Calcium transport and phytate hydrolysis during chemical hardening of common bean seeds.

In this study, two chemical bean seed hardening methods were used to investigate the changes in cooking behavior associated with Ca transport and phytate hydrolysis to better understand their role in the pectin-cation-phytate hypothesis. The texture evolution of fresh and hardened red kidney beans was evaluated, hardening being induced by soaking or in a CaCl solution (0.01 M, 0.05 M, 0.1 M) or sodium acetate buffer (0.1 M, pH 4.4, 41 °C). The beans soaked in a CaCl solution at higher concentrations or in sodium acetate buffer for a longer time exhibited a delayed cooking behavior. This study also explored the bio-chemical changes (calcium content in different bean substructures, phytate content and the pectin degree of methylesterification (DM) in the cotyledons) occurring in the beans during chemical hardening and cooking. The Ca concentrations in the whole beans and cotyledons of beans soaked and cooked in CaCl solutions significantly increased while inositol hexaphosphate IP content showed no significant changes. This indicates that the delayed texture drop in this case results from the influx of exogenous Ca in the cotyledons and seed coats during cooking while the IP was not hydrolyzed and did not release endogenous Ca. For beans soaked in sodium acetate buffer, phytate profiling showed increased hydrolysis of IP with longer soaking time, suggesting the migration of endogenous Ca released from phytate hydrolysis contributing to the delayed cooking of these beans. These results indicate that both an exogenous Ca influx during soaking and cooking and an endogenous Ca replacement resulting from phytate hydrolysis can play an important role in the hardening of beans. In neither of the cases, a significant change in pectin DM was observed during chemical hardening, therefore limiting the delayed cooking to the role of Ca transport. The outcome of both cases is inline with the basic principles of the pectin-cation-phytate hypothesis whereby pectin DM changes are hardly involved and different mechanisms of release/transport are involved.