than in strips containing 5%, when stored at 37C. (98.7F.). While this point may seem a minor one now, the development of this type of off flavor may become more apparent and of increasing importance as progress is made in the control of browning.
Browning of dehydrated potatoes is accompanied by the development of ultraviolet fluorescence of extracts (Doty et al., 1946; Pyke et al., 1946; Patton and Pyke, 1946). The fluorescence of the extracts varies directly with color. The fluorescence varies with pH (Pyke et al., 1946; Patton and Pyke, 1946). The pH-fluorescence curve of the brown pigment ex- tracted from dehydrated potatoes has been shown to be similar to that of the extract of potato chips.
The browning of potato flour is accompanied by a decrease in pH and by an increase in buffering capacity (Burton, 1945a).
A few data have been accumulated on the nature of the brown pigment or pigments produced in potatoes. Gore and Rutledge (1921) reported it to be water soluble, an observation that has been confirmed repeatedly since 1921. The pigment is insoluble in ether (Burton, 1945b), soluble in methyl alcohol, and somewhat soluble in ethyl alcohol (Doty et al., 1946). Dawson (1945) isolated a brown pigment from browned potatoes by a process involving clarification with amylase, dialysis, and precipitation with acetone. The latter treatment rendered the pigment imoluble in
DE1TERIORATION OF PROCESSED POTATOES 277
water. Both the dialyzed and nondialyred pigments reduced Fehling's solution and iodine. The former gave iodine titrations equivalent to 3540% glucose and about a third of the reducing value was retained after prolonged dialysis. The pigment reduced methylene blue and absorbed oxygen directly at 37C. (pH 8.3 and 9.7). After prolonged dialysis the product contained 1.5% nitrogen, of which was amino nitrogen. The latter value was doubled by acid hydrolysis. Doty et al. (1946) stated that a purified preparation of the pigment gave certain carbohydrate and protein tests. As no statement was given as to the specific protein tests made, one cannot conclude that protein itself is a part of the pigment, since some mcalled protein tests give positive reactions with amino acids. A crystalline derivative of the pigment has been obtained by Caldwell el al. (1945) but no analytical data concerning it are available.
Pyke and associates (Pyke et al., 1946; Patton and Pyke, 1946) have re- ported results indicating that reducing sugars and amino acids are both concerned in the browning reaction in potatoes. They worked chiefly with potato chips but were able to show that the browning reaction in this prod- uct was quite similar to that in dehydrated potatoes. Extraction of potato slices with hot water yielded an extract containing considerable amounts of reducing sugars and amino acids. When dried in vacuo the extract yielded a dry powder that browned if moistened and heated. The extracted potato chips did not brown when fried in hot fat but did so if first impreg- nated with the concentrated extract. Impregnation of the potentially white slices with glucose or other reducing sugars failed to produce brown- ing during subsequent frying; similar results were obtained by impreg- nation with glycine or other amino acids. When the slices were impreg- nated simultaneously with both types of compounds, browning occurred during subsequent frying. These results appear a t variance with those of Denny and Thornton (1940) who obtained browning of filter paper discs impregnated with glucose, dipped into potato starch, dried and then fried in hot fat. Moisture relationships were probably different in the two cases and the possibility of the presence of impurities in the potato starch was not eliminated. Unpublished results of the present author indicate that some constituent other than reducing sugars is concerned in the browning of dehydrated potatoes. Samples of low-sugar potatoes were rehydrated in glucose and in fructose solutions and in a solution containing both. Efforts to avoid leaching of other constituents were unsuccessful, for ap- preciable quantities of liquid drained from the potatoes. The samples were redried and when stored at 48.9OC. (120OF.) failed to brown at rates anticipated from their reducing sugar contents. It is concluded that suffi- cient quantities of other constituents essential for browning were leached from the samples, to appreciably affect the rate of browning.
