CHANGES IN THE CHEMICAL-COMPOSITION DURING ULTRAFILTRATION OF SKIM MILK

Five batches of pasteurized skim milk were concentrated to approximately 2-, 4-, and 5-fold by UF. Chemical analyses of the skim milk and retentates were done to determine the change in chemical composition that occurred during UF. Milk proteins were concentrated during UF from an average 3.42% in skim milk to 6.85, 13.51, and 17.1% in 2-, 4-, and 5-fold retentates, respectively. Milk fat similarly was concentrated from .11% in skim milk to .24, .45, and .60% in 2-, 4-, and 5-fold retentates, respectively. Lactose content progressively decreased during UF from an initial 5.06% in skim milk to 4.76, 4.29, and 4.06% in 2-, 4-, and 5-fold retentates, respectively. Total solids content increased to a lesser extent, compared with protein and fat, from 9.19% in skim milk to 12.72, 17.80, and 23.91% in 2-, 4-, and 5-fold retentates, respectively, reflecting the loss of lactose and other soluble small molecular weight components. Analysis of the following minerals indicated partial concentration: Ca++ by 1.6-, 3.0-, and 4.3-fold; Mg++ by 1.4-, 2.3-, and 3.0-fold; Zn++ by 1.8-, 3.4-, and 4.9-fold; Fe++ by 1.9-, 3.5-, and 4.9-fold; Cu++ by 1.7-, 3.4-, and 4.7-fold; and Mn++ by 1.5-, 2.0-, and 3.0-fold in the 2-, 4-, and 5-fold retentates, respectively. Microbiological assays indicated decreases in the following B vitamins: thiamin by 57, 77, and 85%; riboflavin by 41, 62, and 71%; niacin by 63, 81, and 87%; pantothenic acid by 53, 75, and 82%; biotin by 55, 77, and 84% in 2-, 4-, and 5-fold retentates, respectively. Free amino acid analysis of skim milk and retentates indicated that by a 5-fold concentration of milk, amino acids decreased by a minimum of 50% for methionine to a maximum of 98% for serine.