Effect of Cooking on the Pectic Polysaccharides in Cotyledons of Adzuki Beans (Vigna angularis)

Studies of "Ann" (Bean Jam), Part XII

Abstract

Three pectic polysaccharide preparations (HXSP, from uncooked cotyledons; HWIP, from cooked ones; HWSP, from the cooking soup) were fractionated into neutral (I) and acidic (II, III and IV) fractions by DEAE-cellulose chromatography. II and III, the major components, were analyzed in more detail. The uronic acid contents of II and III from HXSP were about 40 and 45%, respectively, while those of II and III from HWSP and also from HWIP were about 46 and 50%, respectively. The degree of esterification of the pectic polysaccharides was generally low; about 20% for II and III from HXSP, about 10-15% for those from HWSP and HWIP. In each case, II was composed mostly of high molecular weight polysaccharides (MW: above 5×10⁵), whereas the greater part of III was low molecular weight polysaccharides (MW: below 1×10⁴). Analysis of sugars showed that xylose and arabinose amounted together to about 70% of the neutral sugars in each fraction, the xylose: arabinose ratio in II and III being about 1:1 and 4.5:1, respectively. In addition, considerable amounts of galactose and mannose, and small amounts of rhamnose and glucouse were detected. After heating in hot neutral solutions, all the fractions gave a negative thiobarbituric acid test for unsaturated pectic acids, the degradation products of pectin by transelimination. These results indicate that the pectic polysaccharides of Adzuki bean cotyledons are fairly stable during cooking.