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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
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instances the entire system from the dump tank to the bottle way to determine the exact course of the phenomenon inis entirely enclosed, largely eliminating possible oxidative volved and methods of prevention. Although homogenizachanges in the milk. The effect of the materials used in tion of the milk largely retards a separation of the fat, there is equipment has been more accurately studied, bringing about a constant tendency for it to occur, a condition augmented by constant improvement. Copper and iron have been largely prolonged storage. This, together with deposition of lime replaced, since these metals promote oxidation of the butter salts, corrosion of tin plate by the action of lactic acid, and fat, giving rise to metallic, rancid, and bitter flavors. h'ickel, the development of metallic flavors frequently encountered chrome-nickel alloys, and enameled iron have proved very in the stored goods, gives rise to complaints. Although satisfactory, although the rapid corrosion and disintegration proper processing prevents bacterial changes, the sterilizaof nickel in surface coolers makes it unsuitable for this par- tion period causes the development of a characteristic cooked ticular equipment. Research is being undertaken in this or caramelized flavor in evaporated milk. This is a serious handicap in wider consumption of the product, and research direction in a number of laboratories. One of the most interesting scientific studies has been the has been done, and more is probably in progress, to eliminate tracing of the incidence to bovine sources of undulant fever this flavor and still maintain adequate bacterial preservation. Sweetened condensed milk, containing approximately carried to the human by raw milk. Fortunately, present pasteurization practice entirely destroys the causative or- 28 per cent milk solids and 45 per cent sucrose, is concentrated ganisms, and the fact that most of the fluid milk consumed by a process similar to that used for plain evaporated milk, in the United States is pasteurized largely eliminates danger the sugar being introduced in the condensing pan with the of this fever. Large producers of certified raw milk now fluid mi&. Unless properly cooled, with sufficient agitation maintain constant bacterial examination of the milk from and seeding with fine lactose crystals, sweetened condensed individual cows in order to exclude from their product the milk on storage develops a sandy or gritty condition. This is due to the growth of large lactose crystals in the lactosepathogenic bacteria causing this disease. Recent work has revealed that pasteurized milk frequently supersaturated aqueous phase. The application of the contains large numbers of minute thermophilic organisms, scientific method has developed manufacturing technic which appear to have little significance from the hygienic which prevents this defect. A progressive thickening viewpoint, although they produce lactic acid in the milk. accelerated at high storage temperatures is one of the major The growth of these types of bacteria during the pasteurizing problems in preserving this product. It is partially due to process is being prevented by improvement in methods of bacterial action and partially to certain physical changes. handling. Considerable research on short-time high-tempera- Sufficiently high sucrose concentrations prevent the former, ture methods of pasteurization is in progress, with the object whereas storage below 13" C. will check the latter. Soof shortening the process and decreasing the somewhat cooked called "buttons," which are curd-like lumps having a cheesy flavor in pasteurized milk as now produced. Of course, such stale flavor, result from mold growth. It is now known that changes in pasteuriaation practice must be as effective in bac- this change can be controlled by high sanitary standards, terial destruction as our present method in order to secure low temperatures of storage, and the exclusion of oxygen. the approval of state and local health authorities. The Rancidity of the butter fat and development of a dark brown increasing use of paper bottles, made up in the milk plant color on aging are problems not yet thoroughly solved. and coated with paraffin at a temperature of 88" C. just The wider adoption of methods of vacuum packing are exbefore filling, is an economic and hygienic development of pected to prove increasingly valuable in the preservation of sweetened condensed milk. importance in the fluid milk field. Dry milk is manufactured by a number of dehydrating processes and has become an important item in the dairy CONCENTRATED MILKS industry, production in 1930 being well over a quarter of a The existence of large seasonal surplus production, to- billion pounds. The character of the problems of preservagether with the expense of transporting over