RIPENING OF CHEESE. Ill 



rennet,* whereby it remains sweet, or less frequently by the 

 application of lactic acid (sour milk). The gradual decom- 

 position of the curd is due to a slight extent to the action of 

 pepsin contained in rennet, as well as to an enzyme (Galactase, 

 Babcock and Russell), although, according to O. Jensen, this 

 is precipitated from milk by bacteria at a very early stage. 

 The action is, however, mainly due to the rich flora of micro- 

 organisms embedded in the cheese. These are derived partly 

 from the rennet, and partly, and, indeed chiefly, from the 

 milk itself, and consist of lactic acid bacteria, peptonising 

 bacteria, butyric acid bacteria, and moulds. 



The basis of bacteriological work on cheese was laid by 

 Cohn, Duclaux, and Benecke, and their researches have been 

 extended by Adametz, Freudenreich, Weigmann, 0. Jensen, 

 and Harding. The main lines of this development must be 

 regarded, according to these workers, as a modification of the 

 albuminoids of the curd brought about during the first short 

 period by peptonising bacteria bacteria that had been active 

 in the milk. Amongst these must be especially mentioned 

 Micrococcus casei liquefaciens, which occur in great numbers. 

 It multiplies at lower temperatures than the true lactic acid 

 bacteria, and, therefore, is found in large numbers in the cooled 

 milk, and is thus transferred to the cheese. It coagulates 



* Rennet is an enzyme which decomposes casein into paracasein and whey 

 albumen. It is secreted in special glands of the stomach of various animals (e.g., 

 the ruminants) ; calves' stomachs are used for the preparation of rennet. In 

 the vegetable kingdom this enzyme is widely distributed. It is found, for example, 

 in Pinguicula, Ficus carica, Galium vervm, in the calyx of the artichoke (Cynara 

 scolymus), and in many bacteria. In 1892, Conn isolated an enzyme resembling 

 rennet from bacteria which were isolated from cream, and completely lique- 

 fied gelatine. They produced the enzyme most rapidly and freely at about 

 20 C. He isolated it from the filtrate of a ten-day-old milk culture. The filtrate 

 was acidified with O'l per cent, of sulphuric acid, and then mixed with an excess 

 of salt. A white foam separated out, which contained the comparatively pure 

 enzyme. The dry foam formed a white powder. This enzyme also occurs in the 

 ubiquitous putrefactive bacterium, Bacillus vulgaris (Prot. vulg.). It further 

 occurs in Bad. prodigiosum (the bacterium of the Bleeding Host), and also in Bac. 

 coli communia, which is always found in the intestines of men and animals, as 

 well as in many of the " potato bacilli." Finally, it has been detected in torula 

 species (Lactomyces), in different species of moulds (e.g., Aspergillus, Monilia), 

 and in certain yeast species. 



