MECHANISM OF PECTIN CHANGES DURING SOAKING AND HEATING AS RELATED TO HARD-TO-COOK DEFECT IN COWPEAS

Cowpea (Vigna unguiculata) seeds were aged at 30-degrees-C and 64% rel ative humidity for 6, 12, and 18 months. Pectin loss in control seeds (-18-degrees-C and ambient humidity storage) was very low after 6 h of soaking and very high after 1 h of cooking. As aging progressed, pect in loss increased slightly after soaking but decreased significantly a fter cooking. The pH of soaking or cooking liquors also decreased, pre sumably resulting from decreased tissue pH reported earlier. With heat ing temperature, pectin loss was lower below 60-degrees-C, higher at 8 5-degrees-C, and maximum at 100-degrees-C. These changes in pectin los s from cowpeas were similar to those in viscosity of pectin solution a s affected by medium pH and temperature in a model study reported earl ier, suggesting that pectin beta-eliminative reaction occurs during he ating of cowpeas. With aging time and heating temperature there was a negative correlation (r = -0.926) between seed texture and pectin loss during soaking or heating. Soaking in CaCl2 or an acidic buffer also caused increased hardness and decreased pectin loss. However, with coo king time, no apparent relationship existed between the two. Overall r esults suggest that the hard-to-cook defect is caused in part by reduc ed pectin beta-degradation during cooking, which apparently results fr om decreased tissue pH during aging.