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% relative 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, pectin loss increased slightly after soaking but decreased significantly after cooking. The pH of soaking or cooking liquors also decreased, presumably resulting from decreased tissue pH reported earlier. With heating temperature, pectin loss was lower below 60 degrees C, higher at 85 degrees C, and maximum at 100 degrees C. These changes in pectin loss from cowpeas were similar to those in viscosity of pectin solution as affected by medium pH and temperature in a model study reported earlier, suggesting that pectin beta-eliminative reaction occurs during heating 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 cooking time, no apparent relationship existed between the two. Overall results suggest that the hard-to-cook defect is caused in part by reduced pectin beta-degradation during cooking, which apparently results from decreased tissue pH during aging