147. The soluble protein fraction of milk

Abstract

1. The literature on the soluble protein fraction of milk has been reviewed to show the difficulties that are encountered in attempting to decide whether the fraction consists, as is usually supposed, of lactalbumin and lactoglobulin only, or whether it consists partly of secondary proteins of a proteose-peptone nature. 2. It has been shown that on boiling the casein-free filtrate of milk, maximum coagulation of albumin and globulin occurs at a pH. of 4–75–4–80. At this pS. prolonged boiling does not cause hydrolysis, and the amount of albumin and globulin N coagulated represents about 70 per cent of the total soluble protein N of milk. 3.By precipitation of the casein-free nitrate of milk with various con- centrations of trichloracetic acid, a partition of the soluble proteins has been effected which strongly suggests the presence of proteose-peptone substances to an extent in agreement with the results of heat coagulation. 4. The maximum amount of albumin and globulin rendered coagulable by heating milk itself has been determined by precipitating denatured albumin and globulin along with casein at pH. 4–7. This maximum represented an average of 76 per cent of the total soluble protein N in a series of normal milk samples, and reasons have been given for considering this maximum as the total of true albumin and globulin present. 5. The conclusion has been reached that the soluble protein fraction of normal fresh milk is composed of approximately 76 per cent albumin and globulin, and 24 per cent proteose-peptone substances.