242 BACTERIA IN CHEESE MAKING. 



ant tastes. There is no doubt that this trouble is due to the 

 development of certain species of bacteria in the cheese. We 

 have learned in an earlier chapter that among the lactic bac- 

 teria there is one type of which B. lactis aero genes is an 

 illustration that has the character of producing large quan- 

 tities of gas (CO 2 , H, etc.). Although the souring of milk 

 is not commonly produced by this group of lactic bacteria, 

 they occasionally become very abundant. 

 It is this group of gas-producing bacteria 

 which is responsible for this trouble of 



swelled cheeses. If the milk is filled 

 chiefly with the common Bad. lactis acidi, 

 which produces no gas, the cheese will 

 A bacillus causing ripen normally, but if there chances to be 



swelled cheese (B. 1 r ,1 i 



shnfferi). large numbers of the gas-producing group, 



the cheese is very liable to develop gas 

 and undergo this abnormal ripening process. Other bac- 

 teria besides B. acrogenes are known to produce swelled 

 cheese (Fig. 36). 



There is no type of abnormal cheese ripening that produces 

 so much trouble as this. No means are known for absolutely 

 avoiding the presence of such gas-producing organisms, for 

 they are liable to appear in milk from unavoidable sources 

 of contamination. Their growth can be, in a measure, 

 checked by the addition of salt to the milk, and this is one 

 of the means suggested for avoiding this type of abnormal 

 ripening. 



A more widely used plan of avoiding this trouble is by 

 testing the milk of individual patrons, and excluding any 

 which is, in the slightest degree, suspicious. This is done by 

 what is called a fermentation test, and is based upon the fact 

 that, at a high temperature, these gas-producing organisms 

 grow rapidly. It is conducted essentially as follows : A 

 small sample of the milk from each patron who furnishes 

 milk to a cheese factory is placed in a vial, warmed up to a 



