CARBOHYDRATES 



49 



CH2OH 



CH2OH 



H OH H OH 



/3-Galactoside part Glucose part 



a -Lactose 



Note that lactose is a galactoside, not a glucoside, and that the disac- 

 charide Hnkage is a /?-l,4-typc. The (potential) aldehyde group is at 

 carbon 1 of the glucose part. In the formula above, the configuration 

 of this reducing group is written as a (—OH down). In j8-lactose the 

 configuration around this carbon is reversed, but the structure is other- 

 wise identical to that of ordinary a-lactose. 



The mucic acid test is positive with lactose because of the galactose 

 component which is set free by hydrolysis during the test. 



THE TRISACCHARIDES, CisH^aOie 



Raffinose 



Rafiinose is the most important trisaccharide. It is found in small 

 quantities (2-5 per cent) in cottonseed, beet molasses, and manna, and 

 to a less extent in barley, wheat, and other cereals. The amount of 

 raffinose in sugar beets increases considerably as a result of abnormal 

 climatic conditions such as drought and freezing. The sucrose from 

 such beets is hard to crystallize properly as it tends to take on an 

 elongated pointed form. Raffinose is not readily fermented, does not 

 reduce Fehling's solution, and on hydrolysis gives one molecule each 

 of fructose, glucose, and galactose. 



Melezitose 



Another trisaccharide that has attracted some notice because of its 

 occurrence in the exudate of the Douglas fir and other coniferous trees 

 is melezitose. In dry seasons the trees become laden with an exudate 

 very rich in this sugar. At such times honey bees gather the material 

 and incorporate it into the honey, where it soon crystallizes and may 

 suggest adulteration. Upon hydrolysis it yields two molecules of glu- 

 cose and one of fructose. 



POLYSACCHARIDES 



The polysaccharides are the most complex, as well as the most numer- 

 ous and abundant, of the carbohydrates found in nature. They are 



