CARBOHYDRATES 33 



oxidized with nitric acid, of an insoluble dibasic acid, mucic acid. The 

 formation of mucic acid is used both as a qualitative and quantitative 

 test for the presence of galactose-yielding compounds. It is of use in 

 showing that milk has been used in the preparation of milk chocolate, 

 infant foods, and other preparations. The following equation indicates 

 the nature of the reaction: 



CH20H(CHOH)4CHO + 30 = C00H(CH0H)4C00H + HoO 

 Galactose Mucic acid 



The occurrence of L-galactose among the hydrolysis products of flax- 

 seed mucilage has been reported recently. Galactose is one of few 

 sugars (arabinose is another) which, thus far, has been found to occur 

 in nature in both d- and L-forms. 



D'Mannose 



Mannose does not occur in the free state in nature. However, it is 

 widely distributed in mannans, polysaccharides that yield mannose on 

 hydrolysis — compare fructosan, galactan, pentosan. That mannose may 

 play an important role in animal physiology is indicated by relatively 

 recent observations. Mannose is a constituent of egg albumin (1.77 

 per cent), serum albumin (0.45 per cent), and many other proteins 

 (0.3^.0 per cent) . 



The hexahydric alcohol mannitol, C6H8(OH)6, corresponding to man- 

 nose, is also widely distributed in nature. It has been found in the 

 pineapple, onion, green bean, cauliflower, olive, mushroom, and in the 

 bark and leaves of many trees. It is the chief constituent of Sicilian 

 manna, a sweet exudate produced by a certain species of ash when 

 incisions are made in the bark. Many other trees and shrubs produce 

 mannas of varying composition as a result of the sting of certain insects. 

 It is supposed that the manna upon which the Israelites subsisted during 

 their wanderings in the wilderness was an exudate secreted by a species 

 of tamarisk tree. In Australia, India, and other countries manna from 

 different species of trees is used as a food by the natives. Many differ- 

 ent kinds of sugar have been isolated from these mannas. 



D-Fructose 



Fructose, also called levulose, is widely distributed ia nature, and in 

 the free state is generally associated with glucose and sucrose. It is 

 particularly abundant in fruit juices, whence comes the name fruit sugar. 

 Vegetables, the nectar of flowers, and the sap of green leaves and stalks 

 also contain fructose and glucose. Honey contains about equal quantities 