long distances a tion vary with the butter fat content of the dry milk. Powfood containing 87 per cent water, led to the devising of means dered skim milk must be protected from moisture, which of concentrating and storing milk nutrients for a period of causes a progressive insolubility of the protein constituents months. Centuries ago, the making of cheese or butter was and hastens the development of a stale flavor and odor. Cold found to be a good way to preserve and these two foodstuffs storage is of value, but, because of high humidity conditions still comprise the major portion of manufactured dairy existing in most cold storage warehouses, the changes cannot products. Within the past 75 years we have learned how to be prevented without humidity control. Oxidation with the preserve milk itself through the removal of part of the water development of tallowy flavors is a major problem with content, and now have three types of such products: plain powdered milk containing butter fat. In small containers evaporated, preserved by sterilization; sweetened condensed, this has been largely eliminated by vacuum packing in an preserved by high sucrose content; and dry milk, preserved atmosphere of inert gas, a process which is being widely apby the reduction of the moisture content below 5 per cent. plied in all branches of the food industry. I n barrel packEach of these products presents many problems in preserva- ages, however, all that has been accomplished to date has been tion, and the work of the chemist and engineer in relation to a retarding of the oxidation through cold storage. Research has shown that the presence of small amounts of copper and them is far from finished. Plain condensed milk is packed in tin and sterilized by hold- iron has a catalytic effect in accelerating the changes due to ing a t 115" to 117" C. for 20 minutes, thus insuring the oxidation. The elimination of these metals from the powdestruction of the bacterial spores. There occurs during dered milk plant has aided keeping quality. A pre-condensamanufacturing a desirable thickening which gives body to the tion and homogenization of milk before desiccation has likeproduct but, upon storage, evaporated milk becomes pro- wise been found to aid in retarding oxidation. Hydrolysis gressively thinner, the change being accelerated by increasing of the fat, giving rise to rancidity, is another defect sometemperatures. This necessitates building up a greater vis- times encountered with milk powders containing butter fat, cosity in the freshly processed milk than is desirable for carry- and is believed to be caused by the presence of fat-splitting ing conditions and is particularly essential in goods destined enzymes, such as lipases, but these can be largely inactivated for export to tropical climates. Cold storage, preferably at through proper heat treatment of the milk in the drying 4" C., retards this change, but considerable research is under process, When butter-fat milk powders are stored at low
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temperatures for long periods of time, they occasionally develop a characteristic fishiness, which is also encountered in butter. The chemical changes leading to the develop ment of this flavor in milk powder are probably identical with those which cause its appearance in butter, tnrnethylamine being formed from the choline set free by hydrolysis of lecithin. 13ETTER The manufacture of butter accounts for approximately ouequarter of all the milk produced in the United States. Methods of manufacture are well standardized today, arid there has been a general improvement in the q u a l i t y of b u t t e r a v a i l a b l e on t h e market. It has become recognized that the p r o d u c t i o n of high quality b u t t e r and its preservation are closely related to t h e q u a l i t y of the eream from which it is made. High-acid cream, which has been separated on tire farm and has acquired offflavors, o r become contaminated with a v a r i e t y of bacterial flora, produces butter of l i m i t e d k e e p i n g q u a l i t y and inferior flavor. For this reason, attention in the p a s t 15 y e a r s h a s been centered on the separation and ehurning of sweet cream, with a consequent greater uniformity of good flavor and better keeping qualities of the butter. Much is still to be learned concerning the problems of acidity and neutralization and their relation to the keeping quality of butt.er. Occasionally one encounters a. caramel or burnt flavor in butter, which the work of Hammer and Cordes (3) has shown is due to a particular organism present. No corrective measures can be applied a t the time of churning. The majority of storage stocks of butter are manufactured during May, June, and early July, and are seldom held more than 9 months. Many experiments have been carried on to determine the best storage conditions for the preservation of butter. Recommendations of the U. S. Department of Agriculture ( 1 ) are that butter