26 CHANGES IN MILK CAUSED BY BACTERIA. 



Preservation by chemicals. Bacterial changes in milk or 

 cream may be retarded by the addition of various' chemicals, 

 such as borax, formalin, and others. They are substitutes 

 for cleanliness and refrigeration, besides being undesirable 

 constituents of an article of food. Their use in milk has been, 

 and needs to be, very vigorously opposed by pure food laws. 

 The campaign against the use of milk preservatives has been 

 very effective. The result has been an increase in the use 

 of another method for compensating for deficiencies in the 

 care of milk, that is, commercial pasteurization. Budde has 

 suggested a method of preserving milk by the addition of 

 hydrogen peroxide, which is ver}^ readily removed from milk 

 by heating. The scheme has not received serious consideration. 

 The matter of the use of preservatives and their detection, is 

 discussed in Chap. XI. The changes in milk constitute a 

 complex biochemic problem, but certain of the more common 

 ones may be discussed in a systematic manner. 



Lactic acid fermentation. The souring of- milk is the mani- 

 festation of the commonest of fermentative changes in milk. 

 As a result of bacterial activities the lactose is transformed 

 into lactic acid, which compound is responsible for the char- 

 acteristic flavor and curdling of soured milk. A considerable 

 number of bacteria are capable of inducing the change, but 

 it has been shown that a very small group of organisms are 

 ordinarily found in milk when souring is well advanced. In 

 fact, one of these, Bad. lactis acidi, may be regarded as 

 the typical lactic acid organism. It has been found in nu- 

 merous examinations made of milk from widely separated 

 quarters of the United States. Its identity with the lactic acid 

 organism described by L,eichmann in Europe has been estab- 

 lished and this fact adds strength to the view that it may be 

 regarded as the common lactic acid organism. This organism 

 has been classified as a rod form (bacterium) because the indi- 

 vidual cells are usually longer than broad. It forms chains 

 of greater or less length and in bouillon culture sometimes 

 appears as chains of undoubted micrococci. Streptococcus lac- 

 ticus Kruse is another widely distributed organism observed 

 to predominate in souring milk. Some writers have insisted 



