Microwave vacuum evaporation as a potential technology to concentrate sugar solutions: A study based on dielectric spectroscopy

Abstract

Traditional vacuum evaporation in the sugar industry generally relies on steam and is characterized by low energy efficiency. Microwaves may be an alternative heat source to steam. In this study, three sugar solutions (glucose, maltose, and trehalose) were concentrated by lab-scale microwave vacuum evaporation (MVE). The dielectric spectroscopy in the frequency range of 0.1-6.5 GHz and the temperature range of 5-90 °C was determined and analyzed. Results showed that the dielectric loss factor increased with concentration and decreased with temperature at 2.45 GHz. The maltose solution had a lower microwave absorption and conversion capacity than glucose or trehalose solution. Meanwhile, the hydrogen bond interactions of trehalose solution were stronger, contributing to higher intermolecular interactions. These results may account for the distinct evaporation efficiencies of MVE on the three sugar solutions. Due to the rapid heating rate (up to 2.86 ± 0.08 °C/s) and massive water loss (closer to 2/3 total mass of the feed solution) over a short period of time (10 min at a microwave power density of 40 W/g and a vacuum pressure of 50 kPa), MVE may be a substitute for conventional methods. Furthermore, the dielectric spectroscopy can be an effective tool to help guide MVE to obtain sugars with high quality. • Dielectric spectroscopy of three sugar solutions was determined and analyzed. • Maltose solution had weaker microwave absorption and conversion ability. • Strong intermolecular interactions in trehalose solution impeded evaporation. • Rapid heating and massive water loss occurred during microwave vacuum evaporation. • Microwave vacuum evaporation may be a substitute for conventional evaporation.