200 AGRICULTURAL BACTERIOLOGY 



process, further expansion occurs, and the loaf becomes 

 light and porous. The yeast may be obtained by saving 

 a piece of the dough from the previous baking, by the pur- 

 chase of dried or compressed yeast, or by furnishing con- 

 ditions favorable to the natural seeding of the materials. 



The leaven most commonly used is compressed yeast. 

 The yeast is secured by seeding a maltose solution, as in 

 the manufacture of alcohol, with a pure culture of yeast. 

 Air is passed through the solution for from six to eight 

 hours. The thorough aeration results in an increased 

 growth of yeast. The cells are removed by centrifugation, 

 washed, and pressed. 



In the baking industry large quantities of compressed 

 yeast are added to the dough, and the rising takes place 

 rapidly. If smaller amounts of yeast are used, as is usual 

 in the home, a longer time must be allowed for the growth 

 of the yeast and the formation of sufficient gas to produce 

 the desired porosity in the bread. In this case the yeast is 

 added to a thin mixture of flour and water, called the bat- 

 ter. After standing for a few hours in a warm place, ad- 

 ditional flour is added, and the yeast uniformly incorpo- 

 rated with the dough by the kneading process. More or 

 less bacterial growth takes place in the dough, depending 

 on the length of time it is allowed to stand before baking. 

 The bacteria have much to do with the flavors of the bread. 

 If the development of the bacteria has been too great, the 

 bread is likely to have an acid or sour taste. 



If no yeast is added, but the mixture of flour and water 

 is allowed to stand, gas will be produced by the gas-forming 

 bacteria that are normally in the flour. The yeasts that are 

 naturally present will also function. Such bread has a 

 quite different flavor from the ordinary product, and is 

 called salt-rising bread. The flavor is far from constant, 

 since there is no control over the kind of organisms that 



