Other Dairy Products
( Originally Published 1939 )
It has been known for a long time that babies fed with boiled milk thrived better than those fed with regular milk. It was supposed that this improved quality lay in the destruction of pathogenic organisms. Gradually a wealth of data has been accumulating which shows that boiling or other treatment of milk increases its digestibility. After Buckley had showed that the physical nature of the curd of milk is important in determining the food value of milk,' and Ladd had published some chemical data that showed that homogenized milk produced a soft curd in the infant's stomach and was similar to breast milk in this respect,' Washburn and Jones showed that homogenization of milk produced curds which were much more flocculent and friable than those of regular milk, although this property was not reflected in any improved nutrition of his experimental animals, young pigs. The recent work of Hill is credited with giving this subject of the digestive quality of milk an emphasis which has found important application in the commercial production of soft curd milk. Interest has been further stimulated competitively by reason of the inroads that the evaporated milk industry has made into the bottled trade, largely by reason of the superior properties of the canned product in infant feeding. This has led to laboratory activity directed toward devising processes for imparting soft curd properties and for measuring curd hardness rather than toward ascertaining to what extent, if any, these treatments actually improve the digestibility of the milk. The scientific literature leaves the subject in a very con-fused state. Whatever improved digestibility there is seems to result entirely from the speeding of the passage of the milk from the stomach and not from any increased food value or degree of assimilation.' The whole subject is excellently reviewed by Doan in the Journal of Dairy Science, 21, 739-756 (1938).
NATURAL SOFT-CURD MILK
Hill found that the milk of different cows possessed unequal digestibility, and that many infants could tolerate milk from certain cows but not that from others. In general, this improved tolerance was associated with milk of relatively low total solids content, al-though this relationship did not seem to be exclusively specific. Soft-curd milk was produced by cows of different breeds and was fairly uniform over the lactation period of a given cow. This property enabled herdsmen to select cows for the regular production of this kind of milk.
Soft-curd milk is more rapidly digested by humans, calves, and rats, and leaves the stomach more quickly than regular milks At the same time, soft-curd milk has a lower content of total solids and a smaller calorific value.
It has been observed that cows suffering with mastitis produce a soft-curd milk. This has led many persons to think that all soft-curd milk is pathologic. Such a belief is erroneous. Soft-curd milk is actually under more stringent control than regular milk because its production is mostly, if not entirely, limited to Grade A and certified herds. However, on account of the widespread prevalence of sub-clinical mastitis, it is recommended that the presence of udder infections be tested for when the curd tension is determined.
Elias- showed 10 that soft-curd milk gave curds in the stomach similar to those of boiled milk. Espe and Dye reported that doubling the curd tension increased the length of the digestive period from 30 to 65 percent, and that boiling markedly lowered the curd tension. Welch and Doan showed that curd tension was greater in milk of high casein content, and that equalization of casein content by dilution with water caused both the curd tension and differences in rates of digestion largely to disappear, although the casein content might exercise only a minor role in the rate of digestion if the curd is artificially softened by heating, homogenization, and other means.
ARTIFICIAL SOFT-CURD MILK
Soft-curd by homogenization. Softness of curd can be imparted to a milk by homogenizing it. This procedure consists in pumping milk under very great pressure through a special valve with small clearance so that the, butterfat globules are broken up and uniformly distributed. The homogenization of skimmed milk does not impart soft-curd properties; at least about 1 percent of butterfat or other oil must be present. Chocolate milk is a soft-curd milk. Therefore, it seems that the imparting of soft-curd properties by mechanical means is a function of the degree of dispersion of discrete particles whereby the curd is mechanically prevented from setting into a solid homogeneous mass. Feeding experiments on rats showed that this homogenized soft-curd milk was digested just about as quickly as boiled milk or natural soft-curd milk. Letters patent 12 have been issued to cover the production of soft-curd milk by homogenization, although the process seems to have been practiced by milk companies for many years previously to the granting of the patent.
