Technical Bulletin No. 832 . August 1942 Effect of Homogenization on the Curd Ten- sion, Digestibility, and Keeping Quality of Milk^ By C. J. BABCOCK Market-milk specialist. Division of Market-Milk Investigations, Bureau of Dairy Industry CONTENTS Page Page introduction 1 Relative digestibility of homogenized and un- Methods 2 homogenized milk 7 Curd-tension measurements 2 Curd area _ 11 Digestion in vitro 2 Keeping quality 11 Determination of curd area 3 Susceptibility to copper-induced oxidized Determinations of keeping quality 3 flavor VI Curd tension of milk 3 Susceptibility to sunlight-induced oxidized Effect of homogenization 3 flavor 14 Effect of fat content 5 Flavor developments during storage 14 Effect of mixing homogenized milk with un- Comparisons of bacterial growth and acid homogenized milk 6 development 17 Effect of partial freezing 7 Conclusions 20' Literature cited 21 INTRODUCTION The use of homogenized milk is increasing rapidly in the United States. One reason is that many consumers prefer milk in which the fat is evenly dispersed; another is that some consumers also find the homogenized milk more palatable than unhomogenized milk; and furthermore, milk distributors have promoted the use of homo- genized milk on the basis of its soft-curd properties. Some authori- ties contend that soft-curd milk is more readily digested than hard-curd milk. Weisberg, Johnson, and McCollum {36) f Wallace (J*^) and Washburn (rSS) were among the first to report that homogenization changes the character of the curd of milk. Hill (lo)^ however, re- ported that the results of his work did not justify the use of the homogenizer for the production of soft-curd milk. More recently Doan and Welch (11) ; Tracy (28) ; Theophilus, Hansen, and Spen- cer {^6) ; Wolman {S7) ; Babcock {£) ; and others have shown that soft-curd milk can be produced by homogenization. 1 Received for publication April 27, 1942. ^ Italic numbers in parentheses refer to Literature Cited, p. 21. 467453°—42 1 2 TECHNICAL BULLETIN 832, U. S. DEPT. OF AGRICULTURE The use of homogenization in commercial practice to produce soft-curd milk is comparatively new. The investigations discussed in this bulletin were made in the market-milk laboratory of the Bu- reau of Dairy Industry to determine the effect of homogenization on the curd tension of milk under different conditions found in com- mercial practice, and to obtain information on the relative digesti- bility and keeping quality of homogenized and unhomogenized milk. METHODS CURD-TENSION MEASUREMENTS The curd-tension measurements were made according to the tenta- tive method recommended by the curd-tension committee of the American Dairy Science Association, as reviewed by Doan (P, f, 71,0). DIGESTION IN VITRO In the studies on the digestion of milk, formol titration was used to measure proteolysis. The method, based on the work of Northrop (^^) and adapted to milk, follows: Place 5 ml. of milk in each of several 50-ml. test tubes con- taining several 4-mm. glass beads. To each test tube add sufficient reagent (N/10 hydrochloric acid containing 1 percent of pepsin (1: 3,000) ) to bring the pH value of the mixture to 3.5. Thoroughly mix the contents of one of the test tubes, allowing the glass beads to break up the curd particles. Add 1 ml. of formol solution. Then, using a burette graduated in hundredths of a millimeter, titrate the mixture with N/20 sodium hydroxide against an identical mixture to which 0.2 ml. of formol solution has been added and which has been completely neutralized (at approximately pH 11). This stand- ard is kept in a stoppered bottle to prevent its color from fading. Place the remaining test tubes in a water bath at 37° C, where the contents of the tubes are gently agitated. At stated intervals (every 15 minutes during the first hour and hourly thereafter) remove one of the test tubes from the water bath and proceed as with the first test tube. The difference between the titration readings on the burette at the beginning and subsequent periods represents the proteolysis expressed in milliliters. In the comparison of the digestibility of pasteurized and pasteur- ized homogenized milk, the 5 ml. of milk in the test tubes was coagu- lated by adding 0.5 ml. of N/10 hydrochloric acid containing 0.45 per- cent of pepsin and placing in a water bath at 37° C. for 10 minutes. At the end of 10 minutes a solution consisting of stronger (0.55 N approximately) hydrochloric acid containing 1 percent of pepsin, was added in sufficient quantity to lower the pH to 2.0. Thereafter the procedure was the same as outlined, except that the contents of the tubes was titrated with N/10 instead of N/20 sodium hydroxide. The formol solution used