Physicochemical Properties of Hydrothermally Treated Cowpea Seeds

ABSTRACT

Cowpeas are a relatively inexpensive source of proteins and vitamin B. Unfortunately, the problem of insect infestation is one of the main constraints to the efficient and widespread utilisation of cowpeas. The hydrothermal treatment which involves the exposure of cowpeas to steam followed by drying has been used effectively in protecting cowpeas against the insect infestations. This study set out to assess the functional, chemical and textural changes associated with the hydrothermal treatment. An alternative steaming system was designed and compared to the current steaming system used in the process. Changes in physical, functional and chemical properties of two (2) cowpea varieties - blackeye and asontem, following steaming for 0, 2, 5,10 and 15 min were investigated. Indices measured were germinating capacity, water absorption capacity, soaked seed hardness, protein solubility, least gelation capacity and trypsin inhibitor activity. Steaming had a significant effect on all the indices. A complete loss of viability following steaming was observed in all the seeds. There were also significant decreases in the water absorption capacity and protein solubility of steam treated cowpea seeds. The steam treatment was effective in inactivating trypsin inhibitors, with complete inactivation being attained after 10 min steaming. Steaming also resulted in increases in leached solids during soaking and the concentration required to form a stable gel. The effect of steaming on textural characteristics was monitored over a 6 month storage period at either room temperature (~28°C) or cold room temperature (6°C). Two test cells - the Kramer Shear and the Warner-Bratzler blade were used for the analysis. The steam treatment resulted in an immediate and pronounced increase in cooked bean hardness, such that peak force values increased from less than 20kg to greater than 25 kg after 2 hours cooking. Storage however, had different effects on the steamed and unsteamed cowpeas. The typical increase in hardness of stored legumes was observed in the unsteamed seeds, whilst the steamed seeds (especially the 10 min sample) showed a decrease in cooked bean hardness after the 6 months of storage. A three factor-three level Box-Behneken experimental design was used to follow the effect of steaming time (2, 6, 10 min), drying temperature (35, 45, 55°C) and drying humidity (0.01, 0.02, 0.03 g water/kg air) on some characteristics of blackeye peas. Indices measured were moisture content, 1000 seed weight, water absorption, dehulling efficiency and cooked bean hardness. Increasing steaming time resulted in increased moisture content whilst the most efficient drying occurred under conditions of high drying temperature and humidity. Steaming did not have a significant effect on the density of the cowpea seeds. The variables (steaming time, drying temperature and drying humidity) influenced the regression models developed to predict product characteristics. A reduction in water absorption capacity probably due to seed hardness was observed in all the steamed seeds. At high drying humidity, increasing drying temperature increased water absorption capacity. The treatments improved the dehulling efficiency with the effect of drying condition, especially temperature, being more significant. The hardening of the steamed seeds as evidenced by the reduced water absorption capacity was also reflected in the cooking characteristics of the seeds. In general, increasing drying temperature resulted in a decrease in cooked bean hardness.