ENZYMES AND OTHER FERMENTS DIGESTION. 135 



mon beer yeast appears in the form of nearly spherical cells 

 having a diameter of 8 to 9 /*. It is active through a com- 

 paratively wide range of temperature. In practice the fer- 

 mentation of malt wort to produce beer is carried out at a 

 very low temperature, while a grain mash to produce common 

 alcohol or whisky is fermented at a high temperature. In the 

 one case, however, weeks are required to complete the change, 

 in the other one or two days. 



Like most similar reactions brought about by living cells a 

 limit to the quantity of product formed is soon reached. With 

 ordinary glucose the reaction follows approximately according 

 to this equation : 



CeH 12 O 6 = 2C 2 H 6 O + 2CO 2 . 



As the alcohol formed accumulates a point is reached where 

 the activity of the yeast cell is impeded and the fermentation 

 finally stopped. This occurs when about 20 per cent of alco- 

 hol has accumulated as a reaction product. Very strong sugar 

 solutions do not ferment at all. Indeed some of the common 

 uses of cane sugar in preserving fruits depend on this fact. 

 Some of the conditions of fermentation may be readily illus- 

 trated by simple experiments. 



Ex. Make a strong cane sugar syrup, by boiling or heating together 10 

 gm. of sugar and 10 cc. of water. Allow to cool and add about a gram 

 of crumbled compressed yeast, and then set aside for several days. The 

 solution should be found free from any signs of fermentation. 



Ex. Prepare a 20 per cent solution of commercial glucose 'and pour 50 

 cc. of it into a small flask. Add some yeast and allow to stand two days 

 in a moderately warm place. At the end of this time it should be found 

 in active fermentation, as shown by the escape of gas bubbles and the odor 

 of alcohol. If allowed to stand several days longer in the ordinary at- 

 mosphere the liquid in the flask usually becomes sour from acetic fermen- 

 tation. 



Ex. Prepare a tube with sugar solution and yeast as in the last experi- 

 ment. Close it loosely with a plug of absorbent cotton and heat to boil- 

 ing, allowing steam to escape through the cotton. If the tube is now 

 left to itself for several days it will be found that fermentation has not 

 taken place, showing that heat destroys the characteristic property of the 

 yeast cell. 



Ex. Prepare another tube with sugar and yeast and add 10 cc. of strong 

 alcohol. Shake the mixture and allow to stand. No fermentation ap- 



