RIPENING PROCESSES OF CHEESES 135 



of fat hydrolysis and ammonia formation to which the production 

 of the sharp taste (not to be confused with the salt taste) is due 

 proceed with extreme slowness in these cheeses. Both of these 

 processes seem also in the case of hard cheeses to start at the 

 surface $nd gradually penetrate inwards. 



While the fat is usually slowly decomposed in cheese, the 

 lactose is the first constituent to undergo change. In hard 

 cheeses it usually disappears in a few days and in soft cheeses in 

 a week or two. Normally it is completely converted into lactic 

 acid, which is neutralised by the lime and other bases present. 

 Calcium lactate is not necessarily the end product, for it is quite 

 frequently more or less completely converted into propionic acid by 

 the fermentation process described on p. 41, whereby the normal 

 " eyes " or cavities are produced in cheese. The butyric acid 

 fermentation of calcium lactate however, must be looked on as 

 a disease of cheese, for the evolution of gas will be too vigorous, 

 while unpalatable or even poisonous substances may be produced. 

 The author's researches on the volatile acids of cheese, summarised 

 in the accompanying table, show that normally no more butyric 

 acid is found in the ordinary rennet cheeses than will originate 

 from fat hydrolysis. 



After these general remarks, we may pass on to consider the 

 microorganisms which play the chief part in the ripening of the 

 different kinds of cheese. The flora of the hard rennet cheeses 

 will be dealt with first. Duclaux, who was the first to investigate 

 this field, found in cheese various sporing rod bacteria which he 

 named Tyrothrix, i.e., " cheese threads." He found both aerobic 

 and anaerobic forms, i.e., what now would be called hay and potato 

 bacilli and anaerobic putrefactive bacteria. As the aerobic forms 

 were able to decompose and dissolve casein, and the anaerobic forms 

 produced the odour characteristic of cheese, Duclaux and all other 

 contemporary investigators were agreed that the ripening of 

 cheese was due to the combined action of these bacteria 1 . The 

 constant success of Duclaux in isolating Tyrothrix bacteria from 

 cheese was due to the fact that he employed an enrichment 

 method which particularly favoured the growth of these bacteria. 

 He introduced a small piece of cheese into broth in which the 

 lactic acid bacteria did not thrive, and it was therefore only 

 necessary to have a few Tyrothrix spores present from the outset 

 in order that a Tyrothrix film should form on the surface of the 

 liquid after some time, after which the anaerobic sporing bacteria, 

 protected from the air, could grow rapidly. 



The careful researches carried out by Freudenreich over a period 



1 Duclaux, "Le Lait," Paris, 1894, pp. 213 258. 



