Microorganisms as Affecting Froze C. A. MAGOON, Bureau of Plant Industry, Department of Agriculture,
T
HE relation
kept for long p e f f r d s u n d e r of microorThree main groups of microorganisms are cold s t o r a g e conditions, have ganisms to our present responsible for spoilage of frozen foods-molds, reasoned that the causal agents frozen food problems is yeasts, and bacteria. The old belief that freezing of food spoilage may be entirely made clear by the consideration destroys these causal agents of decay must be inhibited if not actually deof a few J u n d a m e n t a l facts. renised, for, while the majority of a microbial stroyed by the use of temperaI n the first place, the detures well below the freezing composition of organic matter population is usually killed by freezing temperapoint. The working out of this through the iduence of microtures in food products, there is ordinarily a idea has met with a measurable scopic l i v i n g o r g a n i s m s has small proportion of survivors whose vitality is degree of success; there remain, been going on in nature for ununaffected by subzero temperatures. Some have however, very important probtold millions of years. During been found to resist temperatures as low as lems to be solved, not the least of this time there has developed which arise from the fact that under widely varying environ-252" C. for as long as 10 hours with no chemical reactions will proceed, m e n t a l conditions a m y r i a d apparent harmful effect. Some are not only that some of the enzymes at least of m i c r o f o r m s (representing unharmed by cold but are even able to increase will continue their catalytic acmany families, genera, and in numbers at temperatures well below 0" C. tivity, and that a t least some of species) that by reason of their The changes which are brought about in frozenthe microorganisms will continue enzymic equipment and their to function in spite of low temability to grow and thrive under pack foods diJer with [he nature of the substance peratures. greatly diverse conditions are and with the particular organism concerned. Microbiological researches are able, in the aggregate, to utilize They are both physical and chemical in nature. f a c i l i t a t e d and an u n d e r all manner of organic material Alterations in texture, color, flavor, and healths t a n d i n g of the n a t u r e and a s f o o d , a n d i n s o d o i n g to fulness of the foods resull from these microbial m a g n i t u d e of the problem is bring about its destruction. I n made clearer by the work of other everyday life these serve priactivities. i n v e s t iga t 0 1 s . more than two marilv as the scavengers of the orgar& world and are, therefore, widely distributed. The hundred in number, who during the last sixty-odd years have spoilage of human food by microorganisms is but a minor contributed to a voluminous literature on the effect of low temperature on microorganisms. Time forbids more than phase of this universaliphenomenon of decomposition. passing mention of some of their more outstanding findings. The materials studied included natural and artificial ices, SPECIAL PROBLEMS OF FROZEN FOODS snow, hail, water from glacial streams, and from Arctic and The present discussion will be concerned principally with Antarctic seas, frozen soils, and decomposing animal and frozen foods of fruit and vegetable origin. The specific vegetable matter, frozen meats and fish, frozen vegetables problems encountered in this work arise from the fact that: and fruits, sherbets, ice creams, frozen milk, and other dairy (1) many of the fruits and vegetables grow close to or in products, as well as various other frozen substances, in all of the soil from which they receive their initial contamination; which living microorganisms were found in considerable (2) on the way from the field to the packing plant they are numbers with their vitality apparently unaffected by the subjected to further contamination from dust, containers, and freezing treatment. Of particular significance were the other sources; (3) some of them, particularly the small findings of some of these workers from tests made on the fruits, are extremely delicate in structure and, therefore, are direct killing effects of various low temperatures. The readily damaged in handling; (4) means have not yet been temperatures ranged to as low as -252' C., and it was found found to rid the raw materials entirely