312 PLANT PHYSIOLOGY 



generally not more than 5% of alcohol can be obtained, but yeasts 

 tolerate up to 10-15% without becoming poisoned by their own 

 products. After this percentage is reached, the fermentation is 

 stopped. A prohibition law based upon nature would thus abol- 

 ish all distilled liquors, which are man-made, and permit alcoholic 

 beverages up to the limit found in nature, — up to 15% alcoholic 

 content. This " natural" prohibition law would abolish distilled 

 liquors like whiskey, gin, rum, absinthe, etc., all of which vary in 

 alcoholic content, in the source of the fermentable substrate (corn, 

 barley, fruit, agave juice, etc.), and in the materials added for 

 flavoring (juniper berries, wormwood, etc.). 



In the baking industry, the fermentation is carried on for the 

 purpose of obtaining the carbon dioxide and not for the alcohol. 

 In the dough, the bubbles of carbon dioxide gas rise to the surface 

 and thus "lighten" or leaven the mixture. During the process of 

 baking, both the carbon dioxide and the alcohol are driven off 

 and at the same time the yeast is killed. On the market two kinds 

 of baking yeasts are available. One is dry and contains much 

 starchy material. It is not so efficient since the proportion of 

 yeast present is smaller, but this type of cake has better "keep- 

 ing" qualities and does not deteriorate so rapidly. The other 

 type is a moist cake with much water present. It is relatively 

 richer in yeast cells, but must be kept in a cool, moist place. 

 Both types thus have their advantages and disadvantages. 



Although oxidases are found to only a very small degree in 

 yeasts, reductases are present and, according to Palladin (1908), 

 play an important part in fermentation. The enzyme of chief 

 importance, however, is zymase (Chap. XV). This enzyme may 

 be extracted from the yeast by grinding and crushing and, in fact, 

 is sold upon the market under the name of "zymin." It has an 

 optimum temperature of 30° C. and is killed at 50° if wet and at 

 85° if dry. It acts only on six carbon sugars, but is found in Mucor 

 and Penicillium in addition to the yeasts. 



Phosphates are necessary for the action of zymase and have 

 been called coenzymes of it. According to the researches of Harden 

 and Young (1910), the process of fermentation takes place in two 

 steps as follows: 



2C 6 H 1 20 6 +2MHP04=2C02+2C 2 H 5 OH+2H 2 0+C 6 Hio04(MP04)2 

 C 6 H 1 o04(MP04)2+2H 2 = C 6 H 12 6 +2MHP0 4 . 



