68 PHYSIOLOGY 



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 

 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 protective 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 deriva- 

 tives of proteins, the sole significance of polysaccharides 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 polysaccharides 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 potato. In the plant cells 

 it occurs as concentrically striated grains within minute protoplasmic 

 structures the amyloplasts, the office of which it is to manufacture starch 

 from the glucose present in the cell sap. When freed, by breaking up the 

 cells and washing with water, it forms a white powder consisting of micro- 

 scopic grains, each of which presents the characteristic concentric striation. 

 It is insoluble in cold water. In hot water the grains swell up and burst, 

 forming a thick paste, which sets to a jelly on cooling. This semi-solution, 

 as well as the original starch-grains, gives an intense blue colour on the 

 addition of iodine. On treating starch with cold alkalies or cold dilute acid, 

 it is converted into a soluble modification, the so-called soluble starch or 

 amylodextrin, which also gives a blue colour with iodine. This modification 

 is also produced as the first stage of the action of diastatic ferments upon 

 starch. On boiling with dilute acids, starch is converted first into a mixture 

 of dextrins, then into maltose, and finally into glucose. On acting upon starch 

 with various ferments, such as the diastase which may be extracted from 

 malt or germinating barley, or with the amylase occurring in saliva or pan- 

 creatic juice, it undergoes hydrolysis, the final result of the action being a 

 mixture of four parts of maltose to one part of dextrin. As to the inter- 

 mediate stages in this reaction opinions are still divided. The first product 

 is soluble starch, amylodextrin, giving a blue colour with iodine. This 

 breaks up into a reducing sugar, and another dextrin, erythrodextrin, which 

 gives a red colour with iodine, and this dextrin, on further hydrolysis, 

 yields reducing sugar and achroodextrin, which is not coloured by the 

 addition of iodine. Thus there are a series of successive hydrolytic decom- 

 positions of the molecule, each resulting in the splitting off of a molecule of 

 sugar and the production of a lower dextrin. 



The DEXTRINS are ill-defined bodies which are difficult to separate. 



