Application of dynamic mechanical thermal analysis to the investigation of hard-to-cook defect in legumes

Abstract

The temperature-dependent mechanical stiffness of hard-to-cook (HTC) and control bean cotyledons and seed coats (testae) in the moisture content range 30 to 300 g/kg (wet basis) before and after a cooking treatment was measured using Dynamic Mechanical Thermal Analysis. The testa stiffness decreased less with increasing temperature in the range -40 to 100 °C than that of the cotyledon for cooked or uncooked tissues. A Mattson bean cooker showed that the cooking times of whole beans were longer than those of cotyledons. The room temperature stiffness of cotyledons removed from the beans also increased with induction of the HTC defect. The testa was 5 to 50 times stiffer than the cotyledon at a given moisture content. The moduli of cotyledons and testae decreased with increasing moisture content. Below about 180 g/kg moisture the uncooked HTC cotyledons were less stiff compared to the uncooked controls whereas the converse was observed for the uncooked testae. Above 180 g/kg moisture the modulus of HTC cotyledons was slightly greater than that of the controls whereas the control and HTC testae stiffnesses were comparable. Differences between cooked HTC and control cotyledons appeared to result primarily from the different rates of water loss in response to a particular dehydration regime. © 1997 Academic Press Limited.