should be stored a t -17" C. or below. Eewlander and Ellenberger (6) have recently reported that storage temperatures above - 18' C . cannot be successfully used. The objectionable flavors which develop in butter may be classified into those caused by chemical deterioration and those due to bacterial growth. The former seem to be more prevalent in salted butter made from high-acid cream, whereaq bacterial changes take place more markedly in unsalted butter, particularly when made from sweet cream. Chemical changes are the result of oxidation or hydrolysis, or a comhination of both. These give rise to the appearance of such offflavors as tallowy, fishy, rancid, metallic, or storage. It has been shown by Sommer (7) that fishy flavor is developed through the action of acid upon lecithin, which is a normal constituent of milk, and that this reaction is greatly ac-
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celerated by the presence of salt and traces of iron and copper. Storage and metallic flavors are also the result of chemical action and are not prevented by st.orage at low temperatures or by careful pasteurization of the cream. Rancidity is frequently caused by molds, such as Oidium lactis, which seem to grow better under conditions of high acidity. Wrapping materials, such as parchment paper, carelessly stored, often serve as carriers of mold infection. Bacterial deterioration gives rise to stale, clreesg, and rancid flavors, but does not usually occur in salted butter when properly made, since the presence of the salt has an inhibitive effect on bacterial growth. Unsalted butter, particularly that made from sweet cream, deteriorates quite r a p i d l y from bacterial act.ion. The nice balance of acidity and salt necessary to secure a butter which will a p peal in flavor, and which at the s a m e time stands up well under c o l d - s t o r a g e warehousing, still presents major problems to the industry.
ICECREAM Ice c r e a m i s a distinctively American product, a n d the consumption in the U n i t e d S t a t e s has increased to a p proximately two hundred fifty million gallons a n n u a l l y . Scientific research in this braiich of the dairy industry has been more concerned with improvement of production methods and the development of types of ice cream which will appeal to the consumer than with problems of preservation. Being a frozen food, which under proper handling conditions is kept at quite low temperatures and marketed usually within a short period after manufacture, ice cream itself does not offer very serious problenls in preservation. Sueh off-flavors as develop in ice cream between its manufacture and consumption me usually traceable to the poor quality of the dairy products which have entered into its composition.
CHEESE Although the art of making cheese is an ancient one, the methods used still are largely rule of thumb and comparatively little is understood of the course OS proteolytic action during storage. Freshly made or green cheese has very little appeal to the average person. A definite period of ripening or storage must he used to give play to bacterial growth which develops the flavors characteristic of different types of cheese. I n recent years, it has been established that temperatures of storage of 2" to 4" C . permit slower development of proteolysis and result in a much better flavored product. A recent d e velopment in the field is the packaging of processed cheese, in which cheeses of various stages of ripeness are blended and pasteurized to give a uniform keeping quality and flavor. Phosphates, tartrates, and citrates are incorporated into the mass, giving i t plasticity and preventing crumbling. Such
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processed cheese frequently becomes discolored on the surface during storage, a defect which has been traced to the foil used in wrapping the loaves.
FROZEN CREAM One other dairy product which offers a number of problems in preservation is frozen cream. The separation of cream of 40 to 50 per cent butter fat, followed by pasteurization and freezing, is becoming increasingly important as a means of carrying surplus butter fat over from the early summer to the midwinter season. This frozen cream is used mainly in the preparation of ice cream mixes in which homogenization is employed for re-dispersion of the butter fat. Rancidity, tallowiness, fishiness, and metallic and storage flavors are frequently encountered in frozen cream. These arise from the same chemical and bacteriological changes which have been described as characteristic of other dairy products. Just recently the quick-freezing of cream has received attention because of the present prominence of the general subject of quick-freezing, although so far as known no such product is yet available on the market. Another recent development is the production of a cream containing 80 per cent butter fat, whose storage should present no greater difficulty than does that of butter.
