CARBOHYDRATES 41 



If the percentage of lactose present is low it may be necessary to 

 cool the solution and permit it to stand for some time before the 

 precipitate will appear. It is impossible to differentiate between lactose 

 and galactose by this test, but the reaction serves to differentiate these 

 two sugars from all other reducing sugars. 



Differentiate lactose from galactose by means of Barfoed's test, 

 page 30. 



SUCROSE, Ci 2 H 22 On 



Sucrose, also called saccharose or cane sugar, is one of the most 

 important of the sugars and occurs very extensively distributed in 

 plants, particularly in the sugar cane, sugar beet, sugar millet and in 

 certain palms and maples. 



Sucrose is dextro-rotatory and upon hydrolysis, as before mentioned, 

 the molecule of sucrose takes on a molecule of water and breaks down 

 into two molecules of monosaccharide. The monosaccharides formed in 

 this instance are glucose and fructose. This is the reaction: 



Sucrose Glucose Fructose 



This process is called inversion and may be produced by bacteria, en- 

 zymes, and certain weak acids. After this inversion the previously 

 strongly dextro-rotatory solution becomes levo-rotatory. This is due 

 to the fact that the fructose molecule is more strongly levo-rotatory 

 than the glucose molecule is dextro-rotatory. The product of this 

 inversion is called invert sugar. 



Sucrose does not reduce metallic oxides in alkaline solution and forms 

 no osazone with phenylhydrazine. It is not fermentable directly by 

 yeast, but must first be inverted by the enzyme sucrase (invertase or 

 invertin) contained in the yeast. The probable structure of sucrose 

 may be represented by the following formula. Note the absence of 

 any free ketone or aldehyde group. 



CH 2 OH CH 2 OH 



I 'I 



CHOH CHO 



I I 



CHOH 



CHOH 



CHOH 



CHOH 



I I 



C~ O CH 2 OH 



H 



Sucrose. 



