( Originally Published 1939 )
Several types of concentrated milks are in large commercial production.
Evaporated milk is cows' milk condensed in vacuo in the ratio of about 21/4 parts of fresh milk to 1 part of evaporated milk. This is the unsweetened, canned milk, which is usually packed in 141/2-ounce "tall cans," or, to a limited extent, in 6-ounce cans, and sterilized. It has the appearance and consistency of light cream. This product is marketed almost entirely for household consumption.
Plain condensed milk is the name given to a product made by concentrating cows' milk in vacuo in the ratio of about 3-4 parts of fresh milk to 1 part of the finished product. Its consistency is that of rich cream, although its content of butterfat may vary from about 12 percent down to 0, according to the degree of separation. It is marketed in ordinary 10-gallon milk cans. Inasmuch as it is not sterilized, it must be kept under refrigeration at temperatures just above freezing, and should be used within a week or 10 days after manufacture. "Plain condensed" or "skim condensed" milk is mostly used as an ingredient in the manufacture of ice cream, milk bread, and confectionery.
Condensed milk is the common designation of the sweetened condensed milk which is usually marketed in 8-or 14-ounce cans and, for commercial use, in barrels. This product is cows' milk condensed in the ratio of about 2 1/2 parts of fresh milk to 1 part of the finished product. It is not sterilized by heat, but it is preserved from spoilage by its high content of added sugar. It is used in household consumption and in the baking, ice-cream, and confection industries.
Powdered milk is made by drying whole or skimmed milks by several processes. It is packed in cans, usually 1 pound, for household use, or in large tins or barrels for commercial use in the baking and confectionery industries, in the manufacture of sausage, and in animal feed-mixing.
"Semi Solid" is the descriptive name given to condensed buttermilk from butter manufacture, concentrated in the ratio of about 3 1/2 parts of buttermilk to 1 part of the finished product. It is packed in barrels for animal feeding, especially poultry.
Powdered buttermilk is finding a widening use in the baking and confection trades, although the great bulk is used for animal feed.
A raw-milk supply must be of good quality in order to manufacture it into an acceptable grade of evaporated milk. Freshness alone is not the most important consideration. The cans of incoming milk must be examined for flavor, not only because the presence of an off-flavor might spoil a whole batch, but also because some types of off-flavor indicate that the milk when evaporated would not withstand the sterilization temperature without coagulating. The temperature of the milk as received at the plant may be noted to indicate the degree of proper cooling on the farm, and sediment tests may be run to ascertain the care and cleanliness exercised in production.
Several chemical tests are sometimes used to locate raw supplies which may cause plant difficulties. The acidity test depends on a titration with standard alkali to indicate whether the acidity is above 0.18 percent, the upper limit under average conditions. The alcohol test consists in a mixing of equal parts of milk with 75 percent alcohol; a coagulation of the milk usually indicates its unsuitableness for manufacture, and often points to some unsanitary condition on the farm. Laboratory heat coagulation tests have been devised to help identify unsuitable milk.
Inasmuch as competition is very keen in this industry and there is such a ready market for cream, the milk for manufacture into evaporated milk is usually standardized as to its fat content, although sometimes it is standardized after manufacture.
The milk is heated to near the boiling point in a forewarmer. This treatment contributes to the stabilization of the milk in the later operations and relieves the heating load on the evaporating pans. The condensing is then done under vacuum so that the low temperature of evaporation will not spoil the milk. These vacuum pans are usually made of copper (although new resistant alloys are now coming into use), heated with steam coils, and provided with peepholes and testing sticks, as they are called, for observing and sampling the contents of the pan. The condensing is completed when a test with the hydrometer shows that the proper degree of concentration has been attained.
In the manufacture of plain condensed (unsweetened) milk for bulk shipment there has been a demand that the finished product be given a special treatment with steam, called superheating, to impart a greatly increased "body" or viscosity. It was thought that this property of thickness improved the quality of the milk for making bread of proper texture and ice cream of proper body. It is now known that forewarning, or homogenization after condensing, will prevent or destroy this viscosity without impairing the desired qualities in the bread or ice cream.
