CARBOHYDRATES AND RELATED BODIES. 33 



pure yeast, but by prolonged action of common yeast on a 

 dilute solution of the sugar fermentation appears. This is 

 due to the fact that the crude yeast contains an inverting 

 enzyme known as invertase which produces glucose and fruc- 

 tose which then yield to the true fermentation. Cane sugar 

 gives no combination with phenyl hydrazine, and is not a 

 reducing sugar. These facts point to the absence of aldehyde 

 or ketone groups in the large molecule. An experiment illus- 

 trates this : 



Ex. Prepare a dilute solution of pure cane sugar and boil it with 

 Fehling solution in the usual manner. Observe that no reduction of the 

 copper compound takes place. Next boil a similar cane sugar solution 

 with a few drops of dilute hydrochloric or sulphuric acid several minutes, 

 neutralize with sodium carbonate, and then apply the Fehling test. The 

 characteristic red precipitate now appears. In this reaction the cane 

 sugar is broken up by the acid into a molecule of glucose, and a molecule 

 of fructose, both reducing sugars as explained above. 



The behavior of cane sugar solutions with polarized light 

 is characteristic and affords the simplest and most accurate 

 means for quantitative determination. The specific rota- 

 tion is practically independent of the concentration and is rep- 

 resented by the formula [ a ].o = + 66.5. 



Strong solutions of cane sugar, " syrups," are used in the 

 household and in pharmacy to prevent fermentation. Hence 

 the use of this sugar in the canning or preserving of fruit. 



LACTOSE. This is the characteristic sugar of all kinds of 

 milk, with possibly one or two exceptions. It may be sepa- 

 rated from the " whey " which is the product remaining after 

 skimming and precipitating the casein. It is made commer- 

 cially in large quantities as a by-product in the cheese industry, 

 and in pure crystallized form has the formula C 12 H 22 O ii.H 2 O '. 



Milk sugar resembles cane sugar in respect to the condi- 

 tions under which it may be fermented, but it is a reducing 

 sugar directly, acting strongly on copper or bismuth solutions. 

 In its behavior with polarized light it resembles glucose 

 closely, having a specific rotation, [a] D = -j- 52.5. Inverted 

 milk sugar ferments readily, and products known as kumyss, 

 4 



