Effect of cooking on non-starch polysaccharides of hard-to-cook beans

Experiments carried out to study changes induced by hard-to-cook (HTC) phenomenon in the non-starch polysaccharides of beans stored at 30 °C and 75% RH for 8 months showed that the development of HTC did not affect the amounts of soluble and insoluble fibre in cooked seeds but changed their carbohydrates physical properties. Aged beans non-starch polysaccharides presented lower water-solubility and underwent lower degradation of galacturonans and arabinose-rich polysaccharides when submitted to cooking. The decrease in non-starch polysaccharides water-solubility produced a shift in the polymers fractionation profile which resulted in an increase of weak and middle-alkali soluble polymers bulk as well as in their arabinose and uronic acid contents. Uronic acid contents were higher in polymers released by 1 M NaOH and in the cellulose-rich residues while the arabinose contents were higher in the mild-alkali soluble polymers of aged seeds. Methylation analysis showed no evident alterations in the xyloglucans and arabinans branching degree with beans ageing. However, both, the molecular mass of water-soluble pectins and CDTA-soluble pectins, increased. Even though changes in the non-starch polysaccharide solubility were not related to the decrease in the arabinan and xyloglucan degree of branching they may be related to the formation of new chemical interactions other than hydrogen bond. There was a correlation between acidic and neutral polysaccharides insolubilisation in beans ageing and probably in beans hardening. After processing, aged seeds present higher amounts of insoluble fibre when compared to normal beans.