CONSTITUENTS OF PLANTS 365 



Fermentation is important in the manufacture of cider from 

 apple juice; wine from grape juices; and alcoholic beverages and 

 alcohol from materials containing starch or sugar, such as 

 potatoes, corn, rye, barley, etc. 



Classes of Simple Sugars. Four groups of simple sugars are 

 known to chemists, though only two of these are of agricultural 

 importance. The groups are : ( i ) the trioses, containing three car- 

 bon atoms in the molecule C 3 H 6 O 3 ; (2) the tetroses, containing 

 four carbon atoms, C 4 H 8 O 4 ; (3) the pentoses, containing five 

 carbon atoms; and (4) the hexoses, containing six. All the 

 natural carbohydrates are related to either the pentoses or the 

 hexoses. The principal simple sugars are xylose and arabinose, 

 which are pentoses ; and glucose, fructose, mannose, and galactose, 

 which are hexose sugars. 



Each of these sugars exists in three modifications, namely, one 

 which rotates polarized light to the right, one which rotates 

 polarized light to the left, and one which does not rotate it, and 

 is inactive. Thus we have d-glucose, 1-glucose and i-glucose, 

 dextro-, laevo-, and inactive glucose. Although many of these 

 sugars are known, as a rule only one modification of each sugar 

 is of natural occurrence. Thus natural glucose is dextro-rotatory, 

 and was formerly called dextrose for this reason, and ordinary 

 fructose is laevo-rotatory, and was called levulose. 



Pentose Sugars, C 5 H 10 O 5 . The pentose sugars occur to a very 

 limited extent, if at all, in nature. They are of agricultural im- 

 portance on account of their relation to the pentosans, which are 

 found in large quantities in agricultural products. 



When distilled with strong hydrochloric acid, the pentoses and 

 pentosans are converted into furfural, 1 which distils over with the 

 acid: 



C 3 H 10 8 = C 5 H 4 2 + 3 H 2 

 Pentose Furfural 



The furfural can be precipitated with a solution of phloro- 

 glucinol, the product being furfural phloroglucid. The precipitate 

 is filtered off, dried, and weighed. The quantity of furfural 

 1 Landw. Versuchs-stat. , 42, p. 381. 