The batch is then homogenized to distribute the butterfat evenly throughout the milk and to keep the fat from separating or oiling off during sterilization. The homogenizer is built to operate like a powerful pump which forces a liquid through a minute valve or opening of only about 1/10,000 inch. When any milk product containing butterfat is homogenized, the fat particles are broken down into very small droplets which remain dispersed in relatively stable condition. Pressures of some 2000 pounds per square inch are about the maxi-mum; otherwise the batch may coagulate during sterilization.
Milk is automatically canned by filling machines which deliver it under reduced pressure into the usual 141/2-ounce cans. The hole can, which is the most common type, has a small opening in the top for introducing the milk. The hole is closed with solder, and the can is tested for leaks by immersion in hot water, bubbles being given off if there are any leaks in the can.
The cans of evaporated milk are next sterilized in a pressure retort. Great care is required to prevent coagulation. The higher the temperature, the longer the time, the greater the concentration of milk solids, and the poorer the quality of the milk, the greater is the difficulty of withstanding heating. A small pilot sterilizer, holding a few cans, is commonly used to determine the proper time and temperature of sterilization for the batch in question, and also to indicate how much, if any, soda or disodium hydrogen phosphate must be added so that the batch can be sterilized without excessive coagulation.
In practice, a sterilization temperature of 240° F. for 15 minutes is about the maximum safe heating. A temperature of 245° may be held for a shorter time, or a lower temperature be applied for a longer time, usually by prolonging the temperature rise. Immediately after sterilizing, the cans are cooled to reduce discoloration of the milk to a minimum, and to prevent loss of smoothness. This cooling is effected by turning cold water into the sterilizer tank, sometimes under pressure to prevent bulging of the cans while they are hot.
For the whole procedure of manufacturing evaporated milk, together with full discussions of the many tests applied to the raw materials, the causes of defective packing, and the analytical methods used in the industry, see the treatises by Hunziker and Miyawaki, although they do not cover the latest developments. For discussion of vitamin D evaporated milk, see page 243.
RELATION TO THE PUBLIC HEALTH
Composition. Evaporated milk is defined by the U. S. Department of Agriculture as having the following composition:
Evaporated milk is the product resulting from the evaporation of a considerable portion of the water from milk, or from milk with adjustment, if necessary, of the ratio of fat to nonfat solids by the addition or by the abstraction of cream. It contains not less than 7.8 percent of milk fat, nor less than 25.5 percent of total milk solids; provided, however, that the sum of the percentages of milk fat and total milk solids be not less than 33.7.
Official standards contain no special sanitary requirements for evaporated milk, other than those generally comprehended in the definition of milk itself, as follows:
Milk is the whole, fresh, clean lacteal secretion obtained by the complete milking of one or more healthy cows, properly fed and kept, excluding that obtained within 15 days before and 5 days after calving, or such longer period as may be necessary to render the milk practically colostrum free.
Inasmuch as these conditions of healthfulness of cows, their proper feeding and keeping, and the sanitary handling of the milking operations cannot be determined from an analysis of the milk, official control of the evaporated-milk industry consists in enforcing only a standard of proximate composition. The Commission on Milk Standards recommended supervision as follows:
The Commission recognizes that in the manufacture of condensed milk, evaporated, powdered, and condensed milk products, the sanitary character of the raw milk used affects not only the keeping qualities but also the safety and decency of the finished product. It is clearly to the best interests of the public and the condensed milk industry that condensed milk should be so labeled that the product prepared from fluid milk of a good quality may be distinguished from that prepared from inferior milk.
The Commission therefore recommends the passage of Federal, State, and municipal legislation which will permit the manufacturer to state upon the label that his product has been prepared from Grade A milk, and he shall be protected in the use of such a label.
