204 AGEICULTURAL AND INDUSTRIAL BACTERIOLOGY 



acid bacteria. Usually rennet acts rather more rapidly and 

 more satisfactorily upon milk in which a slight amount of 

 lactic acid has developed. Bacteria of the group Strepto- 

 coccus lacticus and particularly Lactobacillus caucasicus 

 and related species rapidly transform any residual lactose 

 in cheese into lactic acid. The other changes incident to 

 cheese ripening will be discussed in another connection. 



LACTIC ACID IN FOOD PRESERVATION 



Ensilage. Ensilage or silage is prepared by chopping 

 or shredding various green foods and packing them com- 

 pactly into an air-tight structure termed a silo. In general 

 the food plants most used for a silo are those which contain 

 considerable quantities of sugar when green and of starch 

 when mature; such, for example, are corn and sorghum. 

 Other fodder plants may be ensiled but unless the content 

 of sugar or starch is sufficiently high, abnormal or putre- 

 factive changes may occur. This may be obviated by mix- 

 ing with corn or with sorghum. Such, for example, would 

 be silage made from mixtures of Indian corn and soy beans 

 or cow peas. 



The most essential change occurring in the ensilage is 

 the transformation of a part of the carbohydrates present 

 into lactic acid. Carbon dioxide, acetic acid, traces of 

 butyric and other acids and sometimes alcohol may be 

 formed. The lactic acid accumulates in quantities sufficient 

 to prevent the development of putrefactive or other unde- 

 sirable microorganisms, providing conditions are satisfac- 

 tory. 



For proper fermentation in the preparation of silage it 

 is necessary to have, first, material properly prepared by 

 shredding; second, the ensilage packed tightly into an air- 

 tight container or silo so as to leave as little air space as is 

 practicable; third, the presence of an optimum amount of 

 moisture, and fourth, the presence of suitable bacteria. 



