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The effects of pre-treatments on the drying characteristics, water state, thermal properties, and bulk shrinkage of potato slices during microwave vacuum drying (MVD) were investigated. The pre-treatment included ultrasound in distilled water (USOD-0%), and ultrasound-assisted osmotic dehydration in a 60% sucrose solution (USOD-60%). Results showed that the drying time of potato slices was reduced and the drying rate was increased when USOD-0% was used as a pretreatment, whereas USOD-60% had a negative effect on the drying rate of the samples. The Weibull model was effective in predicting the water changes in potato slices during the drying process. NMR analysis revealed that the relative content of immovable water (M22) increased initially, then decreased for drying, while the transverse relaxation time (T2) and the relative content of free water (M23) decreased consistently. The DSC results indicated that the glass transition temperature (Tg) had an inverse relationship with the water content of the samples, yet had virtually no influence on the volume shrinkage. The sample volumes decreased linearly with the decrease in water content during the initial drying stages. USOD pre-treatment lessened the volume shrinkage of MVD potato slices. Static gravimetry was used to determine the moisture sorption isotherms of MVD potato slices at 30 °C within the aw range of 0.113-0.923. The GAB model accurately fitted the experimental sorption data, which showed sigmoid shape curves for the MVD samples. When aw values exceeded 0.7, the USOD-60% treatment significantly reduce the water sorption capacity of MVD potato slices, while USOD-0% treatment was observed to increase the hygroscopic properties of MVD samples. Compared with the control, USOD-0% pretreatment significantly increased the monolayer water content (X0), sorption surface area (S0), the thickness of sorbed water multilayer (tm), and density of sorbed water (Ds) values of MVD potato slices, while USOD-60% decreased these values.
Cheng et al. (Sun,) studied this question.