Evaluation of electrical and other physical properties of heated sweet potato

Electrical impedance spectroscopy is expected to use for quality estimation of agricultural product. In this study, changes in moisture content, porosity and electrical and mechanical properties of sweet potato heated in hot water at 50, 55, 60, 65, and 70 °C were investigated. Additionally, the tissue structure was scanned by X-ray computerized tomography (CT). As a result, the porosity of the sample decreased with treatment time at every temperature while the moisture content changed at only 70 °C. This decrease in the porosity greatly affected the extracellular resistance and the elastic modulus. Moreover, the elastic modulus could be expressed by a logarithmic equation of the extracellular resistance with the high determination coefficient (R² = 0.91). Average CT values calculated from the scanned images of the tissue also indicated good correlations with porosity and elastic modulus. These results indicated that the extracellular resistance and the average CT value were effective for the estimation of elasticity as well as porosity. Practical applications: Sweet potato (Ipomoea batatas (L.) Lam.) is one of the most popular vegetables in Japan with its highly nutritional value and sweet taste. It is widely used for processed foods and shochu (Japanese liquor). To make these products, sweet potato is heated by hot water immersion or steaming in the process lines. Because the processes have been mechanized, technological development which enables to estimate inner state of sweet potato during heating is required. This research investigated potential of electrical impedance spectroscopy and CT scanning as the quality estimation technique for heated sweet potato. The results presented here clarified relationships between electrical properties and other physical properties, and CT value and the properties. The knowledge will contribute to design a quality control system based on the automatically quality estimation.