Abstract
Starches were prepared from the roots of two kinds of cocoyams. Sweet potato starch was used as the control. The rheological properties of starch pastes and gels were investigated. A new method of electrical impedance measurement was developed to measure the spinning characteristics of starch pastes (1–6%), spinning distance, spinning stress and spinning energy. It was confirmed that spinning energy increased with starch concentration and tensile velocity. However, no correlation could be found between spinning distance and spinning energy. The hardness and breaking strength of 10% starch gels were measured by a curdmeter and the two kinds of cocoyam starch gels were found to have approximately the same values, although the sweet potato gel showed much lower values. The creep curves for the 10% starch gels, that showed characteristic values under the small deformation required for linear formation, were analysed using four-element models. The creep compliance curve measured at 10–40°C showed that white cocoyam is the lowest in temperature dependency and sweet potato is the highest. A master curve was based on creep compliance curves and apparent activation energy of flow was calculated. Investigation of the changing ratio between the recovery strain and permanent strain for each starch gel indicated that the value of the recovery strain is more readily affected by temperature.
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