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CONCLUSION It is hoped that sufficient emphasis has been given to the points cited to indicate that a wide field remains for the work of the chemist and bacteriologist Improvements in the condition of dairy products after long storage will go far toward leveling off the seasonal price changes which are characteristic of the industry, and may prove of large economic value in permitting longer storage of surpluses during seasons of low prices, such as have been experienced during the past year. LITERATURE CITED (1) Bur. Markets, Bull. 729, U. S. Dept. of Agriculture (1918). (2) Food Industries, 4, 2 (1932). (3) Hammer, B. W., and Cordes, W. A . , Iowa Agr. Expt. Sta., Bzcil. 68 (1918). (4) Hess, A. F., Lewis, J. M., MacLeod, F. L., and Thomas, B. H., J. Am. Med. Assoc., 96, 370 (1931). ( 5 ) Newlander, J. 8., and Euenberger, H. R., Vt. Agr. Expt. Sta., Bull. 299 (1920). (6) Rogers, L. A,, (Associates of) "Fundamentals of Dairy Science," Chap. XIV, Chemical Catalog, 1928. (7) Sommer, H. H., and Smit, B. J . , Wis. Agr. Expt. Sta., Research BUZZ. 57 (1923). RECEIVED April 7, 1932.
Factors in Commercial Cold Storage JAMESC. IRWIN,JR., United States Cold Storage Corporation, Chicago, Ill.
C
OLD storage, which occupies such a prominent position
in the field of food preservation, may be divided in two general classes in relation to operation and application of development. One is the cold storage operated by the packer in conjunction with his various operations, subject to a unified supervision which controls preparation, handling, storage, and distribution. For him cold storage is a process phase in his program of production and selling, and a regulator rather than a primary source of profit. The commercial cold storage warehouse, on the other hand, is dealing with numerous clients and handling products over which it has little or no control before they are received for storage. While the storage must maintain the material and return it in the best possible condition, its methods are still subject to the established ideas of the storer, who is in general slow to accept changes. The selling of cold storage service is highly competitive, and, in the establishment of fair rates, investment charges have a large effect so that the premium obtained from radical changes in construction or equipment frequently cannot be counted on to be reflected in earnings. I n addition, scheduling of commodities through storage introduces difficulties and limitations on control. Goods received either in car lots or in small lots for car-lot accumulation a t one period may be withdrawn a t any time either in car lots or a few packages a t a time from rooms containing, from necessity of economical operation, from twenty-five to fifty or more cars. In setting up ideal conditions for commercial cold storage, the weight of these factors must be taken into account. Cold storage facilities are of two distinct classes: cooler, covering temperatures above freezing point; and freezer, applying to temperatures below 25" F. In the cooler, no marked physical changes are produced except the delayed maturing of some fruits and vegetables, and the storage life is shorter. I n the freezer, the commodity, if no$ received frozen, is frozen or partly frozen as quickly as conditions
warrant and with sufficiently low temperature the storage life is practically unlimited. Much closer supervision is required during the storage period in the cooler, while for the freezer commodities, emphasis must be placed on processing before being received for storage, and on the freezing period. C O L D STOR.4GE
I N COOLERS
For the cooler, temperatures are pretty well established just a safe distance above the freezing point of the commodity.
I n some cases these are too low, and in other cases higher temperatures give equally good results; but, unless undesirable effects are produced, it is obviously the point which will result in the longest storage life. The tendency is toward somewhat lower temperatures than formerly. Celery, for example, is held a t 33" to 34" F. in many warehouses, but others have found that temperatures which may result in slight external breakdown or show a trace of frost in storage give, over long periods, a better marketable portion. Humidity in. the cooler ranks close to temperature in its effect on out-of-storage quality. I n general, it is advantageous to maintain the highest possible humidity without serious mold growth, but for certain commodities the humidity must be much lower to maintain texture, as in cereals or tobacco; or by those commodities which have a tendency to absorb moisture and which are damaged by moisture absorption, as candy and dried fruits. Humidity control in coolers is comparatively simple, as saturation is rarely required. Methods in satisfactory use range from recirculating air in contact with a brine of the desired vapor pressure through humidifying by vaporizing or atomizing or merely sprinkling the floors, and dehumidifying by passing the air in the room over calcium chloride. The determination of humidity is frequently the weak point in humidity control, as the sling psychrometer requires the greatest care and patience a t 32" F. or below. Other devices
