Effects of different pretreatment techniques on the color indexes, drying characteristics and rehydration ratio of eggplant slices

This study aimed to examine the influence of microwave pretreatment (MWP) for 0, 15, 30, 45, and 60 s, and ultrasound pretreatment (USP) for 0, 5, 10, 15, and 20 min, on the drying time, effective moisture diffusivity (Deff), rehydration rate, color indexes (lightness, redness, and yellowness), and total color change (ΔE) of eggplant slices during dehydration in a hot-air dryer. The results demonstrated that USP significantly reduced the drying time (higher water loss) of eggplant slices (p < 0.05). While, the MWP significantly increased the drying time of eggplant slices (p < 0.05). The Deff values of ultrasonic pretreated slices were considerably higher than those of untreated eggplant slices (p < 0.05). The Deff determined by Fick's second law was varied from 5.95 × 10−10 to 9.56 × 10−10 m2s−1, and from 1.27 × 10−9 to 1.42 × 10−9 m2s−1, for samples pretreated by microwave and ultrasound, respectively. Nine mathematical models (Page model, Quadratic model, Newton model, Verma model, Logarithmic model, Two-term Exponential model, Wang and Singh model, Henderson and Pabis model, and Midilli model) were used to estimate the drying coefficients by following the non-linear regression method. The results showed that the Page model became the most accurate model to show the drying characteristics of eggplant slices compared to eight other confirmed mathematical models with sum of squared error (SSE) values ranging from 0.0014 to 0.0098. The average rehydration ratio of untreated, microwave-treated, and ultrasound-treated eggplant slices were 404.6 %, 360.1 %, and 438.6 %, respectively. The redness, yellowness, and ΔE parameters of pretreated samples by microwave were higher than the untreated slices. It can be concluded that USP proved to be effective in retaining surface color and improving drying speed of eggplant slices compared to MWP. The results showed that the ultrasonically pretreated eggplant slices had shorter drying times, improved Deff values, higher rehydration rates, and appropriate appearance.