Influence of Different Pretreatments on Hot air and Microwave-Hot Air Combined Drying of White Sweet Cherry

Abstract

Microwave (MW)-hot air (HA) combined drying was applied to white sweet cherries besides solelyHA drying at 50, 60 and 70°C in the presence of citric acid, sucrose and freezing pretreatment inthis study. Single power level of MW (90 W) was chosen, and drying behavior of all samples wasmodelled by using eleven thin layer equations. Two-term, rational and sigmoid models were themost suitable models for describing drying phenomena. Effective moisture diffusivities (Deff)ranged from 1.724×10-10 to 5.173×10-10 m2/s in HA drying and from 4.260×10-10 to 1.805×10-9 m2/sin MW-HA drying. Activation energies (Ea) were between 2.785 and 30.541 kJ/mol and 6.929 and42.101 kJ/mol for HA and MW-HA drying techniques, respectively. Total color change (ΔE) levelsof the outer surface of dried cherries were generally higher than the ones of inner surface. Freezingpretreatment had a comparably lower enhancing effect on the total phenolic content (TPC) of HAdried white sweet cherries compared to fresh sample. The TPC of freezing pretreated and HA driedat 50°C and HA dried at 70°C control samples were 1.481 ± 0.398 mg gallic acid equivalent(GAE)/g dry matter (DM) and 6.181 ± 0.012 mg GAE/g DM as the minimum and maximum,respectively. These values were determined as 4.183 ± 1.728 and 8.240 ± 0.502 mg GAE/g DM thatwere belonged to MW-HA dried at 50°C control and freezing pretreated MW-HA dried at 70°Csamples in combined procedure, respectively.