Bacteriology of cheese I. Effect of pasteurizing the milk on the nitrogenous decomposition in Cheddar cheese

The effect of pasteurizing the milk on the nitrogenous decomposition in cheddar cheese was studied with nine series of cheese. Each of six series contained two cheese, manufactured at the same time from equal portions of the same lot of milk, one cheese being made from raw milk and one from pasteurized milk. Each of three series contained three cheese; one cheese was made from raw milk, one from pasteurized milk, and one from 90 percent pasteurized and 10 percent raw milk. Changes in the nitrogen distribution in the cheese were determined by chemical analyses of cheese serum at intervals during ripening. The cheese serum was obtained by submitting a mixture of finely divided cheese and sand to relatively high pressures in a laboratory press. The analyses of the cheese serum included determinations of total nitrogen, amino nitrogen and various nitrogen fractions which were soluble or insoluble in trichloracetic acid, ethyl alcohol, phosphotungstic acid, or tungstic acid. The cheese was scored for flavor and the flavor criticized at the same periods that chemical analyses were made. In all of the cheese there was a steady breaking down of the proteins as indicated by increases in the various nitrogen fractions determined. During the early stages of ripening, there was little variation in the amounts of the various fractions in the serums of the raw and the pasteurized milk cheese. After longer ripening, the amounts of the various nitrogen fractions were definitely larger in the serum of the raw milk cheese than in the serum of the pasteurized milk cheese, which indicates a more rapid and extensive breakdown of the proteins in the cheese made from raw milk. After about 2 months of ripening the flavor scores of the cheese made from raw milk were regularly higher than those of cheese made from pasteurized milk. The cheese made from pasteurized milk was generally characterized by a lack of flavor and a tough rubbery body. The amounts of the various nitrogen fractions in the serum of the cheese made from 90 percent pasteurized and 10 percent raw milk were usually intermediate between the corresponding values for the raw milk and the pasteurized milk cheese, although they more nearly approached the values for the raw milk cheese than those for the pasteurized milk cheese. The cheese made from the mixed milk developed a flavor essentially the same as that of the raw milk cheese and did not lack flavor as did the cheese made from pasteurized milk. The high quality cheese, which was made from raw milk or a mixture of raw and pasteurized milk, was regularly characterized by the presence, in the cheese serum, of relatively large amounts of the nitrogen fraction soluble in trichloracetic acid but insoluble in ethyl alcohol, and also by the presence of relatively small amounts of the fraction insoluble in trichloracetic acid