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The effects of acoustic energy density (6.8-47.4 W/L) and temperature (20-50 °C) on the extraction yields of total phenolics and tartaric esters during ultrasound-assisted extraction from grape marc were investigated in this study. The ultrasound treatment was performed in a 25-kHz ultrasound bath system and the 50% aqueous ethanol was used as the solvent. The initial extraction rate and final extraction yield increased with the increase of acoustic energy density and temperature. The two site kinetic model was used to simulate the kinetics of extraction process and the diffusion model based on the Fick's second law was employed to determine the effective diffusion coefficient of phenolics in grape marc. Both models gave satisfactory quality of data fit. The diffusion process was divided into one fast stage and one slow stage and the diffusion coefficients in both stages were calculated. Within the current experimental range, the diffusion coefficients of total phenolics and tartaric esters for both diffusion stages increased with acoustic energy density. Meanwhile, the rise of temperature also resulted in the increase of diffusion coefficients of phenolics except the diffusion coefficient of total phenolics in the fast stage, the value of which being the highest at 40 °C. Moreover, an empirical equation was suggested to correlate the effective diffusion coefficient of phenolics in grape marc with acoustic energy density and temperature. In addition, the performance comparison of ultrasound-assisted extraction and convention methods demonstrates that ultrasound is an effective and promising technology to extract bioactive substances from grape marc.

In food processing, the applications of ultrasound can be divided into two categories, namely replacing traditional technologies and assisting traditional technologies. In the latter case, the processing efficiency is enhanced and the disadvantageous of traditional technologies during processing are improved. These ultrasonic effects can be defined as ultrasonic enhancement of food processes. This review is focused on the use of ultrasound to enhance various food processes, including extraction, freezing, thawing, brining, oxidation, filtration, and drying/dehydration. The major functions of ultrasound in enhancing these processes and the factors which can affect the ultrasonic enhancement are elucidated. In the meantime, the strategies of modeling these processes enhanced by ultrasound are provided. Future studies should pay more attention to elucidate the ultrasonic effects during freezing, thawing, brining, oxidation, and filtration processes. Furthermore, when it comes to design the ultrasound equipment at the industrial level, it is better to quantify the ultrasonic effects through numerical stimulation.