PHYSIOLOGICAL VARIATIONS 255 



tation, has its reason in the physiology of the bacteria, and in their 

 reaction upon the environment. 



Cider is acid, and acid is not well suited for the growth of most 

 bacteria. The vinegar bacteria can grow in fruit juices, and a few other 

 bacteria, especially those causing trouble in wine, are not retarded by 

 fruit acids, but the common types attacking proteins and causing 

 organic decay are not able to grow on fruits. Yeasts, however, and 

 molds thrive well only in acid media. They can exist in neutral 

 solutions if in pure culture, but in nature they are easily crowded out 

 by bacteria. Acidity of the medium is therefore one of the most 

 important factors regulating the type of microbial decomposition. 

 This principle is commonly utilized by preserving foods of all kinds in 

 vinegar, and by making butter from sour cream rather than sweet 

 cream; the keeping qualities of hard cheeses depend upon their acid 

 content. 



In acid environment, the two most common types of decomposition 

 are oxidation, complete or incomplete, and alcoholic fermentation. 

 The oxidation is brought about by molds or organisms closely allied to 

 yeasts. The latter are very common on all sour foods, especially on 

 foods containing lactic acid, such as cottage cheese or sauerkraut. 

 The kind of acid decides the type of mold; wherever there is lactic acid, 

 there is Oidium, while malic and tartaric acids favor Penicillium and 

 Aspergillus. 



If the decaying materials contain no acid, the type of decomposi- 

 tion depends mainly on the presence or absence of carbohydrates, 

 especially sugar. It is an old experience, recently verified through 

 a large number of experiments by Kendall and Walker, that practically 

 all bacteria will decompose sugar in preference to proteins. If a leaf 

 contains sugar and protein (cabbage) the sugar decomposition will be 

 conspicuous, and the protein is not attacked very readily. Putrefac- 

 tion in the presence of sugar or of acid does not take place. Meat will 

 not putrefy if mixed with sugar, while milk putrefies readily if the sugar 

 is removed by dialysis. The three types of sugar decomposition which 

 come into consideration in neutral media, are the lactic, the acid-gas and 

 the butyric fermentations. The latter is a strictly anaerobic fermenta- 

 tion, and thus limited to special conditions. Of the other two, the acid- 

 gas fermentation is the most common, and the souring of vegetables 

 of all kinds is due to this type of fermentation (pickles, sauerkraut, 



