74 PHYSIOLOGY 



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 laevo-rotatory. On this 

 account the change from free cane sugar to the mixture of mono- 

 saccharides 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 monosac- 

 charides. With yeast, 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 260 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 fermentation can occur the lactose must be 

 split by the agency of acids or by a ferment, lactase, which occurs in 

 the animal body and in certain moulds, into the monosaccharides glucose 

 and galactose. Lactose reduces Fehling's solution and gives with 

 phenyl hydrazine lactosazone, which is easily soluble in 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 protec- 

 tive substances, and many of their reserve materials consist of members 

 of this group. In the animal body, where the supporting tissues are 

 composed chiefly of derivatives of proteins, the sole significance of 

 polvsaccharides lies in their value as food-stuffs. In plants, anhydrides 

 both of hexoses and pentoses occur in bewildering variety. Here, 

 however, we may confine our attention to those members of the group 

 of polvsaccharides which are important as food-stuffs. 



STARCH (C 6 H 10 5 ) is present in large quantities in nearly all 

 vegetable foods, and is an important constituent of the cereals, 

 from which flour and bread are derived, as well as of tubers, such as the 



