An investigation on dielectric properties of major constituents of grape must using electrochemical impedance spectroscopy

摘要

The Australian wine industry has been undergoing an unprecedented growth during the last decade in both winery establishments and export volumes. Though automated controlling processes are useful for maintaining high quality products and reducing costs, the techniques are not widely adopted by small to medium sized wineries due to their relative high initial costs. In this paper, we investigate the dielectric properties of ethanol and organic acids, two major chemical ingredients that need to be monitored during wine fermentation using electrochemical impedance spectroscopy. The relationships between solution concentrations and parameters derived from the measured impedance spectra were investigated by two methods, the Nyquist plot approach and the equivalent circuit modeling approach. The investigation was carried out for both pure solutions including ethanol, malic acid and tartaric acid, the major constituents of grape must, and their various mixed compound solutions to simulate the grape must under fermentation. Strong linear as well as inverse-squared relations were found between the concentrations of the pure solutions and the derived dielectric parameters, e.g., the diameter of the Nyquist plot for ethanol has a correlation with concentrations with a coefficient of determination of 0.9796 and the R s value obtained from electric circuit modeling also shows a strong inverse-squared relationship with the concentration of both malic and tartaric acids. A further investigation suggests that the individual concentration of mixed acid solutions can be handily determined by measuring the impedance of the mixed acid solutions and using the initial concentration value of one of the mixed acids. Furthermore, the dielectric properties of mixed ethanol, malic acid and tartaric acid show a combined effort from mixed acid solutions and pure ethanol. These findings suggest that the electrochemical impedance, which can be easily measured with satisfactory accuracy, is a good indicator of the concentration change of the essential ingredients of grape must and thus can be further utilized to develop a feedback loop for wine fermentation automatic control.