of microorganisms; that some of the dangerous pathogens survived this treatment and (5) freezing, like cooking, brings about certain physical without apparent injury, even though the exposure was changes in the tissues that permit and favor microbial maintained for many hours, in the case of the lowest temperadevelopment. tures, and for months with temperatures still well below 0" C. There are three main groups of microforms that give rise to Repeated freezings and thawings up to forty times in rapid trouble: the molds, that as a group are able to utilize seemingly succession failed to destroy completely the germs of typhoid almost anything of organic constitution as food; the yeasts, and cholera. and their close relat,ives, that attack particularly those subExposure to freezing temperatures was found to result in stances rich in sugars, and therefore are of especial importance heavy mortality during the earlier period of the treatment, in the fermenting of fruits and fruit products; and finally the 90 per cent or more being killed during the first 24 hours; bacteria, that as a group feed upon a wide variety of sub- some species were completely destroyed within a few days. stances, particularly those having relatively low acidity. Usually, however, tens, hundreds, and in some instances The speed of chemical reactions is known to be a function of several thousand per gram of the frozen material were found the temperature, and, since the changes which these micro- alive a t the termination of the tests. It was noted that in organisms bring about are first of all chemical in nature, it some cases there was an apparent increase in the numbers of seems logical to look to the application of low temperatures to organisms during the freezing treatment. Of particular our food products as an additional means of holding their interest is the report of one worker that very low temperaactivities in check. tures-that is, -190' and -252" C.-were found to be no Practical men, knowing that higher forms of life are more effective in killing germs than temperatures only a few destroyed by cold, and observing that foodstuffs may be degrees below zero. 669
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I N P U S T R I A L A N D E N GI i\: E E R I N G C H E M I S T R Y as..*>h F
RAWMATERIALS
ory with frozen fruits and e results of these earlier investihe various parts of thwest-representpacks. Growers, as well as others engaged in frozen-pack research, have cooperated. Studies have begun with the raw material in the field; this has been followed through the packing plant into the storage warehouse; and the effect of the various packing operations and storage treatments on its micro-content has been studied. Much valuable information has been assembled, but it should be noted that merely a fair beginning on the problem has been made. It has been found, as was to be expected, that the raw materials in the field have a high microbial content; that the harvesting and other handling operations, causing more or less damage to the delicate tissues, predispose them to invasion by molds, yeasts, and bacteria; that delays in transit from field to packing plant and a t the packing plant itself, particularly under prevailing high temperatures, result in microbial increase and deterioration of the material; and that the grading, washing, and inspection, while removing most of the undesirable matter, does not always remove all of it, so that the raw product going into the barrel, can, or other package commonly has a very considerable microbial population. This does not mean that such materials are necessarily unfit for food. The microbe content of fresh fruits and vegetables is normally relatively high, certain conditions favor the increase of the organisms, and under practical working conditions sterility of raw products cannot yet be expected. Quantitative determinations by this Bureau on small fruit in various sections of the country have shown widely varying counts in microorganisms ranging from a few thousand to several million per gram of material. As a rule, relatively firm and sound ripe fruits have shown the smallest number of organisms. High counts, however, running well into hundreds of thousands per gram of material have been obtained from fruit (strawberries) judged to be in good market condition. Counts ranging from fifty thousand to several hundred thousand per gram are the usual findings.
