94 CARBOHYDRATES. 



Lactose (milk-sugar, sugar of milk) is found only in milk, 

 although it may occur in the urine of lying-in women and of suck- 

 lings during the early days of lactation. It is crystallizable, less 

 soluble in water than dextrose, and insoluble in alcohol. It is 

 dextrorotatory, its power in this respect being the same as that 

 of dextrose. As above noted in speaking of galactose, lactose is 

 changed into equal parts of galactose and dextrose by boiling it 

 with dilute mineral acids. 



Lactose by itself does not undergo alcoholic fermentation with 

 yeast, but an alcoholic fermentation does take place in milk, as 

 when mare's milk is used for the preparation of kumyss and 

 kephir. This fermentation is due to special ferments, the nature 

 of which is not fully understood. In Russia kephir ferment may 

 be purchased. Lactose readily undergoes the lactic fermentation 

 (see Lactic Fermentation, p. 92). It is this change which takes 

 place in the souring of milk due to the action of certain micro- 

 organisms. The character of the change in the case of lactose is 

 the same as in dextrose and saccharose. Lactose injected into the 

 blood is eliminated by the urine, as are saccharose and maltose, 

 and like them must therefore be changed in the alimentary canal 

 during the process of absorption. This conversion, which is into 

 dextrose and galactose, takes place under the influence of the sugar- 

 splitting enzyme, lactase (p. 119). 



Maltose. When starch-paste or glycogen is treated with saliva, 

 maltose is the principal sugar formed ; prolonged treatment with 

 pancreatic juice will produce, besides the maltose, some dextrose. 

 Although pancreatic juice, on the one hand, acts in this manner, 

 still the tissue of the small intestine or an extract of it has but 

 slight action on the paste. On .the other hand, the pancreatic juice 

 rapidly changes maltose into dextrose. Maltose, like cane-sugar, 

 injected into the blood is eliminated as maltose in the urine. From 

 this fact it would appear that maltose is not absorbed as such in 

 the intestine, but as dextrose. Recent researches show the presence 

 in the succus entericus of lambs, and in the mucous membrane of 

 the jejunum of dogs and new-born children, of an enzyme glucase 

 which changes maltose into glucose, so that the conversion of the 

 maltose may take place both in the cavity of the intestine and 

 while it is passing through the intestinal walls. The action of 

 pancreatic juice on starch in the intestine will be further discussed 

 in the consideration of the enzymes of this fluid. 



Maltose is soluble in water, but it is less soluble in alcohol than 

 dextrose. It is crystallizable, dextrorotatory, and reduces metallic 

 salts. Maltose is distinguished from dextrose (1) by the difference 

 in its rotatory power on polarized light, that of maltose being 

 greater ; (2) by having a less reducing power, as when boiled 

 with Fehling's solution only two-thirds as much cuprous oxide 

 is separated out with maltose as with dextrose ; (3) by Barfoed's 

 reagent, which, consisting of a solution of cupric acetate in water 



