Raman spectroscopy as a rapid tool for monitoring lactic acid concentration during wine malolactic fermentation directly in the winery

Lactic acid is mainly produced during the process of malolactic fermentation and evolution of its concentration is associated with the wine stabilization process and the quality of the final product. The quantitative analysis of lactic acid is carried out offline in the laboratory using various analytical techniques, the most used being high performance liquid chromatography (HPLC). Because of this, there is a clear demand in the winemaking community for analytical tools that allow real-time, fast and inexpensive quantification of lactic acid. An approach using Raman spectroscopy has positioned itself as a feasible alternative in this regard. The primary goal of this work is therefore to monitor the concentration of lactic acid (which changes rapidly during the malolactic fermentation process) in the analysed samples, specifically, Nebbiolo wine samples for making the Barolo wine. The collected Raman spectra using a portable Raman apparatus are processed using an algorithm that applies Partial Least Squares (PLS) regression to determine the lactic acid concentration for each sample. It proves to be a precise and reliable method that leads to the determination of a predictive model characterised by $R^2=0.76$ (on the validation set), $R^2░=░0.94$ (on the test set) and RMSE of the lactic acid concentration predicted by the model of 0.22 g/l (on the validation set) and 0.11 g/l (on the test set) respectively. This approach produces results comparable to those obtained via HPLC. Moreover, unlike the latter, it allows rapid and easy monitoring of the lactic acid concentration during fermentation directly in the winery.