PREPARATION OF FOOD FOR FREEZING Washing experiments with strawberries have indicated a possible reduction of a t least one-half of these numbers, but careful control of the washing operations must be maintained, and an abundance of pure water and proper application of it to the fruit provided. Otherwise, the washing process becomes a means of gross pollution of the material rather than of cleansing. I n the preparation of vegetables for freezing, it has been found desirable in order to check later enzyme action in storage and to make possible a more uniform pack, to blanch or scald the material just prior to packing. This has had the effect not only of helping to prevent chemical changes, but has shown the added advantage of destroying many of the microorganisms present on the fresh material. Blanching, however, cannot be relied on to sterilize raw vegetables. I n the preparation of experimental packs, use has been made of different types of containers, providing for both vacuum and nonvacuum conditions; various types of packs have been employed (in the case of fruits, different proportions of sugar, different strengths of sirup, water packs, dry packs, etc., have been used, and with the vegetables various strengths of brines, as well as water packs and dry packs have been tried). Different freezing and storage temperatures have been employed, and the conditions have
Vol. 24, No. 6
been varied in other ways so as to give as broad a reading as possible on the effect of these different factors on the micro flora and keeping quality of the stored material. It has not been possible yet to evaluate the influence of all these factors, but the data now in hand are highly significant and the work is being pursued as rapidly as possible. One fact of particular importance is that, as in the findings of the earlier workers on the effect of low temperatures, freezing of fruit and vegetable products, and storage in the frozen condition for many months (and even for as long as two years) have not rendered the products free of living microorganisms. In all cases there has been found a very marked $duction in the numbers, amounting to from 90 to 99 per cent or over, but always there have been found some survivors-enough to initiate again the destructive processes on the return of favorable conditions. That is to say, products going into frozen storage with hundreds of thousands of microorganisms per gram have been found to contain usually several hundred living individuals per gram when removed from frozen storage 12 to 15 months later. I n some cases the counts have been considerably higher than this, and in such instances the predominating organisms have usually been found to be yeasts. While not all species of yeasts are markedly resistant to cold, some of them are particularly so, and some of them appear able actually to increase in numbers under these conditions. Strains have been encountered which after two years of frozen storage were capable of making very rapid growth a t temperatures between 0" and 4" C., and which caused the bursting of hermetically sealed cans of fruit a t this temperature. Many molds have been found to make marked growth on fruits apparently solidly frozen, and have carried on their life functions even to fruiting on foods stored a t temperatures well below 0" C . This, of course, has been commonly observed with other frozen foods. One important finding of these studies has been that packing under a relatively high degree of vacuum not only prevents the development of molds during the storage treatment but so weakens them that the thawed products often may be held a t ordinary room temperatures for many days without any appearance of mold development. SIGNIFICANCE OF SURVIVING ORGANISMS IN THAWED
FOODS It may be asked, what is the significance of the surviving organisms in the thawed foods? From the standpoint of the chemist as a scientist the answer must be sought from the chemist himself. The conditions are as follows: I n these food products we have a wide range of complex organic compounds, including carbohydrates, proteins, and fats, while in the microorganisms we have an amazing gamut of enzymes capable of bringing about an unknown number of chemical reactions with the production of a tremendous variety of organic compounds. Some of the products are, of course, well known. The field for research, however, is confitantly widening and will furnish interesting problems to the organic chemist for years to come. From the standpoint of the food manufacturer, the housewife, and the ultimate consumer, the answer is that the appearance, texture, and flavor of the food is injured by them. I n the fruits this means in some cases the loss of natural color, in others discoloration, the loss of natural flavor or the development of abnormal and undesirable flavors, and the deterioration in texture resulting in mushiness. I n the case of vegetable products, discoloration, sliminess, souring, disintegration of the tissues, and putrefaction are common effects. The question is often raised as to the public health aspects of the microbiology of frozen foods. The question is one that
June, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
cannot be answered completely a t the present time, although researches along this line are being pursued vigorously in several laboratories. The findings of the earlier workers would lead us to expect that some of the pathogenic bacteria if present in the raw material might possibly survive the freezing storage treatment and give rise to trouble. With the care that is being exercised by the packers of frozen fruits and vegetables in their use of sound raw materials carefully sorted and cleansed, the likelihood of gross contamination with pathogenic organisms seems small, and the use of freezing and storage temperatures well below 0" C. would tend to reduce their numbers if present. The development of possible survivors in the thawed foods, however, must not be overlooked, and the foods must be kept frozen continuously until needed for immediate use. I n this connection the question of the possibility of botulinus poisoning from the eating of frozen foods has been raised. Research in several laboratories has been going on for some time to investigate the matter thoroughly. The work to date indicates that the Clostridium botulinum (the organism responsible for the development of the botulinus poison) does not grow a t temperatures below 31 O F . ( -0.6" C.) In artificially inoculated materials, however, living spores of the organisms have survived the frozen storage, and it would appear that, given suitable conditions for growth, these might develop, Dfierent investigators who have reported on the detection of toxin in thawed vegetable products, after holding for a number of days a t ordinary room temperatures, are not
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in agreement. This may be due to differences in methods of investigation. However, the information now in hand indicates that sound fresh fruits and vegetables that have been prepared, packed, and frozen in a sanitary manner and kept in the frozen condition until immediately before consumption are as safe as other raw foods; but, if they are allowed to remain thawed for an extended period a t room temperature, the duration of which has not yet been definitely determined because of the rapidity with which bacteria grow in such material, they may become unsafe. These findings clarify to a considerable extent the problem before us and indicate the direction which future research should take. The field for research is broad and the need for work is urgent. Frozen foods in considerable variety have probably come to stay, and it is our duty to see to it that these additions to our national diet shall be as pure and wholesome and safe as possible. ACKNOWLEDGMENT The author wishes to give due credit to his associates, Helen F. Smart of the Washington laboratories, and J. A. Berry of the Seattle laboratories of the Bureau of Plant Industry, for their part in the investigations. He also desires to acknowledge with appreciation the effective cooperation of H. C. Diehl and his associates of the same Bureau. RECEIVED iipril 7, 1932.
