Enhancing Water Evaporation during Apple Juice Concentration Using Microbubbles

As the most common method for juice concentration, evaporation is, however, a process with high energy requirements. In this study, microbubbles (MBs) generated with air at different temperatures (22, 45, 70 °C) were introduced to assist in water evaporation at 40, 55, and 70 °C, with the goal of improving the energy efficiency of juice concentration. The number density of MBs was inversely affected by the liquid temperature. Infusing MBs into apple juice considerably enhanced water evaporation, but the enhancing effect became less significant as the juice temperature increased, ranging from 76% to 104% at 40 °C to approximately 37% at 70 °C. Moreover, increasing air temperature only improved the performance of MB-assisted evaporation to a minor (16%) or negligible extent. Computational fluid dynamics simulations of bubbly flow demonstrated that MBs enhanced water evaporation by improving mixing and, thus, heat transfer in the liquid, and the predicted results reasonably agreed with the experiments. Although air MBs did not deteriorate the sensory qualities of treated juice, they caused marked oxidation of nutrients. Replacing air with nitrogen for MB infusion effectively preserved the nutritional values of the juice. This study demonstrates a promising and easy-to-implement technology that can help the food and beverage industries innovate conventional concentration processes.