Abstract: A new ultrasound-reinforced vacuum drying device was built to investigate the enhancing effect of ultrasound on the vacuum drying of viscous food. Ultrasound-reinforced vacuum drying of liquid whole egg was carried out to explore the influence of drying temperature, ultrasound energy density and irradiation time on the drying characteristics and microstructure. Furthermore, a kinetic model was established. The results showed that the internal mass transfer process could be enhanced by applying ultrasound to vacuum drying. Meanwhile, the drying rate was improved with the increase of ultrasound energy density. The drying process of liquid egg at 50 ℃ was no longer affected by ultrasound treatment at an energy density of 2.0 W/g after 2.5 h. The results of scanning electron microscopy (SEM) showed that many microtunnels were observed in the microstructure of powdered eggs, which was beneficial to enhancing the connectivity and consequently reducing water migration resistance. Nine different thin layer drying models were fitted nonlinearly with the experimental data. Comparison of correlation coefficient (R2), root mean square error (RMSE) and reduced chisquare (χ2) and verification experiments showed that R2 of the Page model was greater than 0.99, RMSE and χ2 were less than 0.01. Then, the Page model was the most suitable to describe the relationship between liquid egg moisture and drying time. The effective moisture diffusivity (Deff) was calculated according to Fick’s second law to range from 1.645 6 × 10-9 to 6.549 7 × 10-9 m2/s, which was increased with the increase of ultrasonic energy density. The linear relationship between the natural logarithm of effective moisture diffusivity and the reciprocal temperature was described by the Arrhenius empirical formula. The activation energy (Ea) was 16.151 2 kJ/mol. These results can provide a valuable basis for optimizing and controlling ultrasound-reinforced vacuum drying of liquid whole egg.

Key words: ultrasound, vacuum drying, drying characteristics, kinetic model, liquid whole egg