34 



CARBOHYDRATES 



(40 per cent each) of these two sugars. The two occur frequently in 

 nearly equal amounts, and since they both are formed by hydrolysis 

 of sucrose, it is supposed that the two originate from the action of the 

 enzyme sucrase on sucrose. In some fruits such as apples and pears, 

 fructose seems to be more abundant than glucose, however. 



Rafiinose and melezitose are two other sugars that yield a molecule 

 of fructose on hydrolysis. The polysaccharide inulin gives only fructose 

 on hydrolysis and thus stands in the same relation to fructose as starch 

 does to glucose. Fructose may be prepared from either sucrose or inulin, 

 but more easily from the latter. 



In recent years considerable effort has been expended in an attempt 

 to produce fructose, or levulose as it is called in trade, on a commercial 

 scale. There would be a great demand for levulose at a reasonable price 

 because of its marked sweetening power — nearly twice that of sucrose. 

 The most promising source is the Jerusalem artichoke, a plant which 

 grows well in temperate climates and yields a high tonnage of tubers 

 per acre. The tubers are sliced and the sugars extracted in much the 

 same way as sucrose is extracted from the sugar beet. After hydrolysis 

 of the juice, levulose is precipitated as the calcium compound. This is 

 removed, decomposed by carbon dioxide, and the free sugar is obtained 

 either in the form of a sirup or, by careful concentration and cooling 

 of the sirup, as the crystallized product. 



Neither glucose nor fructose crystallizes readily, but fructose has a 

 particularly strong tendency to remain in a sirupy condition. This is 

 well illustrated by honey, in which the glucose generally crystalUzes after 

 two or three months storage, while the fructose remains in a sirupy state. 

 Browne states that "the granulation of honey was known to the ancients 

 and crystallized glucose as thus observed was probably the first sugar 

 knowTi to mankind." 



If fructose and glucose are present in the sirup from cane or sugar 

 beet, they interfere with the crystallization of the sucrose. This property 

 is used to advantage in the preparation of cane sirup from sucrose. Su- 

 crase, an enzyme obtained from yeast, is added to the warm sirup and 

 allowed to hydrolyze the sucrose for about 12 hours. At the end of 

 this time the sirup is further concentrated and may be stored without 

 danger of crystallizing. A similar effect is produced by the partial 

 hydrolysis of sucrose in making jelly and fondant. If sufficient fruc- 

 tose and glucose are present, the unhydrolyzed sucrose is prevented 

 from crystallizing and a smooth even texture results. Whenever 

 sucrose crystallizes, it imparts a rough gritty texture to the candy or 

 jelly. 



D-Fructose is a ketohexose with the same configuration as D-glucose 

 about carbon atoms 3, 4, and 5: 