The difficulty of controlling exactly the effectiveness of the homogenizing machine, together with the variability in the composition or physical nature of the milk, particularly the butterfat, precludes the determination of the most efficient temperatures and pressures. Experience has taught that the curd of a given milk cannot be softened beyond a certain point, regardless of the pressure used, and on the other hand, too light a pressure does not insure permanency to the imparted curd softness. In industrial practice, consistent results can be obtained when milk is homogenized at pressures of about 2500 to 3000 pounds per square inch at a temperature of about 145° F. This softens the curd to a tension of about 30 grams, or reduces the curd tension of average market milk about 50 percent.
The homogenization of milk must be carefully conducted if a satisfactory product is to be obtained. Trout and his associates found 13 that some milk upon homogenization developed rancidity within 15 minutes after treatment. This effect seemed to be caused by a lipolytic enzyme which could be inactivated at temperatures of pasteurization. Accordingly, this off-flavor can be prevented by pasteurizing the milk before or immediately after homogenization. The flavor of the finished product is generally considered to be slightly better if pasteurization precedes homogenization, but health officers are inclined to require pasteurization to come last.
Homogenized milk, unless the milk was initially of high quality, may exhibit a smudgy yellow or gray sediment in the bottom of the bottle. It is too finely divided to be revealed on a sediment disc. Babcock 14 reported that it consists largely of leucocytes, epithelial cells, and some finely divided dirt. Charles and Sommer 15 state that sediment may occur in milk of the highest sanitary quality and may come from a healthy udder. It is not seen in unhomogenized milk because the rising of the fat globules into the cream layer sweeps this light material upward. Clarification by centrifugal clarifying ma-chines will remove it.
Soft-curd properties, artificially imparted to milk by homogenization, were studied by Anthony on two adult males who possessed the unusual ability to regurgitate at will without distress. This enabled them to drink the milk, hold it in their stomachs for 30 minutes, and then return it without the aid of a stomach pump or an emetic. These experiments showed that the tests on curd strength made in vitro and determined with the curd knife reasonably evaluated the nature of the curd in the human stomach (except in the case of mineral modified milk). The curd particles of breast milk were minute and soft, and were so finely divided that they could not be separated from the accompanying juices with a 20-mesh screen. On cows' milk, when the curd tension (by laboratory curd-knife technic) was high, the regurgitation specimens of curd in every case were large and leathery. When the readings were low, the curd particles were small and soft. Breast milk registered 0 curd tension, natural cows' milk 50-100 grams, and homogenized milk (processed at 3500 pounds) 15 grams. The patients reported that the milk tasted better (because of the minute division of the milkfat globules) and gave less distress.
However, no digestive advantage is reported by some other investigators who worked on samples in vitro and on experimental animals. The latter work is not so impressive as clinical studies but may be better controlled. Much more fundamental and clinical re-search is necessary before the value of this processing is substantiated.
Soft curd by sonic vibration. A modification of the homogenizing process for the production of soft-curd milk has been developed by subjecting milk to intense sonic vibration. Electromagnetic oscillators, somewhat similar to those used in submarine communications, are constructed to allow the passage of milk in a thin film between the "anvil" and the vibrating diaphragm. Sonic vibration acts directly on the butterfat of the milk to cause a more complete dispersion. The reduction of curd tension is a function of the number of fat particles, and not of the actual fat concentration. A direct relationship seems to exist between the degree of fat dispersion-and the degree of curd-tension reduction. Inasmuch as only a small proportion of the total fat in milk need be finely subdivided to reduce the curd tension, it is possible to produce soft curd by vibration without destroying the cream volume (cream line).
Commercial homogenization. The practice of homogenizing market milk is gradually extending. It is quite general in parts of Canada, and is increasing irregularly in the United States. Fifteen states have no regulations for the control of homogenized milk, 19 states and the District of Columbia permit its sale if properly labeled, 2 states have taken no action but look upon it with disfavor, and 4 states prohibit its sale. It is a useful practice for the treatment of milk which is to be consumed in restaurants, institutions, or wherever the sale of bulk milk introduces the likelihood that the consumer may be served a portion from which a substantial part of the butterfat has separated. Tracy states that the unpopularity of homogenized milk in the past has been due largely to the emphasis placed on the cream line as a measure of the value of a milk, and to the unfriendly attitude of some regulatory officials who felt that homogenization might encourage fraudulent practices. About one-third of the milk-route customers of the University of Illinois changed to this milk for the following reasons: it looked and tasted better; no cream adhered to the bottle cap; no mixing was required; it tasted better for breakfast foods; it removed the temptation to abstract cream; it was easier to prepare for infant feeding; it did not allow rising of cream to top of glass in refrigerator; it made better milk drinks; it tasted better to children; it was more easily digested by infants; and it did not churn out on freezing.
