Bean softening during hydrothermal processing is greatly limited by pectin solubilization rather than protein denaturation or starch gelatinization

Abstract

The rate liming step of bean softening during cooking was evaluated. Red kidney beans (fresh/non-aged and aged) were cooked at different temperatures (70–95 °C) and their texture evolution established. Softening of beans (loss of hard texture) with cooking and increasing cooking temperature was evident at ≥ 80 °C more so for non-aged than aged beans, evidencing hard-to-cook development during storage. Beans at each cooking time and temperature were subsequently classified into narrow texture ranges and bean cotyledons in the most frequent texture class evaluated for the extent of starch gelatinization, protein denaturation and pectin solubilization. During cooking, starch gelatinization was shown to precede pectin solubilization and protein denaturation, with these reactions progressing faster and to a greater extent with increasing cooking temperature. At 95 °C for instance (practical bean processing temperature), complete starch gelatinization and protein denaturation is attained earlier (∼10 and 60 min cooking, respectively and at comparable time moments for both non-aged and aged beans) than plateau bean texture (∼120 and 270 min for non-aged and aged beans)/plateau pectin solubilization. The extent of pectin solubilization in the cotyledons was consequently most correlated (negatively, r = 0.95) with and plays the most significant role (P < 0.0001) in directing the relative texture of beans during cooking. Ageing was shown to significantly retard bean softening. Protein denaturation plays a less significant role (P = 0.007) while the contribution of starch gelatinization is insignificant (P = 0.181). Thermo-solubilization of pectin in bean cotyledons is therefore the rate limiting step of bean softening towards attaining a palatable texture during cooking.