~Rancid change in butter.~ Fresh butter has a peculiar aroma that is very desirable and one that enhances the market price, if it can be retained; but this delicate flavor is more or less evanescent, soon disappearing, even in the best makes. While a good butter loses with age some of the peculiar aroma that it possesses when first made, yet a gilt-edged product should retain its good keeping qualities for some length of time. All butters, however, sooner or later undergo a change that renders them worthless for table use. This change is usually a rancidity that is observed in all stale products of this class. The cause of this rancid condition in butter was at first attributed to the formation of butyric acid, but it is now recognized that other changes also enter in.[171] Light and especially air also exert a marked effect on the flavor of butter. Where butter is kept in small packages it is much more prone to develop off flavors than when packed in large tubs. From the carefully executed experiments of Jensen it appears that some of the molds as well as certain species of bacteria are able to incite these changes. These organisms are common in the air and water and it therefore readily follows that inoculation occurs. Practically, rancidity is held in check by storing butter at low temperatures where germ growth is quite suspended. ~Lack of flavor.~ Often this may be due to improper handling of the cream in not allowing it to ripen far enough, but sometimes it is impossible to produce a high flavor. The lack of flavor in this case is due to the absence of the proper flavor-producing organisms. This condition can usually be overcome by the addition of a proper starter. ~Putrid butter.~ This specific butter trouble has been observed in Denmark, where it has been studied by Jensen.[172] 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. Jensen found the trouble to be due to several different putrefactive bacteria. One form which he called _Bacillus foetidus lactis_, a close ally of the common feces bacillus, produced this rotten odor and taste in milk in a very short time. Fortunately, this organism was easily killed by a comparatively low heat, so that pasteurization of the cream and use of a culture starter quickly eliminated the trouble, where it was tried. ~Turnip-flavored butter.~ Butter sometimes acquires a peculiar flavor recalling the order of turnips, rutabagas, and other root crops. Often this trouble is due to feeding, there being in several of these crops, aromatic substances that pass directly into the milk, but in some instances the trouble arises from bacteria that are able to produce decomposition products,[173] the odor and taste of which strongly recalls these vegetables.