Soft curd by base exchange. Hard waters are softened by the zeolite or base-exchange method, whereby the percolation of water through a bed of zeolite (a sodium-aluminum silicate) effects an ex-change of sodium and calcium. As applied to milk, sodium from the zeolite replaces soluble calcium from the milk. The milk is first acidified to about 0.3 percent as lactic acid (with a dilute nitric acid solution) and then percolated at 64° F. over a granular column of zeolite. During the process, the pH is adjusted to that of ordinary cows' milk (about 6.50), and the acidity is reduced to about 0.15 per-cent as lactic acid. This process is reported 22 to change the taste, appearance, and other qualities very little from those of regular milk. The cream line is practically the same as in pasteurized milk. Bacteria counts are said to be lowered by the filtering effect of the pass-age of the milk through the zeolite bed.
The Hill method cannot be used to measure the curd tension by this process because the Hill technic introduces about ten times as much soluble calcium into 100 milliliters of milk as is removed by the base-exchange treatment. Moreover, it is considered more desirable to use a method which more closely simulates gastric digestion. Such a method has been developed by Miller.
Hess, Poncher, and Woodward 24 studied the nutritional effects of such a milk on an infant on a metabolism frame. They report that, in spite of the decrease of the percentage content of total calcium and phosphorus, 100 milliliters of such milk per kilogram of body weight kept a normal growing infant in a positive calcium and phosphorus balance during the entire time of feeding.
Soft curd by enzymic action. Milk can be given soft-curd properties within a range of 20 to 30 grams by the addition of pancreatic extract, concentrated in the proportion of 1 part of the powder to 10,000 parts of milk. The milk containing the enzyme is heated at a temperature of 42° C. (108° F.) for 15 minutes, and then is pasteurized in the regular way. The preliminary heating brings about a partial digestion of the curd, and the pasteurization inactivates most of the enzyme. The mineral content, the protein, and the formol titration values remain substantially unchanged.
Standards of quality. The quality of curd is usually determined by the Hill test, or some modification of it. Although natural milks may give a range of reading on the scale from 15 to 200 grams of tension, the average of numerous milk supplies has been found to be about 60-70. The American Association of Medical Milk Commissions 27 specifies that a soft-curd milk must show a curd tension be-low 30 grams, determined at least twice at an interval of 1 to 5 days before it can be claimed to be a soft-curd milk, and that the test must be repeated at monthly intervals thereafter.
Determination of curd tension. Hill's method for determining the characteristics of milk curd is based on the measurement of the degree of toughness of the curd which is coagulated by pepsin in calcium chloride solution. The measurement is the indicated pull in grams necessary for a special knife to cut through the coagulated curd. The knife consists of several radial horizontal blades soldered at right angles to an upright slender rod. This knife is placed in a jar containing 100 milliliters of the milk to be tested. A coagulating solution of scale pepsin and calcium chloride is then added. This sets the curd around the knife. The knife is then hooked to a spring balance, and its pull as it cuts upward through the curd is read directly from the dial.
Caulfield and Riddell have shown that it is expedient to make each determination in triplicate, and that temperature of reaction and time interval between the addition of coagulant and cutting of the curd must be kept constant. Miller 23 has modified this method by substituting an acid pepsin solution for the pepsin-calcium chloride solution. The measurement of toughness of curd by this method substantially parallels the digestibility of the milk by animals. See also the method of the U. S. Department of Agriculture, and that of the American Dairy Science Association reported supra by Doan.
