THE MANUFACTURE OF OTHER FERMENTED PRODUCTS. 461 



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 impure 

 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 

 (7o-go) to promote the growth and multiplication of the microorganisms. 

 It is then kneaded, usually with more flour 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 

 and flavorless. It dries out very rapidly and is palatable only when 

 very fresh. 



In the common household method, a smaller amount of yeast is mixed 

 in a thin batter or sponge and allowed to multiply for ten to eighteen 

 hours. This batter is then thickened to a stiff dough and allowed to 

 rise until it doubles its volume. A second kneading is then given and a 

 little more flour added. After rising again for a few hours it is baked. 



Bread made in this way is usually somewhat more open in texture, 

 not so white and with more flavor. It dries less rapidly and remains 

 palatable for two or three days. The difference seems to be due prin- 

 cipally to the action of bacteria on the gluten and other nitrogenous sub- 

 stances in the latter case. The bacterial action, however, is not sufficient 

 to give a perceptible acidity. 



