364 PRINCIPLES OF AGRICUI/TURAI, CHEMISTRY 



Tables have been prepared showing the amounts of copper 

 which were found by experiments to be reduced by given sugars 

 under fixed conditions ; these tables can be used in the estimation 

 of sugars under the same conditions, but the details of the method 

 used in preparing the tables must be followed carefully. 



The copper solutions used ordinarily are Fehling's solution, 

 containing fixed quantities of copper sulphate, sodium potassium 

 tartrate, and sodium hydroxide; and Allihn's solution, containing 

 certain amounts of copper sulphate, sodium potassium tartrate 

 and potassium hydroxide. Other solutions are used. Different 

 sugars require different quantities of copper under the same con- 

 ditions. For example, the same amount of copper will be reduced 

 from Sachsse's solution by the following amounts of sugars : 

 Fructose 213 mg. 

 Maltose 491 mg. 

 Lactose 387 mg. 



Fermentation. Under suitable conditions, yeast converts cer- 

 tain sugars into alcohol and carbon dioxide according to the 

 following reaction : 



C 6 H 12 6 = 2C 2 H 6 + 2C0 2 . 



Yeast is a plant which grows in the solution, and develops an 

 enzyme which changes the sugar as described. Like all plants, 

 yeast must have nitrogenous food, also phosphoric acid, potash, 

 and lime. Yeast will not grow well in pure sugar. An enzyme 

 is a substance which causes a chemical change, without itself be- 

 ing changed in the reaction. 



The simple hexoses are easily fermented. Some of the com- 

 pound hexose sugars ferment readily, while others must first be 

 split up into the simple hexoses. Pentosans do not undergo the 

 alcoholic fermentation. 



There are other kinds of fermentation, probably the most im- 

 portant being the acetic fermentation, in which alcohol is con- 

 verted into acetic acid (vinegar), and the lactic acid fermentation, 

 in which sugar is converted into lactic acid. This takes place in 

 the souring of milk. 



