Pulsed vacuum drying enhances drying of blueberry by altering micro-, ultrastructure and water status and distribution

Abstract

Effect of pulsed vacuum drying (PVD) on the drying kinetics, microstructure, ultrastructure, water distribution, and bioactive compounds of blueberries were investigated in the current work. Experiments showed that compared to hot air drying (HAD), the drying time of blueberries under PVD was shortened by 32.14% at the same drying temperature of 65 °C. Scanning electron microscopy (SEM) observations showed that several microcracks were formed on the surface of pulsed vacuum dried blueberries. The transmission electron microscopy (TEM) observations indicated that PVD resulted in the presence of abnormal cells of irregular shape and caused more serious damage to the cell walls of blueberries than HAD. Low field nuclear magnetic resonance (LF-NMR) and magnetic resonance imaging (MRI) analyses showed that the successive changes between atmospheric pressure and vacuum resulted in the enlargement and interconnection of pores that are beneficial to the moisture transfer in the initial stage of drying. Compared to HAD, PVD had also a positive effect on maintaining higher contents of total phenolics (TP) and total monomeric anthocyanins (TMA). These findings clarified the mechanism that PVD enhances the drying efficiency and quality of dried blueberries.