Mass and heat transfer characteristics and quality optimization of microwave vacuum drying for wakame

Abstract

Objective: This study aimed to improve both the drying rate and quality of Undaria pinnatifida. Methods: Microwave vacuum drying experiment was carried out under different microwave power density, vacuum degree and pulse intermittent conditions. The thin layer drying model was established and the mass and heat transfer parameters were calculated. The drying process parameters were optimized by L9（34） orthogonal test with indicators of drying time, sensory quality, rehydration rate, energy consumption and material temperature. Results: Page model was found fit the drying kinetics well, and the change of drying kinetics showed obvious dependence on energy contribution of drying conditions. The effective moisture diffusion coefficient ranged from 2.38×10-9 to 1.49×10-8 m2/s and increased with the microwave power density. The enthalpy and entropy decreased, while the Gibbs free energy increased with the increase of drying temperature. The drying activation energy of U. pinnatifida was 4.51 kJ/mol. When the microwave power density was 4.5 W/g, with vacuum degree 80 kPa, the moisture content was reduced to 12.30% within 16 min, and the energy consumption was 0.04 kW·h/g H2O, with rehydration rate of 1 885.69% at an average drying temperature of 26.71 ℃. The leaves of U. pinnatifida were in dark green, flat and free of burning spots and bubbles. Conclusion: Microwave vacuum drying was a suitable method for drying U. pinnatifida with high efficiency, controllable quality. Most importantly, few difference between rehydrated specimen and fresh materials was found.

Keywords

• undaria pinnatifida
• microwave vacuum drying
• heat and mass transfer
• quality