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THE CARBOHYDRATES 67 



A very large number of glucosides occur as plant products. Among these we may 

 mention amygdalin, salicin, phloridzin, indican, &c. 



THE DISACCHARIDES 



The disaccharides are formed by the union of two molecules of mono- 

 saccharides with the elimination of one molecule of water, and can be re- 

 garded, according to the manner in which the molecules are combined, as 

 glucosides, galactosides, &c. On hydrolysis, e.g. on heating with acids, they 

 take up one molecule of water and are split up into the corresponding mono- 

 saccharides. Thus cane sugar gives equal parts of glucose and fructose, 

 maltose gives equal parts of glucose and glucose, while milk sugar or lactose 

 gives equal parts of glucose and galactose. 



CANE SUGAR, sometimes known as saccharose, is widely distributed 

 throughout the vegetable kingdom, and forms an important article of diet. 

 It has no reducing power on Fehling's solution. It is strongly dextro-rota- 

 tory and has a specific rotatory power of + 66 '5. On hydrolysis it is 

 converted into equal molecules of glucose and fructose. Owing to the fact 

 that fructose rotates polarised light more strongly to the left than glucose 

 does to the right, the mixture of the two monosaccharides so obtained is 

 Isevo-rotatory. On .this account the change from free cane sugar to the 

 mixture of monosaccharides is known as inversion, and the mixture is often 

 spoken of as ' invert sugar.' The, term ' inversion ' has since been loosely 

 applied to the process of hydrolysis itself, so that we often speak of the 

 inversion of maltose or of lactose, meaning thereby the hydrolysis of these 

 sugars with the production of their constituent monosaccharides. With 

 east, cane sugar first undergoes inversion by a special ferment present in the 

 yeast (invertase), and the mixture of fructose and glucose is then fermented. 



MALTOSE is formed during the hydrolysis of starch by acids or by 

 digestive ferments, and is also the chief sugar in germinating barley or malt. 

 It is strongly dextro-rotatory, ferments easily with yeast, and reduces 

 Fehling's solution ; its reducing power is about 70 per cent, of that of 

 glucose. With phenyl hydrazine it gives phenyl maltosazone, which forms 

 definite yellow crystals with a melting-point of 206 C. 



MILK SUGAR or LACTOSE is found only as a constituent of milk. 

 It forms colourless rod-like crystals, which are much less soluble in water 

 than are the two other disaccharides. On account of this solubility it is much 

 less sweet than either cane sugar or maltose. It is dextro-rotatory and 

 shows bi-rotation. It is not fermented by ordinary yeast. Before fermen- 

 tation can occur the lactose must be split by the agency of acids or by a 

 :erment, lactase, which occurs in the animal body and in certain moulds, into 

 .e monosaccharides glucose and galactose. Lactose reduces Fehling's 

 solution and gives with phenyl hydrazine lactosazone, which is easily soluble 

 hot water and therefore does not come down until the fluid is cold. 



THE POLYSACCHARIDES 



These play an important part throughout the whole vegetable kingdom, 

 where all the supporting tissues of the plants, their protective substances, 



