358 MICROBIOLOGY OF MILK AND MILK PRODUCTS. 



the same products as they would have done in the uncurdled milk. All 

 abnormalities of the cheese so far as they are occasioned by bacteria are 

 due to the abnormal flora of the milk. To the raw material the maker 

 must direct his attention if a fine product is to be prepared. 



GASSY CHEESE. The most frequent trouble encountered and the 

 one of greatest economic importance is the fermentation caused by organ- 

 isms belonging largely to the B-. coli- aero genes group. It has been seen 

 that these produce in milk gases, such as carbon dioxide and hydrogen, 

 and offensive smelling and tasting compounds. In cheese similar com- 

 pounds are formed by these organisms; the gas causes the more or less 

 abundant formation of holes which give to the cheese judge an indication 

 of what may be expected with reference to flavor. All milk contains some 

 of the gas-forming organisms, but it is only when they are numerous that 

 marked injury is done. 



Gassy cheese may also be due to the presence of lactose-fermenting 

 yeasts which are usually found in milk in such small numbers that they 

 cannot compete with the lactic bacteria in the fermentation of the sugar 

 in the cheese. At times the number may be increased to such an extent 

 that the major part of the sugar is fermented by them, alcohol and carbon 

 dioxide being produced. An outbreak of gassy Swiss cheese was found 

 by Russell and Hastings to be due to such yeasts that had gained entrance 

 to the milk from the whey-barrels because of careless washing of the 

 milk cans. The cheese makers of the country are realizing the impor- 

 tance of the contamination of the milk from the transportation of whey 

 and milk in the same can. The most practical means of preventing 

 trouble from this practice is to heat the whey to 68 as it passes from the 

 cheese vat to the storage tank. This temperature destroys the harmful 

 microorganisms, and if the storage tank is kept in a sanitary condition 

 the whey is sweet when returned to the farm in the milk can. It has 

 been demonstrated that such a treatment of the whey results in a marked 

 improvement in the quality of the product. 



MISCELLANEOUS ABNORMALITIES OF CHEESE. Bitter cheese is 

 produced by bacteria that form a bitter principle. An outbreak of 

 bitter cheese investigated by Hastings was found to be due to the replace- 

 ment of the normal acid-forming flora by a lactic organism which pro- 

 duced such an intense bitterness as to mask the acid taste in the milk 

 and cheese. 