consisted of commercial 40-percent form- aldehyde containing basic magnesium carbonate (in a bag) and 0.4 percent of phenolphthalein. A small amount (no definite quantity) of urea was added to the contents of the test tubes just previous to titration. As shown by Line weaver and Hoover (ßl)^ the urea does not affect the results of titration and gives a clearer liquid to titrate. KEEPING QUALITY OF MILK DETERMINATION OF CURD AREA The curd surface or area was determined in a manner similar to that devised by Wolman (37). The author, however, placed 10 ml. of coagulant (N/10 hydrochloric acid containing 0.45 percent of pep- sin) in the latex bag prior to adding 100 ml. of milk. Agitation of the latex bags was started before the milk was added. Ten minutes after the milk was added an additional 10 ml. of the coagulant was added. One hour of agitation was then allowed before concentrated formaldehyde was poured in to stop the reaction and to harden the curds. DETERMINATIONS OF KEEPING QUALITY The relative keeping quality of homogenized milk as compared with that of the unhomogenized milk was determined by studying the development of copper-induced oxidized flavor, the susceptibility to sunlight-induced oxidized flavor, and b}^ comparing the flavor, titratable acidity, pH value, and plate count after various periods of storage. CURD TENSION OF MILK EFFECT OF HOMOGENIZATION While homogenization lowers the curd tension of milk, the extent that the curd tension is lowered depends on the character of the milk prior to homogenization and the manner in which the process is carried out. The following experiments w^ere performed in order to determine the effect of various homogenizing practices on the curd tension : FIOMOGENIZING PRESSURE The extent that the curd tension of milk is lowered by homogeni- zation depends primarily on the pressure of homogenization. Table 1 shows the effect of homogenization at different pressures on the curd tension of pasteurized milk with an average fat content of 4.0 percent. TABLE 1.—Effect of homogenization at different pressures on the curd tension of milk (homogenized at 142° F.) Homogeniz- Curd ten- Homogeniz- Curd ten- ing pressure sion Decrease ing pressure sion Decrease Pounds Grams Percent Pounds Orams Percent 0 35.9 2,000 13.6 62.1 500 29.6 17.5 2,500 13.6 62.1 1,000 19.5 45.6 3,000 13.1 63.2 1,500 16.0 55.4 As shown by table 1, increasing the homogenizing pressure decreases the curd tension. The decrease, however, is not proportional to the increase in pressure of homogenization. Theophilus, Hansen, and Spencer {26) obtained a similar reduction in curd tension when using 1,000 pounds pressure, but about 25 percent reduction when using 500 pounds pressure, and only about 53 percent reduction when using 2,000 pounds pressure. 4 TECHNICAL BULLETIN 8 3 2^ U. S. DEPT. OF AGRICULTURE Table 1 also shows that, from a curd-tension standpoint, the maxi- mum pressure of homogenization need not be greater than 2,500 pounds. Only a slightly greater reduction in curd tension is obtained at higher pressures. This is in agreement with work of Caulfield and Martin (^), who concluded that homogenizing pressures in excess of 2,500 pounds appear to be of little practical value in reducing the curd tension of milk. Tracy (29) also states that a pressure of 2,500 pounds is sufficiently great for all practical purposes as there is little to be gained, as far as reduction in curd tension is concerned, by increasing the pressure to 3,000 pounds. HOMOGENIZING TEMPERATURE The temperature at which milk is homogenized at any given pres- sure affects the curd tension of the milk. As a rule, the higher the temperature of homogenization the lower the curd tension. (See table 2.) TABLE 2.—Effect of homogenizing temperature on the curd tension of milk {homogenized at 2,000 pounds pressure) Curd tension Curd tension Homogeniz- Milk Milk Homogeniz- Milk Milk ing tem- pasteur- pasteur- ing tem- pasteur- pasteur- Unpas- ized TJnpas- perature teurized ized perature teurized ized ized milk after before milk after before homogen- homogen- homogen- homogen- izing izing izing izing Grams Grams Grams °F. Grams Grams Grams 120 26 24 22 160 19 17 15 130 25 22 23 170 15 14 13 140 23 21 24 180 16 13 12 150 21 20 18 Table 2 shows that when unpasteurized milk is homogenized the higher the temperature of homogenization the lower its curd tension. When homogenized milk is pasteurized the same normal reduction in curd tension occurs. When homogenization takes place after pasteur- ization, however, slightly lower curd tensions result at both the lower and higher homogenizing temperatures. This was also found to be true at homogenizing pressures of 1,000,1,500, 2,500, and 3,000 pounds.