Determination of butterfat. Authorities are not in agreement as to the effect of homogenization on the accuracy of the butterfat de-termination by the Babcock method. Babcock found that in every case the homogenized milk averaged in fat about 0.1 percent lower than the same milk before it was homogenized. On the other hand, Tracy' states that homogenized milk can be tested satisfactorily by the Babcock method if both the acid and milk are at about 70° F., if the acid is added in small portions, if slightly less acid (1.5 milli-liters) is used, and if the solution is shaken well after each addition of acid.
Microbiological examination. Inasmuch as natural soft curd has been associated with mastitis, it is advisable in the interest of sanitation and wholesomeness to examine samples of natural soft-curd milk for the presence of mastitis organisms.
1. S. S. BucKLEY, Maryland Agr. Exp. Sta. Bul. 184, 1914.
2. M. LADD, Boston Med. and Surg. J., 173, 13 (1915).
3. R. M. WASHBURN and C. H. JONES, Vermont Agr. Exp. Bul. 195, 1916.
4. R. L. HILL, Utah Agr. Exp. Sta. Bul. 207, 1928; Circular 101, 1933.
5. Council on Foods, J. Am. Med. Assoc., 108, 2040, 2122 (1937).
6. F. J. DoAN and R. C. WELCH, Pennsylvania State College Agr. Exp. Sta. But. 312, 1934. See also F. J. DoAN and C. C. FLORA, ibid., 380, 1939.
7. H. C. HANSEN, D. R. THEOPHILUS, F. W. ATKESON, and E. M. GILDow,
J. Dairy Sci., 17, 257 (1934).
8. R. C. WELCH and F. J. DoAN, Milk Plant Monthly, 22 (11) 30 (1933).
9. W. V. HALVERSEN, V. A. CHERRINGTON, and H. C. HANSEN, J. Dairy Sci., 17, 281 (1934).
10. H. L. ELIAS, Am. J. Diseases Children, 44, 296 (1932).
11. D. L. EsPE and J. A. DYE, ibid., 43, 62 (1932).
12. R. FLUCKIOER, U. S. Patent 1,973,145, Sept. 11, 1934.
13. G. M. Tamil, C. P. HALLORAN, and I. GouLD, Mich. Agr. Exp. Sta. Tech. Bul. 145, 1935.
14. C. J. BARCOCK, U. S. Dept. Agr. Tech. Bul. 438, 1934.
15. D. A. CHARLES and H. H. SOMMER, Milk Plant Monthly, 24, 26, 32 (1935).
16. G. E. ANTHONY, The Bulletin (official publication of the Genesee County Medical Society), 9, March 4 (1936).
17. L. A. CHAMBERS, J. Dairy Sci., 19, 29 (1936).
18. Milk Dealer, 25, 36 (1936).
19. R. H. TRACY, Milk Plant Monthly, 24, 28 (1935).
20. U. S. Patent 1,954,769, assigned to M. & R. Dietetic Laboratories, Inc.
21. J. F. LYMAN, E. H. BROWNE, and H. E. OTTING, Ind. Eng. Chem., 25, 1297 (1933). Also see Milk Plant Monthly, January, 1934, p. 37.
22. H. E. OTTING and J. J. QuILLIGAN, Milk Dealer, 23, 36 (1934).
23. D. MILLER, J. Dairy Sci., 18, 259 (1935).
24. J. H. HESS, H. G. PONCHER, and H. WOODWARD, Am. J. Diseases Children, 48, 1058 (1934).
25. V. CONQUEST, A. W. TURNER, and H. J. REYNOLDS, J. Dairy Sci., 21, 361 (1938).
26. R. L. HILL, ibid., 6, 509 (1923).
27. Methods and Standards for the Production of Certified Milk, Am. Assoc. Med. Milk Commissions, New York, 1936.
28. W. J. CAULFIELD and W. H. RIDDELL, J. Dairy Sci., 17, 791 (1934).
29. Chief of Bureau of Dairy Industry, 1938, J. Milk Technol., 2, 48 (1939).
30. Curd Tension Committee. Rept. Annual Meeting Amer. Dairy Sci. Assoc.,
31. P. H. TRACY, Milk Dealer, 25, 30, 60 (1936).