Cotyledon pectin molecular interconversions explain pectin solubilization during cooking of common beans (Phaseolus vulgaris)

Dynamics of pectin extractability in cotyledons and seed coats were explored for mechanistic insight into pectin changes due to aging and cooking of beans. In addition, changes in mineral distribution during cooking were determined in order to investigate their retention in the matrix. Pre-soaked fresh and aged beans were cooked in demineralized water for different times and the cotyledons, seed coats and cooking water were lyophilized. From cotyledon and seed coat powders, alcohol insoluble residue (AIR) was extracted and sequentially fractionated into water-, chelator- and sodium carbonate-extractable pectin (WEP, CEP and NEP, respectively). Characterization of pectin in AIR and pectin fractions revealed inherent structural differences between cotyledon and seed coat pectin with the latter exhibiting a lower degree of methylesterification (DM) and being more linear. Due to aging, WEP decreased whilst NEP substantially increased and the CEP fraction and DM of pectin in AIR did not change significantly, suggesting a more crucial role of increased covalent bonding than cationmediated crosslinking in aging-induced hardening of beans. During cooking, some NEP was converted into WEP and no pectin depolymerization was observed from molar mass distribution profiles. Pectin changes due to aging and cooking of beans were more pronounced in the cotyledon compared to the seed coat. Whilst Ca2+, Fe(2+ )and Zn(2+ )were largely retained in the bean matrix during cooking, Mg(2+ )was largely leached from cotyledons into the cooking water. In conclusion, aging-induced hardening of beans and softening during cooking were found to be premised on interconversion of pectin fractions in cotyledons.