30 CARBOHYDRATES 



superior to sucrose for the making of fondants, creams, fancy candies, 

 chewing gum, doughnuts, and other products. 



To prepare crystalhne glucose the hydrolysis is carried to completion; 

 the sugar solution is then concentrated in an evaporator to a density 

 of 1.36 to 1.45. The concentrated sugar solution is then introduced into 

 crystallizing vessels that contain some of the crystals from the preceding 

 batch. This practice of seeding the liquor is an essential step in obtain- 

 ing crystals of approximately the desired size and uniformity. The 

 crystals of hydrated glucose are separated from the mother liquor by 

 means of centrifugal machines, washed in the same machines, and then 

 sent through a drier. In 100-pound bags it now sells for about 8 cents 

 per pound. 



The sweetness of glucose is approximately 75 per cent of that possessed 

 by our common sugar, sucrose. Its calorific value, however, is about 

 equal to that of sucrose. Glucose is readily jermented by practically 

 all microorganisms. The spoilage of fruits and vegetables is accom- 

 panied by a destruction of glucose. The manufacture of alcoholic bever- 

 ages is based upon the fermentation of glucose by yeast. 



The most characteristic chemical property of glucose is its reduction 

 of solutions of copper salts with the formation of a precipitate, cuprous 

 oxide. The color of the precipitate varies from yellow to brick red, 

 depending upon the fineness of the particles of oxide. Some of the most 

 common copper reagents used in sugar tests are Fehling's (copper sulfate, 

 sodium potassium tartrate, and sodium hydroxide) , Benedict's (copper 

 sulfate, sodium citrate, and sodium carbonate), and Barfoed's (copper 

 acetate and acetic acid) solutions. 



Glucose reduces these reagents because it is oxidized by the cupric 

 ion (Cu + + ) present. The process is dependent on the presence of the 

 aldehyde group in the glucose molecule or, in general, on the presence 

 in the sugar tested of an aldehyde or ketone group not attached to other 

 atoms in the form of a glycoside (p. 40). However, it is immaterial 

 whether the sugar is in the open chain or oxide ring form. In the latter, 

 the aldehyde (or ketone) group is apparently covered up, but it is still 

 a -potential aldehyde group because of the easy interconversion of the 

 chain and ring forms in solution. 



Many other aldehydes such as formaldehyde, acetaldehyde, and chloral 

 also have reducing power. Reduction of Fehling's solution and similar 

 reagents must, therefore, be recognized, not as the peculiar attribute of 

 sugars, but rather as a general property common to many substances. 



The chemical changes which reducing sugars undergo during the Fehl- 

 ing's test are very complex. Certainly one main reaction is oxidation 

 of the aldehyde group to a carboxyl with the formation of the correspond- 

 ing aldonic acid: 



