CARBOHYDRATES 



53 



is formed that may be weighed. From this weight the quantity of 

 pentosans can be calculated. 



Furfural is also an interesting and important chemical for other reasons. 

 It offers a means of utilizing agricultural waste products such as oat 

 hulls, corn cobs, etc., because these residues contain large amounts of 

 pentosans which can be converted into furfural by a simple, cheap process. 

 The crude furfural so produced is a brownish, oily liquid, which sells 

 for about 10 cents per pound. Large amounts have been used in petroleum 

 refining, and more recently as the starting material for the making of 

 nylon. 



The nutritive value of the pentosans is still an unsolved problem. In 

 passing through the digestive tract, considerable quantities disappear. 

 In herbivora from 50-75 per cent of the pentosans, and in man about 

 15 per cent, appear to be utilized. This utilization must be an indirect 

 one for no digestive enzymes that bring about hydrolysis of pentosans 

 are known to occur in higher animals. It is assumed that the bacteria 

 of the intestine break down the pentosans into soluble products such as 

 acetic and lactic acids, which are then absorbed and utilized. Consider- 

 ing the large amount of pentosans consumed by herbivorous animals, it 

 seems that such a fermentation must be very rapid. 



Hexosans (CeHioOs)^, 



Starch is the most important food carbohydrate. It contributes more 

 calories to the usual diet of human beings than any other single sub- 

 stance. Ordinary starch, as it is found in plants, is a mixture of amylose 

 and amylopectin. Usually there is a much greater proportion of the 

 amylopectin. For example, corn starch and potato starch each contain 

 about four-fifths amylopectin and one-fifth amylose. The so-called waxy 

 corn starch is almost all amylopectin. The two fractions can be separated 

 by dispersing the crude starch in hot water saturated with butyl alcohol. 

 On cooling slowly, the amylose separates as a semicrystalline precipitate, 

 which is easily removed. The amylopectin can then be recovered from 

 the solution. 



Both amylose and amylopectin are polysaccharides made up of anhydro- 

 glucose units attached to each other by a-l,4-linkages. Amylose, as 

 explained on p. 50, is a linear-type polysaccharide, consisting of a long, 

 unbranched chain of about 200-300 glucose units. On the other hand, 

 amylopectin has a branched structure somewhat similar to that of gly- 

 cogen (see diagram on p. 50). At the point of branching, a-l,6-linkages 

 are present. The molecular weight of amylopectin is thought by many 

 investigators to be at least 500,000, which corresponds to 2000-3000 

 glucose units in the molecule. 



Amylose is less soluble in water than amylopectin. It gives a clear 



