An investigation on electrical properties of major constituents of grape must under fermentation using electrical impedance spectroscopy
Creators: name: Zheng, Sicong
Publisher: name: RMIT University
Year: 2024

description: Electrical Impedance Spectroscopy <i>(EIS)</i> has been used for investigating structures of organic and inorganic materials since the 1960s. Recently, research on detecting physical and chemical changes in plants and biological tissues by <i>EIS</i> measurements has become a focus of interest. This research confirmed that <i>EIS</i> is effective in monitoring the ripening stages and physical damage to fruit, such as chilling and bruising. However, the <i>EIS</i> applications on wine fermentation analysis are rarely researched. Due to the huge rise of wine consumption all over the world, analytical tools on wine fermentation monitoring, which can measure major components in fermenting grape juice quickly, is required. As a rapid, inexpensive and simple method, <i>EIS </i>has great potential to replace the current slow response tools, such as High Performance Liquid Chromatography <i>(HPLC)</i> and Gas Chromatography <i>(GC)</i>.<br><br>In this research, an investigation into the application of electrical impedance measurements to simulated solutions and grape musts is carried out. The dissertation outlines the experimental research conducted on relationships between impedance properties and concentration of major components of grape must. <i>EIS</i> measurements for monitoring ethanol, sugar, tartaric acid and malic acid changes in the process of fermentation are investigated. Experiments were performed on single solvent solutions, compound solutions and grape juice under fermentation. <br>As a result, linear relationships between impedance properties (including impedance magnitude on specific frequency, diameter of Nyquist plot and value of components in equivalent electrical circuits) and concentration of ethanol in pure solutions are found. Two major organic acids in grape juice, tartaric acid and malic acid, are also tested. Power relationships between malic and tartaric acid concentrations and solution resistance which is measured by <i>EIS</i> are found. It has also been discovered that the ionization resistance of organic acids has a linear relationship with the concentration of ethanol. Based on these findings, the ethanol concentration and organic acid concentrations in compound solutions can be measured by <i>EIS</i>. Two models containing constant phase elements are established for high and low conductivity solutions and they fit the data better than any previous models, such as the Cole model and the Hayden model, etc. Validation experiments on ethanol solutions and real fermenting grape juice are also carried out to confirm the applicability of ethanol measurement in real condition. <br><br>In conclusion, this research proves that the impedance measurements are suitable to measure ethanol concentrations in fermenting grape must. They are rapid in response and easy to operate. This method may cause a revolution in current fermentation analysis. Further research may consummate the <i>EIS</i> method for industrial wine applications, and spread this method to applications in other alcoholic beverages and food products.