Micronisation and hot-air roasting as pre-treatments to control hard-to-cook phenomenon in cowpeas

Abstract

Cowpea (Vigna unguiculata (L.) Walp.) is one of the most important grain legume in sub- Saharan Africa. It is an important source of affordable dietary proteins for the low-income populations. Cowpeas develop hard-to-cook (HTC) defect during storage at high temperaturehigh humidity (HTHH) conditions experienced in these regions. Cowpeas with HTC defect are characterised by long cooking time that lead to increased energy demands that add constraints on the consumers with limited resources. The main objective of this work was to determine the effect of micronisation and hot-air roasting as strategies in the control of HTC defect development. A preliminary study determined the effect of accelerated storage conditions of 40 °C and 80% relative humidity for 40 days on HTC defect development among 3 cowpea types. The accelerated conditions simulate the HTHH storage conditions. The 3 cowpea types (Bechuana white, Agripienk and Mae–e–tsiliwane) stored at these HTHH conditions developed HTC defect as shown by the increased cooking time. The increase in cooking time was not observed in the control cowpeas stored at 4 oC. The increase in cooking time differed among the 3 cowpea types and this indicated varying degree of susceptibility to HTC defect. HTC defect development at the accelerated HTHH conditions was shown to be due to increased phytase activity that resulted in decreased phytate content and decrease in water soluble pectin. This was in agreement with the "phytase-phytate-pectin" theory. The role of lignification was not established during HTHH storage conditions since peroxidase activity and lignin content did not increase. Agripienk which had the highest increase in cooking time was selected for the heat pre-treatment study. Cowpeas of the Agripienk type were pre-conditioned to 25% moisture before being micronised or hot-air roasted at 150 oC for 5 minutes. Phytase activity was reduced by 70% and 45% by micronisation and roasting pre-treatments respectively. After HTHH storage the cooking time increased by >430%, 243% and 92% for the control, roasted and micronised cowpeas respectively. Micronised cowpeas had the lowest increase in cooking time. Therefore, HTC defect was controlled but not prevented by the heat pre-treatments. Micronised cowpeas which had the lowest phytase activity has higher phytate and water soluble pectin contents when compared to both roasted and control cowpeas. The mechanism of HTC defect control by the heat pre-treatments was attributed to partial phytase inactivation which decreased the rate phytate hydrolysis and liberation of divalent cations available to bind to pectin in the middle lamella. There was higher content of water soluble pectin and lower content of chelator soluble pectin in the heat pre-treated cowpeas than in the control after HTHH storage. Increase in chelator soluble pectin indicated formation of pectates that limit the rate of cell separation during cooking. Based on these findings, cell separation during cooking was investigated further. Confocal laser scanning microscopy showed that there was more cell separation in the heat pre-treated cowpeas when compared to the control after HTHH storage. The control cowpeas showed minimal cell separation even after 2 hours of cooking. The ease in cell separation in the heat pre-treated cowpeas was due to presence of more soluble pectin in the middle lamella when compared to the control that had more chelator soluble pectin. Micronised cowpeas had more cell separation when compared to the hot-air roasted cowpeas. Differential scanning calorimetry of cowpea flours showed that HTHH did not lead to an increase in gelatinisation temperatures (To and Tp) or gelatinisation enthalpy( H) in control, micronised and hot-air roasted cowpeas. Increase in these thermal properties is suggested to lead in increased cooking time. The pasting viscosities of the cowpea flours decreased after heat pretreatments but HTHH conditions did not alter the viscosities of either the control or heat pretreated cowpeas. Therefore the changes in starch thermal and pasting properties due to HTHH storage were not observed. This study indicates that HTC defect development was dependent on phytase activity during HTHH storage. The effectiveness of micronisation and hot-air roasting in preventing HTC defect was dependent on the degree of phytase inactivation. Micronisation was more effective than hot air roasting in controlling the development of HTC defect due to a higher degree of phytase inactivation.