Improvement in the cooking and physico-chemical characteristics of hard-to-cook cowpeas by pre-conditioning and micronization

Abstract

Cowpeas (Vigna unguiculata L. Walp) are legumes widely consumed in developing countries. Because cowpeas are an important source of proteins, calories and vitamins, they have the potential to alleviate protein-energy malnutrition. However, the consumption of cowpeas is impaired by the hard-to-cook (HTC) defect, which develops when cowpeas are stored at high temperature and humidity conditions. HTC cowpeas require extended time to cook and have decreased protein, starch, vitamin availability and poor textural quality. The extended cooking time and poor textural quality reduce consumer preference and acceptability. Soaking cowpeas in water or in a solution containing monovalent cations have been used by other researchers to reduce the cooking time of normal and HTC cowpeas, while micronization has been used to reduce the cooking time of normal cowpeas. Hence preconditioning in water or in a solution containing monovalent (Na+) cations in combination with micronization, could have the potential to help in alleviating the HTC phenomenon in legume seeds, especially cowpeas. This study was undertaken in two parts. The first part consisted of inducing the hard-tocook (HTC) defect and determining its effect on cooking and physicochemicalcharacteristics of cowpeas. The second part consisted of attempting to alleviate the HTC defect in cowpeas by pre-conditioning cowpeas in water or in a solution with monovalent (Na+) cations and its combination with micronization. The effect of these treatments on cooking and physicochemical characteristics of normal and HTC cowpeas were studied. Storing cowpeas at high temperature and high relative humidity, increased the cooking time of cowpeas (Mogwe-o-Kgotsheng) from 89 to more than 270 min. The increase in the cooking time was associated with reduced pectin solubility, which was coincident with a decrease in phytic acid content and an increase in phytase activity. According to the “phytate-divalent cations-pectins” theory, at adverse storage conditions, phytase probably hydrolysed phytate to release divalent cations which migrated to the middle lamella to bind with pectins, reducing their solubility. Because of the reduced pectin solubility of HTC cowpeas, the hardness of cooked seeds increased, the degree of splitting reduced and water absorbed during cooking consequently reduced as compared with normal cowpeas. This research supports the “phytate-divalent cations-pectins” as an important mechanism to explain the HTC-defect in legume seeds. From a practical standpoint, pre-conditioning cowpeas in water on its own was effective in reducing the cooking time of normal cowpeas from 89 to 44 min. This coincided with an improvement of pectin solubility, degree of splitting and decreased hardness. For HTC cowpeas, a combination of pre-conditioning in a solution containing monovalent (Na+) cations and micronization was needed to optimally reduce the cooking time from more than 270 min to 59 min. This coincided with an improvement of pectin solubility, degree of splitting and decrease in the hardness of cooked cowpeas. Pre-conditioning cowpeas in water induced the solubilization of pectins in the middle lamella of normal and HTC cowpeas. Pre-conditioning cowpeas in a solution with monovalent (Na+) cations improved pectin solubility due to the solubilization effect of water as well as a conversion of insoluble pectins to soluble pectins by monovalent (Na+) cations. Micronization improved pectin solubility further by breaking pectin molecules into lower and more soluble fractions, probably via the β-elimination reaction. Micronization also decreased the hardness of cooked seeds and increased the degree of splitting for both normal and HTC cowpeas. However, the reduction in hardness and increase in the degree of splitting were more pronounced in normal than in HTC cowpeas, probably becausemore divalent cations were bound to the pectins of HTC cowpeas. For normal cowpeas, the improvement of pectin solubility, decrease in texture of cooked seeds and increase in the splitting as influenced by micronization was reflected in the increase of the amount of water absorbed during cooking, which could have contributed to the reduction in the cooking time. However, for HTC this was not the case as the water absorbed during cooking decreased. The reduction in the amount of water absorbed during cooking could be associated with protein denaturation during storage at adverse conditions and during micronization. The improvement of pectin solubility was at higher levels when all the treatments were applied in HTC cowpeas than in normal cowpeas. However, this was not coincident with reduction in the cooking times. This suggests that factors (i.e. proteins, starch) other than pectin solubility could have contributed to the cooking time of HTC cowpeas. Pre-conditioning cowpeas in a solution with monovalent (Na+) cations in combination with micronization has a definite potential to help alleviate the HTC defect in cowpeas.</P. Copyright 2011, University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria. Please cite as follows: Salvador, BDV 2007, Improvement in the cooking and physico-chemical characteristics of hard-to-cook cowpeas by pre-conditioning and micronization, MSc(Agric) dissertation, University of Pretoria, Pretoria, viewed yymmdd < http://upetd.up.ac.za/thesis/available/etd-08042008-132413 / > E846/gm