552 MICROBIOLOGY OF SPECIAL INDUSTRIES 



considered desirable. The yeast is sometimes spoiled by bacteria 

 causing sliminess or producing butyric acid, but this does not occur 

 when the process is properly carried out. 



Bread. — The rising of the dough to which the lightness and porosity 



of bread are due is caused by the production of carbon dioxide by yeast 



, fermentation. The yeasts are always accompanied by bacteria and 



the character of the bread is determined in great part by the extent of 



bacterial fermentation. 



If we make a dough of flour and water and allow it to stand in a warm 

 place it will rise slowly. Yeasts and bacteria, occurring naturally in 

 the flour and watet, are the causes. Bread is sometimes made in this 

 way (Graham bread, salt-rising bread). The rising is more or less 

 uncertain and the flavor and acidity of the bread very variable, owing 

 to differences in the kind and degree of bacterial action. Many yeasts 

 and a large number of bacteria have been isolated from the spontaneous 

 fermentation of dough. Among the bacteria are forms producing lactic 

 ' and acetic acids, others which dissolve gluten and transform starch into 

 sugar and others which produce alcoholic fermentation with evolution 

 of carbon dioxide. 



Usually the dough is leavened by incorporating more or less im- 

 pure yeast. Bread yeast may be prepared by allowing a culture 

 medium composed of water, sugar, hops and potatoes with a little salt 

 to ferment spontaneously, but the results are uncertain. 



Usually, in the United States, compressed yeast is employed. In 

 some cases the yeast from breweries is used. In most parts of Europe 

 a leaven made from a piece of dough kept over from the last baking 

 is preferred. 



In a general way, the process consists in making a thick dough or 

 thinner sponge by thoroughly mixing the flour with water, yeast or 

 leaven and a little salt. This mixture is then allowed to stand in a 

 warm place (20°-30°) to promote the growth and multiplication of the 

 microorganisms. It is then kneaded, usually with more Ao\ii and 

 put aside to rise. This kneading with fresh flour and rising may be 

 repeated several times. 



If a large quantity of a relatively pure yeast is used, the rising will be 

 rapid, there will be little bacterial action and only one kneading is neces- 

 sary. This is the method commonly employed in the bakeries in the 

 United States. Bread made in this way is usually of fine' grain, white 



