Convective drying of fruits: Effect of pretreatment, slice thickness and drying air temperature on drying kinetics and product quality.

fruits are regarded as nutrient-dense, capable of promoting nutritional and metabolic human health. However, they are seasonal and highly perishable which limits their consumption and wider utilization. In this study, the effect of slice thickness (3 mm and 5 mm), pretreatments (steam blanching and dipping in ascorbic acid solution) and drying air temperature (40 °C, 50 °C, 60 °C and 70 °C) on drying kinetics, color, β-carotene and vitamin C content of fruits were investigated. The results showed that the drying time increased as slice thickness increased, and decreased as drying air temperature increased but did not follow any trend for pretreatment. The Page model (R values of 0.9998-0.9999) exhibited the best fit to the drying kinetics data. The diffusivity values (5.31 × 10 to 4.14 × 10 ms) increased as the slice thickness and drying air temperature increased but had no linear trends with pretreatment. The activation energy ranged from 14.35 to 44.78 kJmol, with the highest being recorded by 5 mm untreated samples and the lowest by the 3 mm blanched samples. The total color change (ΔE*) of the samples generally decreased as the drying air temperature increased but increased as the slice thickness increased. The ascorbic acid pretreated samples had the least color change, followed by the untreated samples while the blanched samples had the highest change. Overall, the 5 mm ascorbic acid pretreated samples dried at 70 °C had the least color change (13.33 ± 0.52). The blanching and dipping in ascorbic acid solution generally yielded lower β-carotene and vitamin C values as compared to the untreated samples. The 3 mm ascorbic acid pretreated samples dried at 50 °C recorded the lowest β-carotene (42.70 ± 3.21 μg/100 g) while the 5 mm ascorbic acid pretreated samples had the lowest vitamin C (37.50 ± 2.65 mg/100 g) at 70 °C. Pretreatments and drying air temperatures showed mixed effects on the drying characteristics, color, β-carotene and vitamin C contents of fruit slices. The findings, therefore, indicate that a compromise may have to be made on the aforementioned processing conditions in order to meet the desired attributes of one's interest.