278 A. FRANK ROSE
The available data constitute strong evidence that browning in potatoes is due to a reaction between reducing sugars and amino compounds, usu- ally referred to as the Maillard reaction. The disappearance of both re- ducing sugars and amino nitTogen during the browning of potatoes has been demonstrated. For browning to occur in potatoes, both types of compounds that have been leached must be replaced. The high tempera- ture coefficient of the reaction, changes in pH during browning, develop- ment of fluorescence, evolution of carbon dioxide, effects of pH and of moisture on the reaction, and the reducing properties and nitrogen content of the pigment are all consistent with this view. Caldwell et al. (1945) have pointed out that reducing sugars and amino acids are always present in potatoes and undergo an enormous concentration during dehydration.
2. Sulfite Retention during Storage
Variables that effect the rate of browning also appear to affect the rate of sulflte loss. Howard (1945a) measured the rate of sulfite loss in potatoes stored in nitrogen at 48.9"C. (120F.). These samples were dehydrated to 8.0, 6.7, and 4.6y0 moisture (absolute) , respectively. The original SO2 content of about 300 p.p.m. decreased considerably during storage. This is shown below:
Storage life', days Moisture content, % SOs after storage, p.p.m. 18 8 60 26 6.7 25 66 4.6 50
Burton (1945a) noted that sulfite loss from dried potatoes is dependent on both moisture content and temperature. This is indicated by data obtained with a sample of potato flour, containing 340 p.p.m. of SO2, held in storage for 4 months.
Storage temperature Moisture content, % SO2 after storage, p.p.m. "C. OF. 1 (33.8') 1 (33.8O) 37 (98.6") 37 (98.6')
300 200 110 60
The present author (unpublished data) found that sulfite loss was 5 to 10 times as great at 37.8"C. (100F.) as a t room temperature. With samples containing 2.5y0 or more reducing sugars, the rate of loss was about 7 times aa great at 48.9"C. (120F.) as a t 37.8"C. (100F.); but where the ' The time intervah indicated were those at which the respective samples were con-
sidered to have become organoleptically undesirable.
DETERIORATION OF PROCESSED POTATOEB 279
reducing sugar content was less than 2.5%, sulfite loss wm only 3 to 4 times as great at the higher temperature as a t 37.8OC. (100OF.). Other data were obtained indicating an effect of reducing sugar content on rate of sulfite low. Two samples containing approximately 650 p.p.m. of SO2 and 6.19% moisture but differing widely in sugar content were stored at 48.9"C. (120F.). The losses in SO2 observed during storage are shown below:
Potatoes containing 0.46% Potatoes containing 4.7% reducing sugars reducing sugara
13 22 13 w 23 30 23 80 39 47 39 80
Storage period, days SOB loat, '% Storage period, days SO* lost, yo
Green et al. (1946) noted better retention of SO, in storage in potatoes given alkaline dips than in those dipped in acid solutions prior to drying.
Very little information on the nature of the inhibiting action of sulfite can be gained from the work on potatoes, Mackinney (1945a), speaking of vegetables in general, stated that sulfite effects an inhibiting rather than a masking action in the prevention of darkening. It is apparent from the data given by different authors that the formation of the brown color and sulfite loss occur simultaneously, that browning is not completely inhib- ited by the presence of sulfite, and that appreciable amounts of sulfite may be present in the product after it has become unacceptable. The present author (unpublished data) found a fairly good correlation between rate of sulfite loss and rate of color development at a given temperature. Esti- mates of the proportion of the initial sulfite lost a t the point where samples became unacceptable were all very close to 60% in 48.9%. (120F.) storage and to 70% in 37.8OC. (100'F.) storage. This held true regardless of reducing sugar content. This does not necessarily indicate a stoichio- metric relationship between the two. It may be that both are affected independently by the same factors at approximately the same degree.
Burton (1945b) pointed out that the substances known to inhibit the browning reaction are reducing agents as well as compounds capable of reacting with aldehydic or ketonic groups, hence it is possible that this might be the property (of SO2) effective in preventing browning.
