CONSTITUENTS OF MILK 153 



Lactic acid begins to be formed in milk soon 

 after it is drawn, if the milk is not kept below 50 

 F. By the time milk reaches the factory, the nor- 

 mal acidity of the milk is usually increased about 

 0.05 to o.io per cent, corresponding to a total 

 acidity of 0.12 to 0.18 per cent. In warm weather, 

 the acidity often exceeds 0.20 per cent in the case of 

 some herds. The increase of acidity over that ex- 

 isting in fresh normal milk is an indication of the 

 temperature at which the milk is kept and also of 

 the cleanliness observed in milking and in caring for 

 the milk and the dairy utensils with which the milk 

 comes in contact (see p. 4). An acidity equivalent 

 to 0.20 per cent of lactic acid in milk when received 

 at the factory is regarded as the danger line for ched- 

 dar cheese-making. Generally, only a part of the 

 milk taken to a cheese-factory will exceed this limit, 

 so that the average for the day may be considerably 

 below the 0.20 per cent limit. 



MILK-ENZYMS 



Enzyms are chemical ferments; they have the 

 power to produce changes in other substances with- 

 out themselves undergoing change. Enzyms are 

 the products of living cells. According to recent 

 views, normal milk is not to be regarded as an in- 

 active fluid, but possesses certain properties char- 

 acteristic of living substances. Normal milk gives 

 evidence of the presence of several different en- 

 zyms, among which are those called (i) diastase, 

 (2) galactase, (3) lipase, (4) catalase, (5) peroxi- 

 dase and (6) reductase. The subject has not been 

 sufficiently studied to enable one to make anything 



