Gamma-irradiation of cowpeas to understand the mechanisms responsible for the hard-to-cook defect

Abstract

Cowpea is one of the most important food grain legumes, with potential to help alleviate protein-energy malnutrition in Africa. However, its utilisation is limited by its extended cooking time, which is associated with the development of the hard-to-cook (HTC) defect. The HTC defect occurs when legumes are stored under conditions of high temperature and high relative humidity (HTHH). The study focused on the application of γ-irradiation in HTC cowpeas, in order to further elucidate the mechanisms of development of HTC defect. Cowpea varieties, Agrigold (more HTC susceptible) and Bechuana White (less HTC susceptible) were selected for study. The accelerated HTHH storage conditions used were 40ºC and 80% relative humidity (RH). After HTHH storage, γ-irradiation at 11 kGy was applied to the HTC cowpea varieties to determine its effects on cellular structure and pectin solubility. In both varieties, γ-irradiation reduced cotyledon cell wall thickness, increased cell size and intercellular spaces. However, it did not reverse the HTC defect in the HTC susceptible variety. In the less susceptible variety, cold water- and hot water-soluble pectin increased but there was no effect on chelator soluble pectin (CSP). Since changes caused by γ-irradiation were associated with pectin, this supports the phytase-phytate-pectin theory to some extent. However, non-reversal of the HTC defect and absence of effect on CSP indicate that other mechanisms are involved in HTC development in cowpeas, possibly formation of alkali-soluble ester bonded pectins. To further understand the changes brought about by irradiation, γ-irradiation at 11 kGy was applied to the two cowpea varieties which had been stored under accelerated HTHH conditions of 40ºC and 80% RH. The effects of γ-irradiation on the functional and thermal properties of the more HTC susceptible and less susceptible cowpea seeds and cooked prepared pastes were investigated. Gamma-irradiation decreased water solubility index (WSI), swelling power, peak-, breakdown- and setback viscosities in both cowpea varieties, presumably due to starch depolymerisation. Water absorption capacity (WAC) increased in the less HTC variety after γ-irradiation, but with no effect in the more HTC variety. Furthermore, γ-irradiation did not affect thermal properties in either variety. Generally, γ-irradiation had no effect on most of the functional properties of cooked prepared pastes in both varieties, with the exception of WAC, which increased in the more HTC variety. Since γ-irradiation affected all starch related functional properties with the exception of thermal properties, the non-reversal of the HTC defect suggests that changes in starch during HTC development are a consequence and not a cause of the HTC defect. Furthermore, they were no differences in the functional properties of pastes from both varieties. Thus, the preparation of cooked pastes eliminated the HTC phenomenon in cowpeas.