74 BACTERIA, YEASTS, AND MOLDS 



purpose of starting fermentation. Although its use has 

 largely given way to cultivated yeast, it has been employed 

 in the baking of bread up to very recent times, and to a 

 limited extent is still used in France. The difficulty 

 with leaven is that its action is unreliable. The leaven 

 contains bacteria as well as yeast, and these may make the 

 bread sour, or sometimes bitter; and unless the very 

 greatest care is taken in its manipulation the bread pro- 

 duced by means of it is not good. Only very skillful 

 bakers can use it satisfactorily. The use of leaven has, 

 therefore, almost wholly been replaced by the far more 

 easy and reliable method of raising dough with cultivated 

 yeasts. 



The use of yeast instead of leaven in bread making is 

 also old. In the time of the Roman empire it is apparent, 

 from a few references in literature, that the use of yeast 

 was understood. It is stated that the Romans in baking 

 their bread sometimes used a leaven made of grape juice 

 and millet for the purpose of hastening fermentation. 

 We have already seen that grape juice is sure to contain 

 yeast, and this phenomenon, whose nature the Romans, 

 of course, did not understand, is perfectly intelligible 

 to-day. The Romans were unconsciously using yeast for 

 raising their bread. The early bakers soon learned to use 

 yeast in a more accurate and satisfactory manner, and 

 from the time of Rome down through the centuries the 

 use of cultivated yeast products for the purpose of raising 

 bread was more or less common. The methods of pro- 

 ducing and cultivating yeast during these various ages are 

 not known at the present time. It is known, however, 

 that later the use of yeast declined, and bakers returned 



