MILK AND ITS PRODUCTS 151 



The butyric fermentation is rarely noticeable during 

 the early stage of cream ripening and its subsequent 

 development in a highly acid cream is explained by 

 Russell as being "probably due, not so much to the pres- 

 ence of lactic acid, as to the absence of dissolved oxygen, 

 which at this stage has been used up by the lactic acid 

 organisms." 



Butter that is apparently good in quality when freshly 

 made, will usually turn rancid when kept at ordinary 

 temperatures a short time. The quickness with which 

 this change comes is dependent largely upon the amount 

 of acid present in cream at the time of churning. Butter 

 made from cream in which the maximum amount of acid 

 consistent with good flavor has been developed, usually 

 possesses poor keeping quality. This seems to indi- 

 cate that at least part of the rancidity that develops in 

 butter after it is made is due to the butyric acid bacteria, 

 while light and air, doubtless, also contribute much to 

 this end. 



ABNORMAL FERMENTATIONS. 



No trouble needs to be anticipated from these fermenta- 

 tions so long as cleanliness prevails in the dairy. The 

 bacteria that belong to this class are usually associated 

 with filth, and dairies that become infested with them 

 show a lack of cleanliness in the care and handling of the 

 milk. Since milk is frequently infected with one or 

 another of these abnormal fermentations a brief discus- 

 sion will be given of the most important. 



i. BITTER FERMENTATION. 



Bitter milk and cream are quite common and there are 

 several ways in which this bitterness is imparted : it may 



