4 GENERAL CONDITIONS AFFECTING BACTERIA IN MILK 



The percentage constituents of milk may be approximately 

 stated as follows : — 



1. Water ..... 



2. Carbohydrates (lactose, etc.) 



3. Proteids or Albuminoids (casein, albumin, etc.) 



4. Fats ..... 



5. Salts .....* 



84-87 



4-5 

 4.4.5 



3-9 

 0-7 



There are in addition traces of extractives (urea, creatinin, 

 lecithin, cholesterin, etc.), and a small quantity of oxygen, nitrogen, 

 and carbon dioxide. 



Picton and Linder have shown that there is no sharp dividing 

 line between colloidal substances in milk and substances in com- 

 plete solution. But broadly speaking we may represent the various 

 conditions as follows : — The fat is in suspension, the casein in 

 pseudo-solution, the albumin in solution as a colloid, and the 

 lactose in solution as a crystalloid. These four states are probably 

 due to the size of the conglomerates of molecules or particles 

 (Richmond). 



The carbohydrates of milk. — The most important carbohy- 

 drate present in milk is milk-sugar or lactose (C12H29O11+H2O). 

 This is a member of the cane-sugar group and is found in varying 

 quantities in the milk of most, if not all, mammals except the 

 Egyptian buffalo (Richmond and Pappel). It is the most constant 

 constituent of milk, about 5 per cent, being present in human milk 

 and 4 per cent, in that of the cow. It possesses only a faint sweet 

 taste, and is much less soluble than cane sugar, being soluble in six 

 parts of cold, and two or three parts of hot, water. It crystallises in 

 the form of rhombic prisms. Lactose is very resistant to the 

 inverting ferment of yeast and therefore undergoes alcoholic fer- 

 mentation very slowly. It is unacted upon by diastase, rennet, 

 pepsin, and trypsin, but is rapidly inverted by the kephir fungus 

 (lactase). Of all the sugars it is most readily affected by the 

 micro-organisms which are the causal agents in lactic acid ferment- 

 ation. This fermentation occurs in two stages : 



I • C12H22OJ1 + HgO = 4C;,Hg03 



(lactose) (lactic acid) 



2. 2C3H603 = C4H802+2C02+2H2 

 (lactic acid) (butyric acid) 



To this matter we shall have occasion to refer at a later stage. 



A point of some interest and importance is the non-assimila- 

 bility of lactose. Like cane-sugar and maltose, the other two chief 

 members of the cane-sugar group, lactose is incapable of assimila- 

 tion, for when injected into the blood-vessels it appears unaltered 



