92 CARBOHYDRATES. 



arrested by the alcohol produced, before all the sugar has been 

 decomposed. 



2. Lactic Fermentation. When milk sours, the sugar which it 

 contains is converted into lactic acid, constituting the lactic fer- 

 mentation : 



C 12 H 22 U + H 2 ---- 4 C 3 H 6 3 



Lactose. Water. Lactic acid. 



This fermentation is not confined to milk-sugar, but may occur 

 also with dextrose. The change is brought about by the presence 

 of specific micro-organisms. It is stated that there exists also in 

 the mucous membrane of the stomach an enzyme which can change 

 lactose, and possibly dextrose, into lactic acid. 



3. Butyric Fermentation. When the lactic fermentation is con- 

 tinued for some time it is liable to pass into the butyric. This 

 change is due to the action of a ferment (organized) upon the 

 lactic acid. In the change, hydrogen and carbonic anhydrid are 

 given off. 



4C 3 H 6 3 2C 4 H 8 2 + 4C0 2 + 4H 2 



Lactic acid. Butyric acid. Carbonic Hydrogen, 

 anhydrid. 



The optimum (most favorable) temperature for the lactic and 

 butyric fermentations is from 35 C. to 40 C. When the diet 

 consists largely of carbohydrates both these fermentations may 

 occur in the alimentary canal. 



Levulose (left-rotating sugar, fruit-sugar, or mucin-sugar) is 

 found in many fruits and in honey, and occurs in small quantity 

 in blood, urine, and muscle. It is crystallizable, but with diffi- 

 culty. When cane-sugar is treated with dilute mineral acids it is 

 decomposed into equal parts of dextrose and levulose. Cane- 

 sugar has a dextrorotatory action on polarized light, but when 

 changed by the acid the solution becomes levorotatory, the levo- 

 rotatory power of the levulose being greater than the dextro- 

 rotatory power of the dextrose, and the cane-sugar is said to be 

 " inverted ;" hence the mixture of dextrose and levulose is some- 

 times spoken of as " invert-sugar." As will be seen in the con- 

 sideration of cane-sugar, " inversion" takes place in the alimentary 

 canal. Although in many respects levulose is very similar to 

 dextrose, still its action on polarized light serves to distinguish 

 the two. 



Galactose. When lactose is boiled with dilute mineral acids 

 it is changed into dextrose and galactose : 



C^O,, + H 2 = C 6 H 12 6 + C 6 H 12 6 



Lactose. Water. Dextrose. Galactose. 



A similar change takes place under the influence of certain 

 enzymes. 



