177. The protein distribution in normal and abnormal milk

Abstract

The N distribution in terms of casein, albumin, globulin, proteose-peptone, and non-protein N is given for a number of samples of normal milk, of milk from cows with mastitis, and of milk persistently low in solids-not-fat content. Improved methods of determination have been used. The average N distribution in normal milk is as follows: casein N 78·5, albumin N 9·2, globulin N 3·3, proteose-peptone N 4·0 and non-protein N 5·0% of the total N. The casein N figure is the least variable, ranging from 77·3 to 80·7. On this basis, normal milk contains, as an average: casein 2·63, albumin 0·31, globulin 0·11, and proteose-peptone substances 0·13%. The total protein content is 3·18%, and the casein: albumin: proteose-peptone: globulin ratios are 23·8: 2·8: 1·2: 1·0. The milk from cows with clinical mastitis is low in solids-not-fat content, and characterized by a decreased content of casein and an increased content of albumin, globulin, and proteose-peptone substances, particularly of globulin. The milk samples of low solids-not-fat content were of normal appearance and reaction, and produced by apparently healthy cows known to have yielded milk of low solids-not-fat content for some time. The samples were examined by strict bacteriological methods for the presence of mastitis streptococci, and formed two distinct groups: (1) samples from cows with subclinical mastitis, having the protein distribution of the mastitis samples described above, the cause of the low solids-not-fat being pathological; and (2) samples from uninfected cows, having the protein distribution of normal milk, the cause of the low solids-not-fat being physiological. The two groups of low solids-not-fat cases are discussed, and the conclusion drawn that subclinical mastitis may account for a large proportion of these widespread and puzzling cases. Many cases of mild subclinical mastitis cannot be detected by the simple chemical tests which will reveal pronounced infection, and satisfactory bacteriological tests, when available, demand much skill and time. For the detection of milk from cows with subclinical mastitis, and for the differentiation of milk samples with pathological and physiological low solids-not-fat, the solids-not-fat, the figure is suggested, and the advantage of this “casein number” is illustrated.