190 AGRICULTURAL AND INDUSTRIAL BACtERIOLOGY 



or gluten, and increases at the same time the size of the gas 

 bubbles. 



In some eases bacteria have been substituted for yeast. 

 Certain organisms belonging to the genus Bacterium are 

 known to be able to split starch with the formation of car- 

 bon dioxide and hydrogen. "Salt-rising bread" is made 

 by utilizing microorganisms of this type. 



In order to prevent rapid deterioration of the bread after 

 baking it is necessary that a certain amount of acid be pres- 

 ent in the dough. This is usually formed as a result of the 

 growth of certain lactic acid bacteria. Occasionally the acid 

 is added by bakers as such. Proper observance of the 

 acidity improves the texture of the loaf and renders impos- 

 sible the development of ropy or stringy bread. 



Alcohol Production in Other Foods. — More or less alco- 

 holic fermentation may be incident to manufacture by fer- 

 mentation of certain articles of food for man and domestic 

 animals. During the process of fermentation of sauerkraut 

 and in the preparation of ensilage more or less alcohol may 

 be formed. Usually, however, the lactic bacteria outgrow 

 the yeast, and alcoholic fermentation is secondary. 



The Commercial Preparation of Yeast. — Originally it 

 was customary for the housewife or the baker to carry along 

 his own culture of yeast. Such a culture was ordinarily 

 prepared by inoculating a mixture of potato water, sugar, 

 hops, salt, and sometimes other ingredients, with a little of 

 a preceding batch of the material. The culture consisted 

 of a mixture of alcohol-forming yeasts and lactic acid bac- 

 teria. Such a home-made culture is, in the United States, 

 usually termed a starter. In Great Britain it is usually 

 called a barm. 



In the commercial manufacture of yeast, it is necessary to 

 grow large quantities of the yeast plants, free them from 

 the material in which they have been growing, and press 

 them into cakes. This is usually accomplished by seeding a 



