COMMERCIAL TESTING OF MILK AND CREAM I77 



containing any undesirable form of fermentation, is 

 comparatively valueless for direct use, however rich 

 it may be in fat and solids-not-fat. The keeping power 

 of milk depends upon (i) the number and kind of 

 bacteria present and (2) the temperature of the milk. 

 Knowledge of the keeping quality of milk can be 

 gained (i) by knowing the number of bacteria pres- 

 ent, (2) by determining the acidity, (3) by estimating 

 the amount of dirt suspended in milk, and (4) by mak- 

 ing fermentation tests of the milk. Since considerable 

 time and expert skill are required to determine the 

 number of bacteria in milk, the determination of this 

 factor may commonly prove impracticable in the case 

 of ordinary market milk; but the acidity of the milk, 

 the fermentation test and the amount of dirt in sus- 

 pension will afford a satisfactory basis for judging 

 the keeping power of milk. The acidity may be 

 quickly learned by the methods given in Chap. VII. 

 The fermentation test can be made in the manner de- 

 scribed on p. 106. The amount of dirt in milk may be 

 easily estimated as described on p. 109. 



How many points shall be allowed for the keeping 

 quality of milk? Owing to the importance of this 

 quality, it would seem as if it should be given at least 

 35 points for perfection, which should mean entire 

 freedom from dirt in suspension, a total acidity not 

 exceeding. 18 per cent., and no development of gaseous 

 or offensive fermentation. Deduction of points for 

 dirt would have to depend to some extent on the judg- 

 ment. For acidity, there should be deducted one point 

 for each .01 per cent, of acidity above .18. In case 

 the number of bacteria is determined, the milk should 