V. CONTROL OF BROWNING
1. Methods of SulJiting
There is considerable variation in the methods used for sulfiting. In England, where hot water blanching is practiced, sulfite salts are usually
280 A. FFlANH ROSS
added to the blanching water. In the United States, most plants blanch the potatoes with steam. For tray blanching, Mackinney (1945) recom- mends application of sulfite in the form of a spray. Subject only to the stipulation that the tray should emerge without dripping, Mackinney found that the later the spray is applied in the blancher, the more efficient will be the penetration and absorption of sulfite. Where potatoes are blanched on continuous belts, a similar procedure is satisfactory provided the belt is made of stainless steel. If corrosion of the belt is a factor, Mackinney (1945b) recommends the application of the sulfite solution as a drip from a perforated pipe while the potatoes are on the vibrating screen (Syntron).
The technological problems involved in obtaining the desired concentra- tion of SO2 in the dry product have been discussed by Mackinney (1945b), Beavens and Bourne (1945), Wager et al. (1945), and by Green et al. (1946). Blanched potatoes absorb SO2 much more readily than raw ones whereas the pH of the sulfite solution has little or no effect on rate of absorption (Green et al., 1946). Approximately neutral sulfite solutions generally are recommended, and for a given plant, the desired SOa content in the dried product is obtained by proper adjustment of the concentration of the sulfite solution. Sulfite solutions apparently do not cause serious corrosion if trays or belts are made of stainless steel, galvanized iron or iron with Bakelite-type finish (Mackinney and Howard, 1944; Western Regional Research Laboratory, 1945).
8. Attainment of Low Moisture It is agreed generally that moisture levels less than 6% cannot be ob-
tained by the usual dehydration methods without damage to the product. Howard (1945a,b,c) has developed what appears to be a satisfactory method for reducing the moisture contents of potatoes well below 6% moisture. This method, termed in-can desiccation, involves placing calcium oxide (in a porous package) in the container at the time of packag- ing. This entails a sacrifice of about 14% of the-container space. This worker clearly demonstrated that this method greatly extends the storage life of dehydrated potatoes. Diced potatoes containing 300 p.p.m. SO2 packed in nitrogen together with CaO were of good quality after 80 days at 50C. (120F.) while a similar sample packed without CaO was unac- ceptable in about 25 days. Nonsulfited potatoes lasted 6 times as long when CaO-packed as did those containing 6% moisture.
It was found, however, that the effectiveness of in-can desiccation depends in part upon the reducing sugar content of the sample. When the latter was quite high (10.7%) storage life a t 37.8C. (100F.) was a little more than doubled, at a medium sugar level (3.5%) storage life was increased 3 to 4 times, and
These data have been substantiated by the present author.
DETERIORATION OF PROCESSED POTATOES 28 1
at a low sugar level (0.8%) storage life was increased 6 times or more. The high sugar samples dried at a slower rate than the others. The storage life of suliited samples (2.3 to 3.6% reducing sugars, 216 to 504 p.p.m. SO2) waa increased about 5 fold by in-can desiccation. Potatoes subjected to in-can desiccation and then held a t a moderate temperature for 3 months before storing at 37.8"C. (100F.) were somewhat more stable although the storage life was not always prolonged by following this procedure. During storage at moderate temperature the moisture content dropped to slightly less than half of the original value during the first 42 days. An additional 42-day period caused the moisture content to drop by about 0.6%. Dry- ing was much faster and more complete at higher temperatures.
Certain dehydrated vegetables that are easily compressed may be dried to low moisture levels by high frequency radiation, preferably at reduced pressures (Sherman, 1944; Rushton et al., 1945). No satisfactory process, however, for compressing the usual forms of dehydrated potatoes has been developed (Proctor and Sluder, 1943; Magoon and associates, 1946). It has been found that dehydrated potatoes are too brittle and that breakage releases free starch, causing gumminess in the reconstituted product. Whether or not mashed potato powder or other powdered or granulated potato products can be satisfactorily compressed is yet to be demonstrated. Drying of this type of product to low moisture levels by the usual methods has not been fully investigated.
