FERMENTATION ORGANISMS 107 



and 3, until a firm dough is made. Cover the beakers with a plate 

 of glass and place in the incubator. Examine each dough every 

 three hours, to determine which rises most rapidly. Note the 

 texture, viscosity, and odor in each case. 



Exercise 148. The Fermentation of Cider 



1. Grind a few ripe apples in a meat cutter. Press the pulp 

 through a cloth and then filter through paper. 



2. Fill six fermentation tubes with juice. Place four of them 

 in the Arnold sterilizer and steam them for half an hour. Inoc- 

 ulate two of the sterilized tubes with yeast. Keep the six tubes 

 in the incubator for twenty-four hours. 



3. Examine the tubes for fermentation. In which tube is the 

 most gas present ? When the arm of the tube is about half full 

 of gas, test it with 10 per cent NaOH. How much of the gas 

 is carbon dioxide ? What caused fermentation in the unsterilized 

 tubes ? 



Exercise 149. The Fermentation of Wine 



Wine is usually defined as the fermented juice of some berry 

 or grape. The grape is the fruit most commonly used, because 

 the juice is rich in sugar and is easily pressed from the crushed 

 fruit as a clear liquid. Among the microorganisms which adhere 

 to the skin of the grape there are always numerous yeasts. The 

 yeasts start fermentation in the saccharine juice pressed from 

 the grape. These wild yeasts are sufficient to start the process 

 of fermentation, although better results could undoubtedly be 

 obtained from the use of cultures of selected yeasts. 



If practically all the sugar has been fermented (only about 0.1 

 per cent remaining), the wine is classified as dry ; the wine is 

 mwt if the fermentation is checked at an early stage by the 

 addition of alcohol or brandy ; the wine is till if the carbon 

 dioxide produced by the fermentation has been allowed to escape 

 before bottling. 



The principles of the fermentation of wine have been demon- 

 strated by Exercise 146 and need not be repeated here. 



