46 



CARBOHYDRATES 



Malted milks and certain infant foods contain the water-soluble ma- 

 terial of germinated barley. The water extract, evaporated to dryness 

 and mixed with the other ingredients, imparts the peculiar malt flavor 

 to these products. Pure maltose can be prepared by digestion of starch 

 with diastase, followed by evaporation and crystallization from 60 per 

 cent alcohol. It is not much used in the crystalline form. 



As is indicated by its formula, maltose is a reducing type disaccharide: 



CH,OH 



CH2OH 



OH 



OH H 



Maltose 



Note that carbon 1 of the right-hand glucose unit, as the formula is 

 written above, is an aldehyde group in the oxide-ring form (compare with 

 the formula of a-D-glucose, p. 25). It therefore gives a positive test 

 with Fehling's solution and an osazone with phenylhydrazine. The 

 osazone is rather soluble in water but usually separates on cooling in 

 the form of daisy-like crystals (Fig. 3-1). Note also that the two 

 glucose units are held together in an a-l,4-linkage. This same type of 

 linkage is present in several of the more common polysaccharides such 

 as starch and glycogen. 



Yeasts, bacteria, and other microorganisms ferment maltose with 

 about the same ease that they ferment glucose. It is assumed that the 

 maltose is first hydrolyzed and then fermented. 



Cellobiose 



Like maltose, this disaccharide, which does not occur free in nature, 

 is composed of two glucose units attached through the 1 and 4 positions, 

 but unlike maltose, the disaccharide linkage is the /3-type: 



CH2OH 



CH2OH 



OH 



Cellobiose 



Note that in the left-hand glucose unit, as the formula is written above, 

 the configuration of carbon 1 is jS, whereas in maltose it is a. This 

 is the only structural difference between cellobiose and maltose. Cello- 