9. Use of Potatoes Low in Reducing Sugars a. Availability. Few data have been published on the reducing sugar
content of fresh potatoes at the time of removal from common storage. In Maine, appreciable quantities of potatoes reasonably low in reducing sugars were found during the first half of the winter (Ross, unpublished data). Considerable variation was found in the Pacific Northwest by Bedford and Lusk (1946) but their results indicate that it is possible and practical to obtain potatoes fairly low in reducing sugars during the winter. In Idaho, the average sugar content for potatoes from 15 cellars had definitely increased by November and continued to increase through March (Stamberg and McKinnon, 1946). Large quantities of low sugar potatoes should be available for drying during the winter and spring, provided they can be conditioned for 2-3 weeks at warm temperature. On the other hand, it is a common experience to find occasional lots of potatoes ex- tremely high in sugar content. The present writer has analyzed commer- cially dehydrated samples containing over 12% reducing sugars. More southernly areas are more fortunate in this respect. Southern grown potatoes are, however, as a rule, lower in specific gravity than northern sown ones aud hence are less suitable for dehydration. Potatoes of low
282 A. FRANK ROBS
specific gra i ty give high dehydration ratios and may result in a product of inferior quality (Caldwell et al., 1943a,b).
b. Factors Influencing the Reducing Sugar Content of Potatoes. The most important factors affecting the reducing sugar content of potatoes are storage temperature and variety. That potatoes in cold storage ac- cumulate sugars has been known for many years (Miiller-Thurgau, 1882); numerous papers have been published on the effect of different tempera- tures on the rate and extent of sugar accumulation (Appleman, 1912; Wright, 1932; Wright et al., 1936; Denny and Thornton, 1941b, 1942a; Ross et al., 1946; Campbell and Kilpatrick, 1945). In general, it has been found that both sucrose and reducing sugars accumulate a t low tempera- tures and that the lower the temperature, the faster is the rate of accumu- lation and the greater is the amount accumulated. This, however, is not always the case since with at least one variety it has been found that re- ducing sugars are formed at a faster rate a t 4C. (40F.) than at 0C. (32F.) (Campbell and Kilpatrick, 1945). Reducing sugars and sucrose, generally, but not always, show parallel changes.
Varieties differ greatly in the rapidity and extent of the accumulation of reducing sugars a t low temperature (Denny and Thornton, 1941b, 1942a; Ross et al., 1946). Fertilizer practices apparently have little or no effect on sugar-forming characteristics of potatoes (Denny and Thornton, 1941b; Ross, unpublished data). Some investigators report that tubers of the same variety grown in different locations show no important differences in the amounts of reducing sugars developed in cold storage (Denny and Thornton, 1941b) while others report significant differences (Wiegand et al., 1946). Potatoes held in an atmosphere containing 4.9% carbon dioxide while in cold storage form reducing sugars at a lower rate than when stored in air (Denny and Thornton, 1941a, 1942b, 1943b). Pre- storage of tubers a t warmer temperatures before they are placed in cold storage causes a small reduction in the rate of reducing sugar formation during subsequent low temperature storage (Barker, 1939; Denny and Thornton, 1943a).
The temperature range at which there is little or no change in reducing sugar content has been variously placed between 4.4" and 10C. (40" and 50F.), (Wright et al., 1936; Butler, 1919; Barker, 1938). Denny and Thornton (1942a) found a variation from 7"-8"C. (44.6"46.4"F.) to be critical for reducing sugars as well as for sprouting. Reducing sugar values were about twice as high at 7C. (44.6"F.) as at 8C. (46.4"F.) and at 7C. (44.6"F.) none of the varieties tested showed an excessive amount of Bprouting in about 7 months.