Digestibility. The boiling or the homogenization of milk greatly improves its digestibility. In the manufacture of evaporated milk, both these treatments are used. The curd of this milk that is formed in the stomach by the digestive juices is much finer than that of untreated raw milk. This property makes evaporated milk very easily digested and has led to numerous studies on its use in infant feeding.
Rice 4 and Tobey 5 have reviewed this clinical work at length. They report that Marriott, Brennemann, and several other pediatricians found that, when infants were fed on formulas prepared from evaporated milk, there were no evidences of retarded development. The ready digestibility of this milk made it particularly suitable for premature and sick infants. They found that resistance to infection was not impaired, and that the infants were particularly free from the fermentative diarrheas and digestive disturbances. Dentition, sitting, and walking developed normally. They report that infants fed evaporated or powdered milk had less intestinal indigestion and diarrhea, and showed greater average weekly gains, than those on breast or bottled milk.
Nutritive value. With regard to the nutritive value, especially the utilization of the minerals in evaporated milk, it is reported that this milk compares favorably with other forms of cows' milk. Jeans and Stearns 6 found that, under carefully controlled conditions, infants fed with evaporated milk diluted with an equal amount of 12 percent corn syrup and acidified with lactic acid grew excellently in weight and length and exceeded the ratios of the average male infant in Iowa. Retention of nitrogen, calcium, and phosphorus was high and approximately the same as for the undiluted acidified whole milk. There were high retention of nitrogen, high excretion of creatinine, and good physical progress, all of which are considered evidences of good muscular growth. High retention of calcium and nitrogen, early car-pal ossification, rapid growth in body length, and absence of chemical or clinical signs of rickets were considered good evidences of bone growth.
Vitamins A and D are present in evaporated milk to the same ex-tent as in the original milk; 4 the D content, however, is too small to be depended upon as an adequate amount. The heat-labile fraction of vitamin B is reported by Samuels and Koch to be reduced by about 20 percent in making evaporated milk from raw milk. Vitamin C is completely destroyed,' but this is of only academic interest be-cause even fresh milk does not contain enough vitamin C to be a dependable source of this vitamin. Vitamin G was shown by Tod-hunter 9 and also by Samuels and Koch' to be retained in full strength.
An extensive investigation on the effect of sterilization on the nutritive values of milk has recently been reported by S. K. Kon and his associates in England. The milk was flash-pasteurized, homogenized, bottled, heated in the open bottles at 212° F., and then sterilized in an autoclave at 230° F. (This treatment is not the exact equivalent of commercial sterilization of ordinary canned evaporated milk.) They found that the biological value of the proteins was slightly decreased, probably by partial destruction of one or more essential amino acids; the vitamin C was decreased by one-half; vita-min B was decreased by 30 percent; and vitamins A and B2 (flavin) were not affected. From the nutritional aspect, such sterilized milk is not the equivalent of raw or pasteurized milk, although it remains a valuable foodstuff.
Evaporated milk is proving beneficial in adult feeding. Soper reports that he has used evaporated milks satisfactorily in cases of gastric and duodenal ulcer, ulcerative colitis, and catarrhal colitis.
Tobey points out that, in the last several years, numerous con-trolled clinical studies on infant feeding have been reported in which about 2700 infants were fed on evaporated-milk formulas. The unanimity of the favorable findings attests the nutritive value of the product. These facts, together with its constancy of composition, availability, convenience of keeping without refrigeration, and ease of preparation into formulas, render it particularly useful whenever fresh market milk is not readily available, or where some pathologic condition precludes the use of fresh milk. Many pediatricians prescribe it in preference to bottled milk.
The changes which the manufacturing process effects in the composition or physicochemical condition of the proteins seem to make evaporated milk suitable for persons who are sensitive or allergic to raw milk.
The flavor, of fresh raw milk is changed by its manufacture into evaporated milk. This effect is due almost exclusively to the heat treatment during sterilization, but many persons do not object to this flavor. The color is somewhat darkened, but this is considered an advantage because it makes the milk look more like cream.
