Thin-layer modelling of sweet potato slices drying under naturally-ventilated warm air by solar-venturi dryer

The drying of agricultural produce is an important food preservation measure. However, high energy requirements for drying, limited access to energy sources, small-medium scale farmers experience high post-harvest losses of their produce. Therefore, there is a need to develop and optimize low-cost food preservation technologies. In this study, eleven mathematical models were used to estimate the drying coefficients following nonlinear regression method, in hot-air oven and naturally-ventilated solar-venturi drying to find the best fit of the moisture ration models. Sweet potato tubers were sliced at 3, 5 and 7 mm thickness sizes with and without pre-drying treatments. The prepared sweet potato slices were dried in a hot-air oven dryer at a constant temperature of 70 degrees C in comparison to the naturally-ventilated solar-venturi dryer with a heated ambient air that varied according to the outside environmental conditions. The drying rate of the samples in a hot-air oven dryer was higher than for those in the naturally-ventilated solar-venturi dryer. The results showed that the drying time was significantly (P < 0.05) affected by the thickness size of sweet potato slices (SPS) and that the drying took place at the falling rate period and a constant drying time. The Midilli et al. model was the best fit for predicting the moisture ratio of SPS dried in hot-air oven dryer and naturally-ventilated solar-venturi dryer based on statistical analysis (R-2 = 0.982-0.999, chi(2) = 4.60 x 10(-6)-5.56 x 10(-5) and RMSE = 0.011-0.067). The D-eff was 3.32 x 10(-9) - 6.31 x 10(-9) m s(-1) for the naturally-ventilated solar-venturi dryer and 1.02 x 10(-8) - 2.19 x 10(-8) m s(-1) for the hot-air oven dryer. According to the results obtained, naturally-ventilated, solar-venturi dryer and lemon juice pre-drying treatment are the suitable application for small-medium scale drying of SPS under Pietermaritzburg conditions.

Automated summary

Drying of agricultural produce is important for food preservation, and the development of low-cost preservation technologies is necessary. This study investigated the drying of sweet potato slices using mathematical models and found the best-fit model. The results showed that naturally-ventilated solar-venturi dryer and lemon juice pre-drying treatment are suitable for drying sweet potato slices on a small-medium scale.