MICROBIOLOGY OF FERMENTED FOODS 567 



tors. Other bakeries use stationary ovens of various types and handle 

 the bake pans by hand labor. 



During baking, the gas pockets in the dough expand with the 

 heat and increase the size of the loaf. The gluten is coagulated by 

 the heat and thus retains the size and shape of the gas bubbles at the 

 moment of coagulation. The yeast activity is for a short time stimu- 

 lated but the cells are killed when the dough reaches 60. Peptic 

 activity is for a time stimulated, resulting in some softening of the 

 gluten. Diastase increases in activity until 65 to 70 is reached and 

 then decreases until the temperature is reached which destroys this 

 enzyme. It hydrolyzes or gelatinizes some of the starch on the surface 

 of the loaf. Heat dextrinizes part of the starch in this locality also, and 

 the drying out of this layer of dextrinized starch gives the crust. The 

 interior of the loaf probably does not reach a temperature much above 

 1 00 during baking because the evaporation of water maintains the 

 dough at the boiling point of water. The carbon dioxide and alcohol 

 formed during fermentation are driven off although some of the less 

 volatile esters and organic acids formed by yeast or bacteria remain in 

 the bread to give it its characteristic flavor. 



Bread made by this method should have a flinty nut-brown 

 crust which cracks when broken; the crumb should be porous, not soggy, 

 free from large gas pockets, of even fine grained texture and elastic; 

 the flavor should be clean and sweet and the color white or creamy 

 white. It should not possess a disagreeable yeasty or butyric odor. 

 A yeasty odor usually comes from stale yeast fresh clean yeast can 

 be used in very large quantities without imparting a yeasty odor or 

 flavor. 



In the ''sponge dough" method commonly used by housewives and 

 by some bakers a thin batter of yeast, flour, and other ingredients 

 is prepared on the day before the dough is to be made. Potato flour or 

 grated potato is often added to furnish yeast food. Malt syrup is used 

 for the same purpose. Vigorous fermentation and probably consider- 

 able growth of yeast occurs in this batter w r hich is softened 

 through peptic and diastatic activity and considerable opportunity for 

 bacterial growth is given. After about twenty-four hours standing it is 

 mixed with flour and handled as described above in the straight dough 

 method except that usually only one " cutting down" of the dough is 

 employed. Bread made in this way usually possesses a more agreeable 



