Effects?of?ultrasound?pretreatment?on?drying?time?for?intermediate-dried?litchi

The purpose of this study is to explore the effect of ultrasound pretreatment on the drying time for intermediate-dried litchi drying and its mechanism. Based on the results of single factor experiments, the conditions of ultrasound pretreatment were optimized and the predictive regression model was established by response surface methodology. The results showed that the efficiency of litchi drying was improved indicated by the reduction of drying time. The optimal drying conditions of intermediate-dried litchi were concluded as follows: ultrasound frequency 40 kHz, ultrasound pretreatment time 32.6 min and ultrasound power 354 W, the time needed for the wet basis moisture content of litchi pulp reached 32% was 15.65 h in the predicted optimal condition, compared to 26.80 h in control experiment. The observed mean value in the confirmation experiments was 15.93 h, which was close to the predicted value. The results indicated that the established optimized model was reliable. The observation of microstructure by scanning electron microscope showed that different ultrasound pretreatment conditions led to the damage of cell structure of litchi exocarp and endocarp, which allowed increasing water diffusivity and therefore reduced the drying time.