106 A MANUAL OF BACTERIOLOGY 



bacteria will be found. Certain of the bacteria have the power to 

 transform starch into sugar and to dissolve gluten ; others form 

 lactic and acetic acids. The amount of acid formed is not usually 

 perceptible unless the dough stands too long before baking or is 

 kept too warm, in which case sour bread is obtained. B. mesen- 

 tericus vulgatus Fliigge is sometimes the cause of viscid bread. 

 2. Soak a dry yeast cake in distilled water which has been 

 boiled and cooled to 40 C. After six to eight hours make 



PIG. 40. Bread dough raised with yeast from different sources 



Each beaker (400 ec. capacity) received 75 g. of flour and enough yeast culture to 



make a stiff dough. The yeast in No. 1 came from a compressed yeast cake ; in No. 2, 



from a dried yeast cake ; in No. 3, from a wild yeast culture 



microscopic examination for the organisms present. Is the 

 greater number of bacteria in the compressed yeast or in the 

 dry yeast cake ? 



3. Make a culture of wild yeast as follows : Cook 200 g. 

 of potato in a tin or granite-ware vessel until it can be mashed. 

 Add 5 g. of sugar and 5 g. of ammonium tartrate. Stir in enough 

 water to make a thin paste. Keep the culture at 25 C. until 

 fermentation is evident. Examine microscopically for bacteria 

 and yeasts. 



4. Weigh off 100 g. of wheat flour into each of three beakers. 

 Stir into each a yeast culture made as directed in paragraphs 1, 2, 



