568 MICROBIOLOGY OF FOODS 



flavor and texture than bread made by the straight method and the 

 bread remains fresh for a longer period. Where dry yeast cakes are 

 used, the sponge method is to be preferred. 



In many European countries a "sour dough ' : starter is used 

 to leaven the loaf. According to the French method described 

 by Boutroux* a little of the dough ready for baking is set aside and 

 mixed with flour and water and permitted to stand four or five hours. 

 This operation is repeated several times before the " leaven" is ready 

 for mixing with the dough. The repeated addition of flour invigorates 

 the yeast by supplying it with food, thus maintaining active yeast 

 growth, which tends to discourage the growth of less desirable ferments. 

 Fresh dough with about one-third its bulk of leaven is mixed for the final 

 baking and fermentation is allowed to proceed again for a short time. 

 In spite of the repeated additions of flour during preparation of the 

 leaven there is a rapid growth of lactic and^other bacteria resulting in 

 the development of the acid and characteristic flavor of genuine 

 French bread. The long fermentation results in considerable softening 

 of the gluten, causing large holes to form in the dough, and in marked 

 diastasic action which also affects the texture of the loaf. Imitation 

 "French bread" as usually sold in the United States is made with 

 compressed yeast and does not resemble true French bread in flavor. 



Sour dough breads are common in Italy and southern Europe. 

 Usually the fermentation is less carefully controlled than in France, 

 imparting a more acid and to the American taste, a less agreeable flavor. 



In some localities an impure natural yeast is prepared for bread 

 making. Flour, hops, hot water, and ground malt, are mixed together, 

 and allowed to stand until the diastase converts most of the starch to 

 sugar. The sweet liquid is drawn off and allowed to ferment. The 

 yeast so grown is known as " virgin barm." If a starter of yeast from a 

 previous lot is added, the yeast is then termed u Parisian barm." The 

 sweet liquid, according to Jago, is of about 14 Brix when freshly 

 prepared. The hops serve to check the growth of undesirable bacteria. 

 Nevertheless, the barm is a mixture of several yeast varieties and 

 bacteria. 



^The fermented liquid and yeast are mixed with the flour in making 

 up the dough. 



The Mexican tortilla contains no leaven and is baked as soon as 



* Boutroux, L. Le pain et la panification. 



