Pectins and hemicelluloses from cell walls of hulls from developing soybean seeds

Soybean is the most cultivated legume in the world because of the nutrition quality of its seeds. Developing seeds of soybean crops can be damaged by stink bugs (Nezara viridula), which must penetrate hulls to feed on them. In this work, hulls were extracted sequentially with aqueous solvents of increasing alkalinity, and the major extracts were further purified. The fine structure of fractions was studied by chemical and spectroscopic methods. Pectins were obtained in all the extracts but containing different amounts of homogalacturonans (HG) and rhamnogalacturonans I. Galactomannans (GM) were extracted with water and with 4 M KOH (molar ratio Gal:Man, 1:1.8 and 1:3, respectively); glucuronoxylans (GX) and xyloglucans (XG) were also present, but in minor amounts. The residue obtained after exhaustive extraction contained 72 % cellulose, but also HG, GM, and GX. Estimation of the contribution of the different polysaccharides showed that they are composed of around 54 % cellulose, 31 % pectins, and 15 % hemicelluloses, and they represent as a whole, lesser amounts than those reported for mature hulls. This work would indicate more hydrated cell walls with lesser proportions of secondary structures in immature hulls. This characterization could help understanding the susceptibility of soybeans seeds to the cosmopolitan stink bugs.