BACTERIA AND MILK FERMENTATIONS 25 
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 deperident 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 
