Image analysis and dynamic modeling of thin layer drying of osmotically dehydrated pumpkin

Pumpkins are traditionally dried with either solar or hot-air dryers, which call for lengthy drying times that degrade product quality. To improve the quality of dehydrated pumpkins, the concept of osmotic dehydration followed by hot-air drying has been tested in recent years. The objectives of this study were to investigate the effect of osmotic dehydration as a pretreatment on the hot-air drying kinetics of pumpkin, to evaluate the best model for hot-air drying of pumpkin, and to apply a computer vision system (CVS) to study the color changes during drying. Pumpkin cubes were treated in 50% w/w sorbitol or sucrose solutions at 50°C for up to 6 hours, followed by hot-air drying at 60°C and air velocity 1 m/s. Drying data were fitted to eleven drying kinetic models. The goodness of fit was determined using R², λ², RMSE and E%. A CVS was used for color evaluations, during which the color images were converted to L*/a*/b* values. The color changes of dried samples were evaluated with total color change (ΔE*), which was fitted to a linear equation. Samples pretreated with different osmotic solutions exhibited different equilibrium moisture contents. The two-term model and the modified Henderson and Pabis model were found to have the highest R² and lowest λ², RMSE and E% values and were hence considered as the best fit for pumpkin drying. Hot-air dried samples pretreated with sorbitol had a higher L* values compared to the hot-air along drying while that pretreated with sucrose exhibited lower L* values. The ΔE* values increased during drying in a Sequence of ΔE*_{sucrose solution} < ΔE*_{sorbitol solution} < ΔE* _{sucrose + hot-air} < ΔE*_{sorbitol + hot-air}.