General use. Although evaporated milk is finding increasing acceptance for infant feeding, it is probable that 90 percent of all that is sold is utilized for general household purposes. Many coffee drinkers substitute evaporated milk for cream. Some persons add water to dilute it back to the strength of regular milk and drink it by the glass. Large amounts are used in the kitchen in the preparation of various foods for the whole family.
Quality of supply. The production of some brands of evaporated milk utilizes much of the surplus milk which is not marketed in bottles. This surplus milk constitutes a reservoir, so to speak, to be drawn on to supply all the milk that the bottled trade will consume. The fluctuating demand for bottled milk, together with the great seasonal variation in milk. production, operates to create a period of peak production of evaporated milk, followed by a period when the plants close down. During all this time, the milk supply of the condenseries is available for bottled use if the great municipalities should require it. Accordingly, this milk is produced under all the public-health safe-guards which surround the production of milk for the bottled-milk trade. Not all the milk that goes to condenseries is produced under this degree of sanitation. The lower prices (as compared with those for bottled milk) which are paid for milk for condensing necessarily limit the source of supply either to the surplus or to supplies from back-country. In the more remote districts there is neither the regulatory pressure from health officers nor the price stimulus to support the additional production cost which must attend improved quality.
Epidemiology. Only two outbreaks of disease attributable to evaporated milk have been located by the author. Both of these are alleged to have been caused by Clostridium botulinum. Schoenholz, Esty, and Meyer 12 report an outbreak of botulism which is alleged to have been caused by evaporated milk, spoiled in both appearance and odor. They experimentally inoculated spores and soil into cans of evaporated milk, incubated them at a temperature of 37° C., and stored them for later observation. Spoilage was irregular; the numbers of surviving organisms were not significant; many of the cans, though normal in appearance, were weakly toxic, others were abnormal in appearance but non-toxic, and still others were neither toxic nor spoiled. They report definite indications that toxin once formed in evaporated milk may deteriorate on prolonged incubation.
Geiger and his associates report four cases with typical symptoms of botulism supposedly the result of consuming spoiled canned milk, although none of the suspected cans of milk was available for laboratory tests, and therefore this cause was not proven.
An extensive study of the microbiological condition of evaporated milk was made by Savage and Hunwicke. They reviewed the work published up to that time, and confirmed the findings of other investigators that cans of evaporated milk often are not sterile. Usually these viable organisms are sporing aerobes, yeasts, micrococci, and thermophilic bacteria, but absence of oxygen causes them to die off. The increase in viscosity of evaporated milk is a function of time and temperature, and is not believed by them to be associated with microbial activity. They point out that inspection of the unopened tins alone does not always indicate whether the contents are spoiled. Evidence of rust, visible leaks, dents, bulging and other signs of ill usage, together with a laboratory examination of the contents of a representative number of cans, are sufficient to determine defectiveness of a batch.
Sterility. Although evaporated milk in hermetically sealed cans is given a heat treatment called sterilization, the product itself has not always been actually sterile. Deming and Davis 15 have reviewed the previous work, and report that some investigators apparently found cans of evaporated milk to be sterile whereas others found them not sterile. The organisms were mostly Gram-positive cocci and spore formers, such as the hay and potato bacilli. They found no record of any report of the presence of any pathogenic organisms. They .themselves examined 154 cans of evaporated milk purchased on the open market. They observed no organoleptic irregularity. Only one can incubated at 55° C. for 7 days was found to be unsterile. It contained large Gram-positive rods which when grown anaerobically coagulated the milk. They found no pathogenic organisms. Improvements in the milk production and manufacturing procedure have resulted in an improved bacterial quality of commercially evaporated milk.
The freedom of evaporated milk from viable pathogenic organisms is probably accounted for by the fact that these bacteria are killed in the heat processing long before spores are killed. As this milk can be kept on hand without refrigeration, it constitutes a safe milk supply for ordinary uses.
Types of adulteration and spoilage. Evaporated milk does not always have the same degree of creamy appearance. Batches differ somewhat in color and consistency according to the conditions of manufacture.
