114 MODERN METHODS OF TESTING MILK 



been kept. The cleanliness of the milk-pails and other 

 vessels and utensils with which the milk comes in con- 

 tact is another important factor in influencing acidity. 

 The average acidity of English market milk, sup- 

 posed to be 12 to 18 hours old, is .18 per cent, and of 

 German milk, .13 to .18 per cent. Market milk should 

 not, in any case, contain over .2 per cent of total acid- 

 ity when it reaches the consumer, and should generally 

 be under .15 per cent. This (.2 per cent.) is also the 

 highest limit allowable for milk that is to be used for 

 cheese-making. The test for acidity can thus be made 

 a very useful indication of the bacterial condition of 

 milk so far as it relates to the acidity, and to the 

 abundance of those forms that produce lactic acid. 



THE FERMENTATION OR WISCONSIN 

 CURD-TEST 



Milk frequently contains objectionable forms of 

 organisms or ferments that are not made perceptible 

 by ordinary methods of observation. The condition 

 arises particularly in milk used for cheese-making and 

 may result in serious injury to the quality of the cheese. 

 The Wisconsin Experiment Station (Wis. xp. Sta. 

 I2th and I5th Annual Reports, 1895 and 1898) has 

 applied certain principles to the development of a 

 test that enables one to identify milk containing cer- 

 tain forms of undesirable ferments likely to do serious 

 injury. This method is based, in general, upon the 

 plan of making conditions favorable for the rapid de- 

 velopment of the ferments present in milk. 



Apparatus. The apparatus consists of the follow- 

 ing parts: (i) Pint glass jars or tin cans with covers; 



