d 



6 GENERAL CONDITIONS AFFECTING BACTERIA IN MILK 



is not a chemical entity sui generis, it and the fat together con- 

 stituting a highly complex organic substance.^ 



Details concerning the exact chemical constitution of casein, 

 lactalbumin, lacto-globulin, and mucoid are not necessary to our 

 purpose here, which is merely a brief survey of the general con- 

 stitution of milk. Such details may be obtained from any standard 

 work on organic chemistry. 



The fats of milk. — Milk fat consists of palmitin (about 25 per 

 cent), myristin (about 20 per cent), and olein (about 35 per cent.), 

 with small quantities of the tri-glycerides (ethereal salts of gly- 

 cerol), of butyric, caproic, caprylic, capric, stearic, and other fatty 

 acids. The amount of fat in cream varies from 15 to 50 per cent. 

 In butter, besides fat, there are small quantities of proteid and 

 lactose, and it is important to remember that the fat itself is char- 

 acterised by the presence in considerable quantity of glycerides of 

 the fatty acids, the most important being butyric acid. The fat of 

 cow's butter averages 68 per cent of palmitin and stearin, 30 per 

 cent of olein, and 2 per cent, of the specific butter fats. Their 

 melting point is 31-34° C. The composition of good butter is, 

 however, variable. It should not contain more than about 12-14 

 per cent of water. The amount of butter fat in butter prepared 

 from cow's milk is about 85 per cent, and there are, of course, small 

 quantities of casein and salts. 



Fat, as is stated above, occurs in milk in the form of small 

 globules, varying in size from -oi/z to -ooidyu in diameter. Accord- 

 ing to Rothschild a drop of milk not larger than a pin's head 

 contains 1,500,000 separate fat globules. Milk fat is not soluble 

 in water, but owing to its fine division is easy of digestion. Its 

 decomposition is the first step in the rancidity of butter, which is 

 set up by fat being split up into fatty acids and glycerol. 



As, of course, is perfectly well known, the fat of milk, being 

 lighter than the water, rises to the surface as cream whenever milk 

 is allowed to stand. Such cream may be skimmed off, leaving 

 behind skim milk which, although being milk devoid of its cream, 

 still retains a small percentage of fat By centrifugal separation 

 the fat of milk may be almost wholly removed. The result is 

 known as separated milk. Forty-five to fifty per cent of fat in 

 cream has been found to be the normal (Vieth, Richmond, etc.). 



By a simple experiment Lloyd - has shown that it is the large 

 globules of fat which form the cream. He took a vessel 15 inches 



^ Brit. Med. Jour., 1898, vol ii., p. 779. 



^ Jour, of Bath and West of England Society, vol. xii. (1902), p. 129. 



