Influence of hot water blanching and saline immersion period on the thermal effusivity and the drying kinetics of hybrid solar drying of sweet potato chips

Abstract

The purpose of this study was to investigate the effect of adding pre-treatment to solar dried sweet potato chips. The tested pre-treatments were blanching using hot water and saline immersion. Indeed, the potato chips were dried using a convective indirect solar dryer, equipped with a wind generator and glycerine as thermal storage. The system was built locally at the University of Agriculture in Umuahia, Abia State, Nigeria. The obtained experimental drying curves and kinetics were directly affected by the weather conditions, as the drying rate increased with the increase of the radiation and the ambient temperature and decreased with their decrease. Drying of the various pre-treated potato slices took 8–14 h to attain equilibrium moisture level under the variable external drying conditions. The drying curves were fitted to semi-empirical models. The results showed that the two-term exponential model was the best model that fitted the experimental data. Shrinkage was a vital phenomenon observed during the drying process, as the volume of the chips was reduced by 50%. The volume of samples treated by saline immersion was higher by 6% than non-treated samples. Shrinkage phenomenon was then introduced to determine the thermos-physical properties of sweet potato chips, with particular attention to the particle density, the bulk density, shape analysis, specific heat capacity, thermal diffusivity, and thermal effusivity. The particle and bulk density peaked at 1.41–1.44 g/cm3, and 1.52–1.53 g/cm3 respectively for all treatments. The thermal effusivity ranged from 17.6 to 7.93 W.S1/2.m−2.K−1 while the specific heat capacity ranged from 1028 to 2288 J/kg °C. All the thermo-physical properties were presented as a function of the volume or moisture content variation with a high degree of association. The results demonstrated the importance of shrinkage and its direct effect on the determination of the thermos-physical properties of the studied product for both pre-treated and untreated samples.