198 ECONOMIC BACTERIA IN MILK AND MILK PRODUCTS 



milk, others are inhibited by salting, and yet others find an un- 

 favourable nidus in butter. Reference will be made subsequently 

 {see p, 220) to the presence of B. tuberculosis and its allies in 

 butter. Various pathogenic organisms have been found but only 

 rarely. Butyric fermentation and rancidity are more common. 

 The latter condition is probably due to micro-organisms (possibly 

 Oidiuni lactis and B. fluorescens liquefaciens and its peptonising 

 allies). Putrid butter is caused by various putrefactive bacteria, 

 vcioXViAva^ B.foetidus lactis (Jensen). Lardy and oily butters have 

 been investigated by Storch and Jensen, and traced to bacteria. 

 Bitter butter is due to fermentative changes. 



Bacteria in Cheese-making 



Cheese is made by the precipitation of casein, in the form of 

 a curd. This is usually done by adding rennet to milk ; but 

 the precipitation may also be accomplished by allowing acid to 

 develop in the milk, such lactic acid producing curdling. The 

 former method is that usually adopted, though some sour milk 

 or cottage cheeses are made by the latter process. After the 

 curd is produced it is collected in moulds for shaping and press- 

 ing, and is eventually set aside for ripening. It is the ripening 

 process which gives to cheese its characteristic flavour, and upon 

 which depends its marketable value. It has been suggested that 

 bacteria, contained in the rennet, cause the ripening, and that 

 thus rennet is responsible for the primary precipitation as well 

 as the secondary fermentation of ripening. That bacteria per- 

 form the major part of the ripening process, and are essential to 

 it, is proved by the fact that when they are either removed or 

 opposed, the curing changes immediately cease. If, for example, 

 the milk itself be sterilised (Freudenreich), or if antiseptics, such as 

 thymol, be added (Adametz), the results are negative. Rennet 

 exerts a digestive effect on casein in cheese, due to the presence of 

 peptic enzymes contained in rennet extracts, the action of which 

 is intensified by the development of acid in the curd. But as far 

 back as 1875, F. Cohn held that the ripening of cheese was a true 

 fermentation, due to organisms present in the rennet (termed 

 " lab bacilli "), and several years later Duclaux isolated from 

 Cantal cheese a number of species of the Schizomycetes which he 

 classified under the generic term of Tyrothrix, and associated with 

 that group of bacilli of which the type is B. subtilis. Since 

 Duclaux's work many investigations have been made. 



