Microorganisms in Relation to Man 567 



Still different varieties are used in the fermentation of milk. 

 One which was originally cultivated by a Russian tribe dwell- 

 ing by the river Kuma has come into wide use in preparing 

 the fermented milk known as Kumiss. Another variety pro- 

 duces the well-known Bulgarian milk beverage, called Matzoon, 

 which has become popular largely because of the theory of 

 Metchnikoff as to its value in prolonging life. These are only 

 a few of the many varieties discovered by modern scientific 

 investigation and made useful to man. 



282. What occurs in rising bread. We may now resume 

 the study of the process which takes place in rising bread or 

 sponge. This is already recognized as complex, involving 

 several different varieties of yeast and bacterial action. The 

 yeast plants begin to develop immediately in the right condi- 

 tions, utilizing the available sugar which they decompose. 

 Flour contains a small amount of sugar besides the large amount 

 of starch. The sugar, which is a carbohydrate composed of 

 carbon, hydrogen, and oxygen (pages 129, 515), is converted by 

 the yeast plants into carbon dioxid and alcohol. Alcohol also is 

 composed of carbon, hydrogen, and oxygen. The carbon dioxid 

 is imprisoned by the dough which is made to swell or rise by 

 the multiplying and expanding bubbles of gas. The alcohol 

 combines with the water in the dough and is evaporated in 

 the baking. While the alcoholic fermentation is going on, 

 another kind of ferment called diastase (page 518), which is 

 contained in the. flour, becomes active and changes part of the 

 starch into a form of sugar. If the process of rising is of the 

 slower kind and made with a relatively impure yeast, as in the 

 homemade bread, it allows certain bacteria to develop which 

 act upon the nitrogenous substances, such as gluten, and produce 

 in the bread a peculiarly fine flavor. 



If a dough is allowed to stand too long or is kept at too high 

 a temperature, it becomes more or less sour or acid. This 

 change is effected by other kinds of bacteria. These begin to 

 grow and multiply rapidly in such conditions and to produce 



