2CX3 ECONOMIC BACTERIA IN MILK AND MILK PRODUCTS 



of this great increase was found to be composed of the peptonis- 

 ing species. Russell has pointed out that "tides" of organisms 

 occur in cheese-ripening much in the same way as we have seen 

 to occur in milk and cream. He points out that there is always 

 at first a marked increase in the number of micro-organisms 

 which is soon followed by a more gradual decline. While the 

 casein-digesting and gas-producing species suffer a more or less 

 rapid decline, the lactic acid species develop to an enormous 

 extent, from which fact it would appear that cheese offers favour- 

 able conditions for the development of the latter, and the latter 

 by their acid production, create unfavourable conditions for the 

 former. Russell divides the ripening process, as regards bacterial 

 content, into the three following divisions : — ^ 



(i) Period of initial bacterial decline in cheese. — Where the 

 green cheeses were examined immediately after removal from 

 the press, it was usually found that a diminution in numbers had 

 taken place. This period of decline lasts but a short time, not 

 beyond the second day. Lower temperature (than that present 

 in the milk) and expulsion of the whey would account for this 

 general decline in all species of bacteria. 



(2) Period of bacterial increase. — Soon after the cheese is removed 

 from the press a most noteworthy change takes place in green 

 cheese. A very rapid increase of bacteria occurs, confined almost 

 exclusively to the lactic acid group. This commences in green 

 cheese about the eighth day and continues more or less for twenty 

 days. In Cheddar cheese it commences about the fifth day, 

 reaches its maximum about the twentieth day, declines rapidly to 

 the thirtieth day, and gradually for a hundred following days. 

 During the first forty days of this period the casein-digesting and 

 gas-producing organisms are present and at first increasing, but 

 relatively to only a slight degree. With the increase in the number 

 of micro-organisms the curd begins to lose its elastic texture, and 

 before the maximum number of bacteria is reached the curing is 

 far advanced. Freudenreich has shown, as we have already stated, 

 that acid inhibits the growth of the casein-digesting microbes ; 

 and in cheese as in milk it is found that the maximum develop- 

 ment of the lactic acid bacteria marks the practical elimination of 

 the peptonising organisms although small traces may persist for 

 some time. 



(3) Period of final bacterial decline. — The maximum development 



^ Thirteenth Annual Report of Agricultural Expt. Sta., Univ. of Wisconsin, 

 1896, p. 105. 



