336 PHYSIOLOGICAL CHEMISTRY 



cellular activity of the mammary gland. It has strong reducing power, 

 is dextro-rotatory and forms an osazone with phenylhydrazine. Lac- 

 tose is not fermentable by the ordinary baker's yeast. For changes 

 which lactose undergoes in lactic acid fermentation see page 330. The 

 crystalline form of lactose is shown in Fig. 109. 



Casein, the principal protein constituent of milk, belongs to the 

 group of phosphoproteins and contains 0.7 per cent of phosphorus. 1 

 It has acidic properties and combines with bases to produce salts. 2 

 It is probably present in milk in the form of neutral calcium caseinate 

 (Casein Ca 4 ). 3 It is not coagulable upon boiling and is precipitated 

 from its neutral solution by certain metallic salts as well as upon satu- 

 ration with sodium chloride or magnesium sulphate. Its acid solu- 

 tion is precipitated by mineral acid. 



Lactalbumin and lacto-globulin, the protein constituents of milk, 

 next in importance to casein, closely resemble serum albumin and 

 serum globulin in their general properties. 



Butter (milk fat) consists in large part of olein and palmitin. 

 Stearin, butyrin, caproin and traces of other fats are also present. 

 An important growth-promoting substance (vitamine) called "Fat 

 Soluble A" is also present in butter fat. 4 When butter becomes 

 rancid through the cleavage of certain of its constituent fats by bacteria 

 the odors of caproic and butyric acids are in evidence. 



The pigment of the fat of cow's milk is made up of carotin and 

 xanthophylls . The principal pigment is carotin, an unsaturated 

 hydrocarbon pigment which is widely distributed in plants. 5 The 

 pigment of the fat of human milk is made up of carotin and xantho- 

 phylls in about equal proportions. Carotin is also probably the 

 pigment of human fat. The pigment of body fat, blood serum, corpus 

 luteum and skin secretions of the cow is principally carotin. 



Colostrum is the name given to the product of the mammary gland 

 secreted for a short time before parturition and during the early period 

 of lactation (see Fig. 104, page 331). It is yellowish in color, contains 

 more solid matter than ordinary milk, and has a higher specific gravity 

 (1.040-1.080). The most striking difference between colostrum and 

 ordinary milk is the high percentage of lactalbumin and lacto-globulin 

 in the former. This abnormality in the protein content is responsible 

 for the coagulation of colostrum upon boiling. 



Such enzymes as lipase, amylase, galactase, catalase, oxidases, 



1 Bosworth and Van Slyke: Jour. Biol. Chem., 19, 67, 1914. 



2 Van Slyke and Bosworth: Jour Biol. Chem., 14, 207-227, 1914. 

 Van Slyke and Bosworth: Jour. Biol Chem., 20, 135, 1915, 



4 See Experiment i, page 580. 



6 Palmer and Eckles: Jour. Biol. Chem., 17, 191, 1914. 