Preservation Problems in the Dairy Industry JOHNH. NAIR,The Borden Company Research Laboratories, Syracuse, N. Y.
A
CCORDING to estimates of the Foodstuffs Division of the Department of Commerce ( 2 ) , dairy products constitute 47 per cent by weight of the annual consumption of foodstuffs by the American people. The number of cows required to produce the milk, the raw material, was approximately twenty-two million in 1930. These cows produced a total of sixty million tons of milk in that year (the last period for which figures are available), which had an estimated farm value of about two and a quarter billion dollars. Approximately one-half of this milk is consumed in the household in the form of fluid milk or cream, whereas the other half is manufactured into butter, cheese, ice cream, and other dairy products. The ultimate retail value may be arrived at from estimates, one-fifth of the per capita expenditure for food, calculated as $178 annually, going to pay for dairy foods. On this basis over four billion dollars annually are paid by the American people to this industry. These figures make it apparent that dairying constitutes the largest single industry in the United States by a wide margin. Biochemical studies during the past 25 years ( 6 ) have determined the nutritive value of milk for animal and man and have established its importance as the most essential single article of food. As secreted by the cow, milk is the most complete natural food known, and all efforts in the dairy industry are directed toward maintaining this value. Bacterial and chemical changes occur readily, whereas physical changes are relatively unimportant. At the present time efforts to improve the vitamin value of milk through improved feeding of the cow are attracting much attention. During the past year Hess and his coworkers (4) have reported success in increasing vitamin D in fluid milk through the feeding of irradiated yeast to the cow.
FLUIDMILK This present paper is concerned, however, with the preservation of dairy products rather than the improvement of the raw milk. The importance of milk products in the diet and the fact that milk is one of the most perishable of foods make the matter of preservation of dairy products a problem of vital importance to the entire population. The seasonal surpluses, which occur during the late spring and early summer, necessitate means of storing and preserving milk until periods of lesseped production, and offer a very real problem. The dairy industry is one whose origin is lost in the mists of antiquity. The art was firmly established long before any applications of science were made. This retarded the introduction of scientific methods and knowledge, but in the past half century great strides have been taken in the improvement of the keeping qualities both of milk and its manufactured products. Education of the dairy farmer to the necessity of bacterial cleanliness in his barns, milking equipment, and utensils, and of proper cooling methods, followed by constant supervision through dairy inspectors, now insures delivery to the modern milk-receiving station of a much higher quality raw product than was formerly available. This is particularly true in those territories constituting the milk sheds of our large urban centers. There still exist, however, large producing areas mainly supplying milk for manufactured products, in which the standards of farm production are much lower, resulting in a lower quality of the raw material available. The larger dairy companies are constantly striving to improve conditions in these territories. Many mechanical developments have contributed to make the modern fluid milk plant a model of sanitation. In some
