Solvent effects and mass transfer on aroma extraction during solid-state distillation

Steam distillation is an important unit operation to extract valuable compounds from natural solid-state substrates. Solid-state distillation with steam can extract aroma from fermented solid substrate such as fermented grains of Chinese liquor fermentation. However, this procedure is still operated based on generations of empirical skills rather than a technique under process control based on scientific principles. The resulting drawback is poor mass and heat transfer with undesired consequences concerning productivity, quality and sustainability. Mathematical modelling can be a powerful tool to get insight into such an empirical process for potential rational design, optimization and control. Therefore, we established a mathematical model to describe the mass transfer based on kinetic parameters. The simulation results show that the distillation had a natural column plate structure that more effectively concentrated compounds. Furthermore, this structure resulted in a high ethanol concentration in the distillate. To evaluate subsequent effect of high ethanol concentration, we analysed aroma compounds in the distillates under simulated conditions with or without ethanol. The hydrophobic compounds were fitted well under conditions with ethanol, whereas hydrophilic compounds had similar outcomes under both conditions. The results show that ethanol could accelerate the release of hydrophobic compounds in Chinese liquor distillation. Therefore, solvent could be one of the important factors to optimize the distillation process. Our findings provide a reference for improving the extraction during solid-state distillation.