The reducing sugar content of potatoes from low temperature storage can be lowered by subsequent storage at higher temperatures (Wright,
DETERIORATION OF PROCESSED POTATOES 283
1932; Wright et al., 1936; Denny and Thornton, 1941a; 1942b; Ross et al., 1946). The rate of loss is much faster at 21.1C. (70F.) than at 15.5C. (60F.) but only slightly more rapid at 26.7C. (80F.) than at 21.1C. (70F.) (Ross, unpublished data). The rate at which reducing sugars dis- appear under such conditions depends upon the variety and upon the length and temperature of previous storage (Denny and Thornton, 1943a; Ross, unpublished data). Excessive sprouting may cause an increase in reducing sugars (Stamberg and McKinnon, 1946). Data from a number of potato producing areas show that conditioning in a warm room can be depended upon to appreciably lower the reducing sugar content of a11 varieties (Bedford and Lusk, 1946; Wiegand et al., 1946; Stamberg and McKinnon, 1946). In a test conducted on a pilot-plant scale the present author demonstrated that conditioning of the raw material for 2% weeks at 21.1OC. (70F.) increased the storage life of the dry product from 2% to 3 times.
c. General Considerations. The use of potatoes low in reducing sugars has advantages other than those cited. Wright et al. (1936) have shown that the culinary quality of potatoes in general is inversely correlated with their sugar content. They found that as sugar accumulates, the texture becomes soggy or watery and the flavor unpleasantly sweet. Later it was demonstrated that dehydrated potatoes prepared from potatoes of high sugar content are also of poor culinary quality, for in addition to being dis- colored, they are soggy and sweet (Wright et al., 1945). Frequently an un- desirable yellow color develops in potatoes held in cold storage (Caldwell et al., 1943a, 1943b, 1945). Finally losses during the processing of potatoes high in soluble sugars may be high because of the removal of soluble ma- terials by leaching.
Conditioning of raw stock at warm temperatures is a common practice among manufacturers of potato chips. It wa8 not possible, however, for dehydrators to rely on this procedure alone during the war years because of large volume of production, In a set of recommendations drawn up for dehydrators, Ross et al. (1945) pointed out the advantages of obtaining desirable potatoes by selective buying as well as the use of warm storage conditioning when necessary. Rapid sugar tests have been developed (Peacock and Brunstetter, 1931; Ross et al., 1946) that make possible the procurement of potatoes on the basis of their reducing sugar content. In addition, the results of Denny and Thornton (1940) suggest the possibility of using a chipping test to estimate the reducing sugar content of potatoes. This chipping test involves preparing potato chips under standardized conditions and comparing their color with similar ones pre- pared from potatoes of known reducing sugar content.
Sprouting is the chief difficulty encountered in storing potatoes so that
284 A. FRANK ROSS
reducing sugars do not accumulate, during conditioning. While it is pos- sible to avoid sprouting by proper choice of time and temperatures, the use of sprout inhibitors is also promising. The methyl ester of a-naph- thaleneacetic acid appears to be the most useful and satisfactory (Guthrie, 1939; Denny, 1942; Denny et al., 1942). These authors have shown that potatoes treated with this chemical can be stored from autumn to spring at temperatures ranging from 10"-22"C. (50-71.6"F.) without appreciable sprouting. The most suitable storage temperature (for treated tubers) to avoid shrinkage and accumulation of reducing sugars is in the range 1Oo-15"C. (15O-59"F.). The sugar forming characteristics of the potatoea are not altered by the treatment with the sprout-inhibiting chemical.
More recently Denny (1945) has found that lots of potatoes treated with sufficient chemical to prevent sprouting when stored for 5 months at 12.5"C. (54.5"F.) will remain firm and contain only traces of reducing sugars after storage, and when made into potato chips the product has a good color. Potatoes treated with the methyl ester of a-naphthalene- acetic acid do not appear to be toxic. Finch and Harteell (1945) fed mice with treated potatoes and with diets containing up to 90 times the amount of the chemical in treated tubers. No harmful effects attributable to the chemical were found.
