156 Dairy Bacteriology. 



From the fact that any condition which restrains or 

 inhibits the growth of micro-organisms has a tendency to 

 improve-the keeping quality of butter, it would appear 

 that the detrimental changes in the quality of butter are 

 due to biological causes. The most common defect 

 known is that usually referred to as rancidity. There 

 are, however, different types of changes that are proba- 

 bly included under this head and it is very probable that 

 different causes are operative in their production. True 

 rancidity is probably due to biological causes; the so- 

 called tallowy change, in which the butter acquires the 

 odor of tallow is probably due to the combined action of 

 light and air on the fat. 



Bacterial defects in butter. There are a number of 

 defects in butter that are positively known to be due to 

 the growth of bacteria in the milk or cream, or in the 

 butter itself. The lack of flavor is looked upon as a de- 

 fect in the case of ripened-cream butter. It may be due 

 to insufficient ripening of the cream, or to the lack of 

 acid-forming bacteria that produce the desirable flavor- 

 forming compounds. Not all acid-forming bacteria are 

 able to produce favorable, flavor-giving compounds; 

 hence, sour cream butter may sometimes be deficient in 

 flavor by reason of this fact. 



Putrid butter. This specific butter trouble has been 

 observed in Denmark, where it was first studied by Jen- 

 sen. Butter affected by it rapidly acquires a peculiar 

 putrid odor that ruins it for table use. Sometimes this 

 flavor may be developed in the cream previous to churn- 

 ing. It may be caused by a number of bacteria. 



Turnip flavored butter. Butter sometimes acquires 

 a flavor resembling turnips. This trouble may be due 

 to the feeding of such roots, the aromatic substances pe- 