Cans of spoiled milk should always be examined bacteriologically. Sometimes cans have been observed in which the milk appeared normal but possessed an off-flavor. Other cans seem to be normal in that their ends are flat, but, when they are opened, the contents may be curdled, sour, or otherwise clearly spoiled.
Sometimes a can may be found to be contaminated with thermophilic organisms which are inactive at cold or moderate temperatures but which proliferate and cause spoilage at summer temperatures.
Cans with bulged ends usually indicate that the contents are under pressure and that there is an excess of gas. This gas may come from the fermentation of the unsterile contents, the chemical action between the contents and the walls of the can, or a change in the storage temperature or altitude. Regardless of these possibilities, a bulged can is an abnormal can, and its contents should not be used without a careful examination as to wholesomeness.
Several types of microbic spoilage have been recognized. Often they are caused by the survival of unusually heat-resistant organisms in a whole batch, or sometimes only individual cans are so affected. Defective closures may admit organisms which spoil the contents with a variety of reactions. A bitter flavor has been found to be imparted by several well-recognized organisms. A spore-forming anaerobe, such as Clostridium putrificum or members of the proteus group," produce changes like those of a true putrefaction. Other types produce gaseous fermentation, curdling with or without flavor changes, and fishy flavors.
Some types of spoilage are caused by strictly chemical effects. Most batches contain more or less coagulum during the sterilization process. This varies in texture from a temporary soft or loose curd, which on shaking entirely disappears, to a hard and tough curd which is permanent. The tendency to form this hard type of curd can be reduced by a proper adjustment of the salt balance of the milk.
Sometimes a granular, insoluble, light-colored deposit will be noticed in the bottom of cans of evaporated milk which have stood a long time. This consists of a mixture of minerals which settle out.
The temperature of sterilization imparts a desirable slightly brown color to evaporated milk. When samples have been stored above 45° F., the rate of darkening increases, depending on the time and temperature. This discoloration is not now considered to be due to a caramelization of the lactose, as was formerly supposed, but to be the result of a reaction between the sugar and the proteins, forming a lactose-amino-acid complex. Storage below 45° F. reduces it to negligible amount.
Spoilage from purely physical effects may come from a separation of the butterfat, the fat emulsion having been broken and the fat granules rising to form an unsightly layer on the surface.
Some of the above types of spoilage really do not affect the nutritive or sanitary value of the evaporated milk. However, they do change the nature of the product so that it looks different from the normal package. The consumer is seldom equipped to subject such a product to critical examination to determine just why such an abnormal condition obtains. Inasmuch as experience teaches that a really dangerous condition sometimes accompanies such a change, the food-control officer is certainly warranted in excluding such products from sale. Any abnormality of the product is sufficient cause for its rejection, and warrants a demand for proof by the merchant that the product is safe.
Filled milk is the name given to milk or milk products in which some fat or oil, other than milkf at, has been substituted for the natural butterfat, yielding a product which looks and tastes like regular evaporated milk. The Federal Filled Milk Act of 1923 prohibits the interstate distribution of such a product, although the law exempts a distinctly proprietary food which is labeled to indicate its special use in infant feeding and which is shipped exclusively to bona fide users such as physicians, wholesale druggists, asylums, and infant welfare associations. The Secretary of Agriculture is authorized to enforce such regulations as he may judge necessary to enforce the act. The U. S. Supreme Court has recently upheld the constitutionality of this act, so that the Food and Drug Administration can now proceed to enforce its provisions. This action does not prevent the sale of filled milk within the confines of those states which do not prohibit its distribution.
Chemical examination. 17 Fat. A 5-gram sample is weighed directly into an extraction flask, 5 milliliters of distilled water added, and the fat is determined as in milk (see page 94) by the official Roese-Gottlieb method or the official Babcock method.
Total solids. The total solids are determined as in milk, except that a 1-gram sample is used. It is diluted with about 1 milliliter of water.
