184 MODERN METHODS OF TESTING MILK 



Keeping-power is an expression used to indicate 

 in a general way the length of time milk remains 

 sweet and palatable for direct consumption. In esti- 

 mating the commercial value of milk, this is an im- 

 portant factor; since milk that is sour or otherwise 

 unpalatable, or milk containing the products of any 

 form of undesirable fermentation, is comparatively 

 valueless for direct consumption, however rich it may 

 be in fat and other solids. Keeping-power is a matter 

 of much importance in warm weather, especially in 

 the case of milk that is transported long distances 

 before reaching the consumer. 



Testing keeping-power. The keeping-power of 

 milk is tested by making determinations of (i) the 

 acidity (p. 96), (2) the dirt in suspension (p. 117), 

 (3) the fermentation test (p. 114), and (4) the num- 

 ber of bacteria (when practicable). 



Terms describing keeping-power. The terms used 

 in describing the keeping-power of milk are (i) per- 

 fect, (2) acidity, (3) dirt in suspension, (4) unde- 

 sirable fermentations, and (5) number of bacteria per 

 cubic centimeter. 



(1) Perfect. Milk is called perfect in keeping- 

 power (a) when its acidity is not above 0.18 per cent., 

 (b) when it contains no dirt in suspension, (c) when 

 the fermentation test reveals nothing abnormal, and 

 (d) when the number of bacteria does not exceed 

 100,000 per cubic centimeter. 



(2) Acidity is used to mean the amount of appar- 

 ent total acid calculated as lactic, as shown by the 

 amount of alkali neutralized. For discussion of the 

 relation of acidity to the temperature and cleanliness 

 of milk, see pp. 113-114. 



