MICRO-ORGANISMS AND ENZYMS 295 



The lactic acid fermentation we have been con 

 sidering is what we may call the normal form, the 

 particular form we desire to have present in milk in 

 cheese-making. Their presence is insured by the use 

 of good starters (p. i8). Milk in which this form 

 of lactic fermentation has occurred produces, in 

 souring, a firm curd free from gas bubbles and with 

 only a little whey on the surface. When agitated, 

 the curd breaks apart readily into small particles, 

 which settle slowly and leave a clear whey. The 

 milk should have a pleasant, clean, acid taste, en- 

 tirely free from anything resembling a tainted flavor. 

 So far as we know, the lactic acid bacteria belonging 

 to this normal group never form products of a poison- 

 ous character. 



GAS-PRODUCING BACTERIA 



Some of the bacteria that decompose milk-sugar 

 with formation of lactic acid are usually grouped with 

 the lactic acid bacteria, though they possess distinguish- 

 ing characteristics which mark them as abnormal, so 

 far as their behavior in cheese-making is concerned. 

 While they decompose milk-sugar and produce lactic 

 acid, they produce other products besides, especially 

 gases ; they may also produce volatile products that 

 are offensive. These bacteria are responsible for many 

 of the defects in cheese (pp. 116-130). When gas- 

 producing ferments are present in milk, they are 

 usually responsible for increased losses of fat in the 

 cheese-making process. 



DIGESTING BACTERIA 



A large group of bacteria curdle milk without sour- 

 ing it and then slowly digest or dissolve the curd; 



