BACTERIA IN CHEESE 



239 



cases, the development of the gas is accompanied by an unusual 

 fermentation and an unpleasant taste and smell. The appear- 

 ance of much gas always injures and sometimes totally ruins 

 the cheese. The cause of the trouble is the development of 

 gas-producing bacteria. Several different species are known, 

 which have this power of developing gas in great quantities in 



f 



* t 



FIG. 6 1 PIECES OF CURDLED MILK 



One curdled by the common Bad. lactis acidi, showing no gas; the other 

 by Bact. aerogencx, filled with gas bubbles that would spoil a cheese 



the ripening cheese. Most of them, perhaps all, belong to the 

 type which have been referred to in Chapter II as Bact. 

 aero genes type. (Fig. 61.) As pointed out in that chapter, the 

 different varieties of this type vary much in the amount of 

 gas they produce; sometimes the quantity is very slight, some- 

 times it is extraordinarily large, and it is easy to understand 

 how different strains and different quantities of bacteria will 

 produce grades of gasiness in cheeses. It is to be remembered 

 that the gas which produces the swelled cheese is a different 

 thing from the gas which produces the normal holes in the Swiss 



OF THE 



UNIVERSITY 



