92 BACTERIA, YEASTS, AND MOLDS 



impurities than a fresh one. Moreover, in an old cake, 

 as we have seen, the number of living yeast cells is less 

 than in a fresh one, and so the undesirable germs have a 

 better chance to grow in the dough. The use of an old 

 yeast cake is therefore unwise, since the bread may thus 

 be ruined. The fermentation does not progress rapidly 

 enough, the bread must be kept longer at a warm tem- 

 perature, and during this whole period the other yeasts or 

 bacteria have a chance to develop and produce a variety 

 of bad flavors. If one uses fresh yeast cakes, there is little 

 probability that any trouble will arise from the action of the 

 smaller number of bacteria or molds that may be present. 

 Sour Bread. The impurities from the yeast or from 

 some other source do, however, occasionally produce 

 trouble, two types of which are so common as to demand 

 notice. The raising of dough by means of yeast some- 

 times causes it to become sour. The dough rises in the 

 proper manner apparently, but the bread when baked is 

 found to have an unpleasant, sour taste. This is espe- 

 cially likely to happen if the bread is raised too long. 

 By some this sour taste is regarded as an improvement to 

 the flavor. It is due to the development, during the fer- 

 mentation, of certain acids in the dough, which come, not 

 from the action of yeasts, but from the growth of bacteria 

 that are present either in the yeast or flour. It has been 

 a disputed question whether the acid produced is lac- 

 tic, acetic, or butyric. (Lactic acid is like that formed in 

 sour milk, acetic acid is formed in vinegar, and butyric 

 acid is the acid found in old rancid butter.) It is fre- 

 quently a mixture of all three, but ordinarily it is prob- 

 ably mostly lactic acid. Each of these acids is known 



