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 probably 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 defect 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 Jensen. 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 churning. 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 peculiar to them being absorbed directly by the milk and thus transferred to the butter. Weigmann traced a similar flavor to certain bacteria that entered the milk from barn filth. =Cowy odor in butter.= There is sometimes to be noted an odor in butter as in milk that resembles that of the cow stable. Usually this defect has been ascribed to the absorption of these odors directly by the milk. Organisms have also been described that impart to the butter a very similar odor. Bitter butter may be due to the feed that is consumed by the cow, or it may be due to those forms of bacteria that produce a bitter fermentation of the milk.