Acidity. The acidity is determined on a weighed sample of about 9 grams, diluted with an equal volume of recently boiled, distilled water, and then diluted and titrated as for milk.
Shrader reports that, when evaporated milk is made from a milk of high bacterial content, a determination of the ammoniacal nitrogen will indicate the approximate extent of this contamination.
Bacteriological examination. The American Dairy Science Association has published a suggested procedure for the bacteriological examination of evaporated milk .
Standard plate colony count. This determination is made by the official procedure as for liquid milk (see page 94).
Sterility. After cans are incubated at 37° C. for 1 week, they are plated as above. If no colonies are found on any plates, including a plating of 1 milliliter direct, the can may be considered sterile for all practical purposes.
Direct microscopic count. This determination is useful to ascertain something of the quality of the milk prior to manufacture, and also in some cases of spoilage.
Anaerobic agar plate method. The plating follows the official methods and generally recognized anaerobic technic. It is useful for the examination of some abnormal products.
Thermophilic bacteria. When these organisms are determined, a special tryptophane-yeast-dextrose-agar medium is used, and the plates are incubated at 55° C. for 48 hours.
Spore-forming bacteria. The sample should be heated in a sterile container on a water bath at 80° C. for 10 minutes, then cooled, plated by the standard technic, and incubated for 3 days at 37° C.
Control procedure. Although evaporated milk may be sterile in the can and therefore harmless from the standpoint of producing a communicable disease, it is highly desirable that the conditions at the plant where it is packed be clean and sanitary. No examination of the product can determine these facts. Only an inspection of the plant and its operations will reveal these conditions.
1. O. F. HUNZIKER, Condensed Milk and Milk Powder, 5th ed., 1934; A. MIYAwAKI, Condensed Milk, John Wiley and Sons, New York, 1928.
2. Service and Regulatory Announcements, Food and Drug 2, fifth revision, November, 1936.
3. Commission on Milk Standards, U. S. Pub. Health Repts., Reprint 634, 1921.
4. F. E. RICE, Am. J. Pub. Health, 24, 194 (1934).
5. J. A. ToBEY, Arch. Pediat., 50, 183 (1933).
6. P. C. JEANS and G. STEARNS, Am. J. Diseases Children, 46, 68 (1933).
7. L. T. SAMUELS and F. C. Kocx, J. Nutrition, 5, 307 (1932).
8. C. B. HART, H. STEENBOCK, and D. W. SMITH, J. Biol. Chem., 38, 305 (1919).
9. E. N. TODHUNTER, J. Am. Diet. Assoc., 8, 42 (1932).
10. S. K. KoN (and associates), J. Dairy Research, 9, 207 (1938), from Chem. Abs., 32, 8607 (1938).
11. H. W. SOPER, Am. J. Digest. Dis. and Nutrition, 2, 113 (1935).
12. P. SCHOENHOLZ, J. R. EsTY, and K. F. MEYER, J. Infectious Dis., 33, 289 (1923).
13. J. C. GEIGER, E. C. DICKSON, and K. F. MEYER, Pub. Health Bul. 127, p. 34, U. S. Pub. Health Service, 1922.
14. W. G. SAVAGE and R. F. HUNWICKE, Food Inspection Bd., Special Rept. 13, London, 1923.
15. J. DEMING and H. DAVIS, Arch. Pediat., 48, 42 (1931).
16. F. W. TANNER, The Microbiology of Foods, Twin City Printing Co., Champaign, Ill., 1932, p. 278.
17. Official and Tentative Methods of Analysis, 4th ed., Association of Official Agricultural Chemists, Washington, 1935, p. 279.
18. Standard Methods of Milk Analysis, 6th ed., American Public Health Association, 1934, (a) p. 81; (b) p. 77; (c) p. 77.
19. J. H. SHRADER, 17th Ann. Rept. Internat. Assoc. Dairy and Milk Inspectors, 1928, p. 261.
20. Subcommittee on Bacteriological Methods of American Dairy Science Association, J. Dairy Sci., 18, 647 (1935).