52 



CARBOHYDRATES 



a double anhydride as gluco-xylan; arabinose may be paired with galac- 

 tose as a galacto-araban. 



Table 3-5 

 Pentosans in plant materials 



[Undried basis] 



per cent 



Navy bean 8.4 



Com meal 5.0 



Corn (whole) 7.4 



Dried peas 7.2 



Barley (whole) 11.1 



Cottonseed flour 5.6 



Beets 1.7 



Spinach 1.0 



Cabbage 1.0 



Wheat bran 22.0 



Wheat straw 27.1 , 



Corn fodder 21.8 



Com cobs 35.0 



Gum arabic 26.0 



Cherry gum 52.0 



White pine wood 7.0 



Maple wood 21.7 



Although the physiological function of pentosans is still obscure, it 

 is doubtful that they are merely the result of an accumulation of waste 

 material. Their very general occurrence in plant material probably 

 indicates that they perform an important function in the life of the plant. 

 Their close relation to cellulose suggests the possibility that they are 

 particularly concerned with structural requirements. Pentosans may 

 also serve as a reserve carbohydrate in the metabolism of the apple 

 tree and thus play an important part in the bearing of fruit. 



On hydrolysis, pentosans give pentoses, as is represented by the follow- 

 ing equation: 



(C5H8O4), + a:HoO = a:C5Hio05 



When substances containing pentosans are boiled with relatively con- 

 centrated solutions of mineral acids (HCl, H2SO4, or H3PO4), the pen- 

 tosans fire hydrolyzed to pentoses, and the pentoses are converted into 

 furfural, as already explained. The red color obtained when furfural 

 reacts with aniline acetate (p. 26) thus serves as a good qualitative 

 test for pentosans. The presence of pentosans in vegetables and whole 

 cereals may be easily demonstrated by holding a piece of filter paper 

 moistened with aniline acetate in the vapors which are evolved when 

 the sample is boiled with 20 per cent hydrochloric acid. By condensing 

 the vapors containing the furfural and adding phloroglucinol, a precipitate 



