Influence of Pretreatments on Microwave Vacuum Drying Kinetics, Physicochemical Properties and Sensory Quality of Apple Slices

The objective of this study was to determine and compare the effect of five different pretreatments including sulfite treatment (ST), osmotic dehydration (OD), steam blanching (SB), steam blanching plus osmotic dehydration (SB+OD), and ultrasound treatment (UT), on the microwave vacuum drying (MVD) kinetics, the physicochemical properties, and sensory quality of the apple slices. The results showed that the pretreatments prior to MVD could reduce the drying time of apple slices by 25-45% as compared to the non-pretreated apple slices, and the drying time in the SB+OD apple slices sample was shortest. Whether pretreated or not, MVD process was controlled by diffusion and characterized by a two-stage falling-rate drying. As calculated according to the Fick's law of diffusion, the moisture diffusivities ( D eff ) ranged from 1.64×10 -8 to 3.46×10 - 8 m 2 /s. Different pretreatment methods had a significant influence on the physicochemical properties and sensory qualities of the dried products (p<0.05). The SB+OD-pretreated apple slices showed the lowest shrinkage (59.5%) and the highest total sugar content (77.90 g/100 g dry matter). Besides, the OD-pretreated apple slices showed the highest density (0.953 g/cm 3 ) and the lowest titratable acidity (1.67 g/100 g dry matter). In addition, the ST-pretreated samples showed the highest titratable acidity (3.21 g/100 g dry matter) and vitamin C content (12.74 mg/100 g dry matter), while the sample pretreated by SB showed the highest total phenolics content (18.37 mg/g dry matter). Non-sulfite-pretreated samples were superior to the sulfite treatment or the control in flavor but inferior to the sulfite treatment in color. Results of preference sensory evaluation showed that the dried apple slices were preferred by the panelists in the following order: SB+OD>SB>UT>OD>ST>the control. ntrol.