Improving the textural properties and storage stability of cowpeas
Affrifah, Nicole Sharon
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The hard-to-cook defect is a well-known probl em in stored legumes, which reduces the consumption and production of these nutritious f oods. It is a generally held view that phytase is involved in the development of the defect. The main objectives of this dissertation were to devel op a heat process targeted at inactivating phytase and consequently improving the textural and storage qualities of cowpeas by preventing/reducing hardening. The inactivation kinetics of phytase was determined at different moisture contents (10-35% d.b.) in the temperature range of 70-95°C. Phytase exhibited a high thermo-stability, retaining 63-95% activity at 10% moisture. Increasing temperature and moisture content enhanced enzyme inactivation. Inactivation kinetics was expressed by a fractional conversion model, which was success fully used in predicting phytase activity in cowpea flour. The estimated E a were 33.29, 37.87 and 43.4 KJ/mol at 10, 25 and 35% moisture, respectively. Phytase inactivation in cowpea seeds was spatially dependent; the regions close to the surface showed a higher inactivation than the interior. Phytaseinactivation predicted using a finite element model was slightly lower than experimental values. Twelve treatments were designed by varying moisture content, steaming temperature and time. Their potential in preventing hardening of stored cowpeas was investigated. The effect of the drying conditions applied after steaming was also studied. Changes in textural and physicochemical indice s were measured after storage at either 4°C or 42°C/80%RH. Generally, steaming significantly reduced the cooked texture before storage; however, only steaming at 121°C for 4 or 6 min was successful in preventing/reducing hardening during storage at 42°C/80%RH. Significant decreases in water absorption, phytate c ontent and phytase activity were recorded whereas increases in solids lo ss, electrolyte leakage and pectin loss were noted. Drying conditions contributed signi ficantly in maintaining the textural characteristics of stored cowpeas. Cooked texture, phytase activity and phytate content significantly decreased as drying temperatur e increased. Increasi ng drying temperature and humidity had positive effects on water ab sorption and electrolyte leakage. Pectin loss during soaking and cooking was however not significantly influenced by any of the drying parameters considered.