134 GROWTH AND WORK OF PLANTS 



Sugars are fermented into alcohol and carbon dioxide. Alcoholic 

 fermentation is caused by the action of yeasts on carbohydrates, 

 and yeasts are employed in the commercial process of brewing 

 beer. The same yeasts are also used in bread making. When 

 the "dough" is set aside to " rise," the yeast plant in the 

 "yeast" which is added, ferments the sugar into alcohol and 

 CO 2 o The latter being a gas forms gas cavities in the dough 

 which causes the bread to " rise." It is then placed in the 

 oven, the heat expands the gas and causes the bread to "rise" 

 still more so that it becomes "light" and the baking makes 

 the loaf rigid in this form. Acid fermentation is another 

 kind of fermentation caused by bacteria. Alcohol is fermented 

 by the acetic acid bacteria into acetic acid. This process 

 takes place in the making of vinegar from cider. The lactic 

 acid bacteria (Bacillus acidi lacti) cause the souring of milk. 

 Sugar and cellulose can also be fermented into butyric acid by 

 certain bacteria. 



210. Unorganized ferments. The yeasts and bacteria which 

 produce fermentation are called organized ferments, since they 

 are living organisms. There is another kind of fermentation 

 caused by what are termed unorganized ferments. These are 

 usually known as enzymes or diastases. They are substances pro- 

 duced in different parts of plants and animals which act on 

 starch and other substances in such a way as to digest or dis- 

 solve them. A well-known animal diastase (ptyalin) is present in 

 the saliva of the mouth, which is necessary as one step in the 

 digestion of starch by animals, thus the importance of thorough 

 mastication of the food to mix the saliva with it. Leaf diastase 

 is formed in the leaves of green plants to change the starch 

 formed during the day into sugar so that it can be transported 

 to other parts of the plant. Malt diastase is formed in seeds and 

 is especially abundant in barley which is used to make " malt" in 

 the breweries. The diastase dissolves the starch to sugar, and then 

 the yeast (an organized ferment) ferments the sugar to alcohol. 

 Taka diastase is a special diastase formed by a mold fungus 

 (Aspergillus oryzae) which grows on rice grains. The fungus 



