Texture classification and in situ cation measurements by SEM-EDX provide detailed quantitative insights into cell wall cation interaction changes during ageing, soaking, and cooking of red kidney beans

Abstract

Hard-to-cook (HTC) is a textural defect that delays the softening of common bean seeds during cooking. While this defect is commonly associated with conventionally stored beans, soaking/cooking of beans in CaCl₂ solutions or sodium acetate buffer can also prolong the cooking time of beans due to formation of Ca²⁺ crosslinked pectin retarding bean softening during cooking. In this study, the role of the cell wall-bound Mg²⁺/Ca²⁺ content and the degree of pectin methyl esterification (DM) was quantified, as important factors for bean texture-related changes stipulated in the pectin-cation-phytate hypothesis, the most plausible hypothesis of HTC development. Evaluation of texture changes during cooking of conventionally aged beans (35 °C and 83% RH for up to 20 weeks), beans soaked/cooked in CaCl₂ solutions (0.01 to 0.1 M) or soaked in 0.1 M sodium acetate buffer (pH 4.4) revealed large bean-to-bean variations. Therefore a texture-based classification approach was used to better capture the relation between texture characteristics and cell wall polymer, in particular pectin, related changes. While cell wall-bound Ca²⁺ and pectin DM did not change/were not related to the texture variation during cooking of fresh beans, increased cell wall-bound Ca²⁺ and decreased pectin DM were associated with prolonged conventional storage of beans and their texture changes during subsequent cooking (due to pectin cross linking, retarding its solubilization during cooking). Exogenously added Ca²⁺ from pre-treating beans in CaCl₂ solutions promoted to a great extent the cell wall-bound Ca²⁺ during soaking but even more so during cooking, complementing the harder texture associated with these beans during cooking (compared to conventionally stored and fresh beans). Similarly, free Ca²⁺ endogenously generated by phytase-catalysed phytate hydrolysis (beans treated by acetate buffer) promoted crosslinking of pectin by Ca²⁺ (cell wall-bound Ca²⁺), delaying softening of beans during cooking.

Keywords: Bound Ca(2+); Chemical soaking/cooking; Conventional ageing; In situ localisation; Pectin DM; Red kidney beans; Texture classification.