THE CARBOHYDKATES 19 



With yeast, cane sugar is first inverted by means of a special 

 soluble ferment produced by the yeast cells, and then there is an 

 alcoholic fermentation of the glucoses so formed. 



Lactose, or milk sugar, occurs in milk. It has also been described 

 as occurring in the urine of women in the early days of lactation or 

 after weaning. 



It crystallises in rhombic prisms (see fig. 3). It is much less 

 soluble in water than cane sugar or dextrose, 

 and has only a slightly sweet taste. It is 

 insoluble in alcohol and ether; aqueous 

 solutions are dextro-rotatory. 



Solutions of lactose give Trommer's 

 test, but when the reducing power is tested 

 quantitatively by Fehling's solution it is 

 found to be a less powerful reducing agent 



than dextrose. If it required seven parts FIG. 3. Miik-sugar crystals, 

 of a solution of dextrose to reduce a given 



quantity of Fehling's solution, it would require ten parts of a solution 

 of lactose of the same strength to reduce the same quantity of 

 Fehling's solution. 



Lactose, like cane sugar, can be hydrolysed by the same agencies 

 as those already enumerated in connection with cane sugar. The 

 glucoses formed are dextrose and galactose, 



[lactose] [dextrose] [galactose] 



With yeast it is first inverted, and then alcohol is formed. This, 

 however, occurs slowly. 



With the lactic-acid organisms which bring about the souring of 

 milk the lactic-acid fermentation is produced. This may also occur 

 as the result of the action of putrefactive bacteria in the alimentary 

 canal. The two stages of the lactic-acid fermentation are represented 

 by the following equations ; 



(1) C^HsaOn + HjO^CaHeOa 



[lactose] [lactic acid] 



(2) 4C 3 H 6 03=2C 4 H 8 O 2 +4C0 2 + 4Hj| 



[lactic acid] [butyric acid] 



Maltose is the chief end product of the action of malt diastase on 

 Starch, and is also formed as an intermediate product in the action 

 of dilute sulphuric acid on the same substance. It is also the chief 

 sugar formed from starch by the diastatic ferments contained in the 

 saliva (ptyalin) and pancreatic juice (amylopsin). It can be obtained 



e2 



