Hard-To-Cook Defect in Black Beans — Peroxidase Characterization and Effect of Heat Pretreatment and Storage Conditions on Enzyme Inactivation

Abstract

Black beans (Phaseolus vulgaris) were studied in order to characterize the soluble peroxidase (POD) activity and to determine the effect of heat pretreatment and storage conditions on hardening and enzyme inactivation. Bean POD activity demonstrated a pH optimum from 6.4 to 6.8, an optimum temperature between 40 and 45°C, an activation energy of 1.19kJ/mol, a strong dependence on water activity and little sensitivity to added calcium ions. Beans stored at 30°C/85%RH exhibited a 30–50% reduction in POD activity compared to samples kept at 15°C/35%RH but this may not reflect in vivo rates. Thermal inactivation of POD activity in bean flour was facilitated at increasing moisture levels. A heat pretreatment of beans at moisture contents not exceeding 13% failed to control hardening in beans subsequently stored at temperatures of 27°C or above and moisture levels of 13%. While no POD activity was found at heat pretreatment equilibrium temperatures above 102°C and none of the samples that had acceptable hardening values demonstrated POD activity, increasing equilibrium temperature above 105°C continued to reduce hardness. Thus, heating to only inactivate POD was not sufficient and higher temperatures were required to inactivate the hardening mechanism.