Effects of pre-treatments on drying kinetics and energy consumption, heat-mass transfer coefficients, micro-structure of jujube (Zizyphus jujuba L.) fruit

Abstract In this study, jujube fruits were investigated effects on drying kinetics and energy consumption (total-specific), heat-mass transfer coefficients, micro-structure of pre-treatments 2% ethyl oleate, 540 W microwave and freeze-thaw + 720 W microwave. Initial moisture of fruit from 0.75 ± 0.2 g (water).g-1 (dry matter) to 0.15 ± 0.1 g (water).g-1 (dry matter). The shortest drying durations were observed in 2% ethyl oleate pre-treated samples. The greatest total power consumption (1.502 kW) was observed in control samples The greatest specific energy consumption (158.98 kW.kg-1) was observed in freeze-thaw pre-treated samples. The effective diffusion values varied between 9.53 x 10-8-5.26 x 10-8 m2.s-1. Lewis and Jena-Das models the best estimated time-dependent moisture ratios in freeze-thaw pre-treated samples. The average speed values was determined between 0.0025-0.0005 (gr db.minute-1) at the beginning of the drying and decreased to about 0.0005 (gr db.minute-1) at the lasting of the drying. The largest mass transfer coefficient value was found depending on the time varied between 1.035 x 10-7-8.256 x 10-12 m.s-1. in the samples dried by dipping into a 2% ethyl oleate solution. The average heat transfer coefficient value is calculated 0.204 W.m2 °C-1. With regard to micro-structure of the dried samples, 2% ethyl oleate pre-treatments yielded the least deformations and had the closest structure to fresh samples.