MILK. 221 



the agency of the bacterium lactis. Putrefactive bacteria in the 

 alimentary canal may bring about this same reaction. Lactose is 

 not fermentable by pure yeast. It was recently claimed that lac- 

 tosin, a new carbohydrate, had been isolated from milk. 



Caseinogen, the principal protein constituent of milk belongs to 

 the group of phosphoproteins. It has acidic properties and com- 

 bines with bases to produce salts. It is not coagulable upon boiling 

 and is precipitated from its neutral solution by certain metallic salts 

 as well as upon saturation with sodium chloride or magnesium sul- 

 phate. Its acid solution is precipitated by an excess of mineral acid. 



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

 next in importance to caseinogen, closely resemble serum albumin 

 and serum globulin in their general properties. According to Wrob- 

 lewski, a protein called opalisin is also present in milk. 



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. 74, p. 219). 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 differ- 

 ence 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 colos- 

 trum upon boiling. 



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

 peroxidases and reductases have been identified in milk, but not 

 all of them in milk of the same species of animal. 



Among the principal preservatives used in connection with milk 

 are formaldehyde, hydrogen peroxide, boric acid, borates, salicylic 

 acid and salicylates. 



EXPERIMENTS ON MILK. 



1. Reaction. Test the reaction of fresh cow's milk to litmus. 



2. Biuret Test. Make the biuret test according to directions 

 given on page 92. 



3. Microscopical Examination. Examine fresh whole milk, 

 skimmed or centrifugated milk and colostrum under the microscope. 

 Compare the microscopical appearance with Fig. 74, page 219. 



4. Specific Gravity. Determine the specific gravity of both 

 whole and skimmed milk (see p. 218). Which possesses the higher 

 specific gravity? Explain why this is so. 



5. Film Formation. Place 10 c.c. of milk in a small beaker 