4. Combined Treatments The method used to prevent browning depends upon the degree of
stability that is required. From a standpoint of military requirements, it generally is considered that a product should have a storage life of 6 months at 37.6"C. (100F.) (Wodicka, 1945) and needless to say, a longer storage life would be even more desirable. This degree of stability may not be essential for the civilian market. According to unpublished data of the present author, if dehydrated potatoes are to have a storage life of 6 months at 37.6"C. (100F.) the reducing sugar content cannot be ignored, even when sulfiting and in-can desiccation are utilized. It was found that for a sample of dried potatoes containing 7% moisture to be able to withstand storage for 6 months at 37.6"C. (lOOF.), the reducing sugar content should not be over 1.2% if unsulfited or over 2% if sulfited. For similar storage requirements, dried potatoes containing 6% moisture must not contain over 1.6% of reducing sugars if unsulfited or 2.7% if sulfited. CaO-packed (in-can desiccation), unsulfited samples containing over 3.5% reducing sugars will not tolerate 6 months of storage at 37.6"C. (100F.). Samples receiving both sdfiting and in-can desiccation are not considered suffi- ciently stable if containing as much as 6% of reducing sugars. In aotual practice, the storage life might be greater because dried potatoes would rarely if ever be removed from the dehydrator, packed, and then subjected
DETERIORATION OF PROCESSED POTATOES 285
to continuous storage at 37.6"C. (100F.). If the combined treatment is used, it would then be necessary to avoid only those potatoes which are excessively high in reducing sugars.
Gas packing does not appear to be justifiable for the purpose of retard- ing discoloration. Additional data of the effects of this treatment on flavor at low moisture levels are needed.
Browning is frequently encountered in most types of processed potatoes. It may occur during processing or during subsequent storage. The avail- able data indicate that browning of dehydrated potatoes is due to a reac- tion between reducing sugars and amino acids. It is most severe with potatoes of high reducing sugar content and is very slight if the reducing sugar content is low. The rate of formation of the brown color is directly proportional to the reducing sugar content. Thus the storage life of a sample containing 2% of reducing sugars will be twice that of one contain- ing 4%, provided other conditions are the same.
The rate of discoloration also is affected by moisture content and temperature. The time required for a given color development varies exponentially with the absolute temperature if the moisture content is constant, or with the moisture content at constant temperature. Thus storage life of the product is doubled by a 3.4"C. (62F.) drop in tempera- ture or by a 2% (absolute) drop in moisture.
Sulfiting inhibits the browning reaction in dehydrated potatoes during processing and also during storage. Sulfite at concentrations ranging from 200-500 p.p.m. results in a 50-100010 increase in the storage life of dried potatoes relatively high in reducing sugars. In cases where the reduc- ing sugar contents are lower, much greater increases in storage life can be expected. Packaging in inert atmospheres has little or no effect on the rate of browning. The effects of sulfiting, of the use of potatoes with low reducing sugar contents, and of attainment of low moisture contents are additive in the sense that a combination of any two methods is more effec- tive than either alone and a triple combination is the most effective.
Low moisture levels can be reached satisfactorily by packaging the product together with a porous container containing calcium oxide (termed in-can desiccation). The reducing sugar content of potatoes is affected principally by storage temperature and by variety. Raw stocks suitable for processing can be obtained by selective buying and by conditioning lots with high reducing sugar content at warm temperature.
Dehydrated potatoes occasionally turn gray during low temperature storage. This type of discoloration is inhibited by sulfite. Mashed potato powder sometimes develops an "off" flavor other than that asso-
286 A. FRANK Roaa
ciafed with browning. The reaction is retarded by high moisture con- tent.
Potatoes high in reducing sugars generally give products with inferior texture and color. If canned, such potatoes may develop a pinkish-amber color in their centers and be of poor flavor. The browning of potato chips and of French fried potatoes is correlated with the sugar content of the tubers used for their manufacture.
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DETERIORATION OF PROCESSED POTATOES 287
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288 A. FRANK ROSS
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