The quantification of red wine phenolics using fluorescence spectroscopy with chemometrics

Thesis (MScAgric)--Stellenbosch University, 2021.ENGLISH ABSTRACT: The organoleptic and perceived quality characteristics of red wine are largely influenced by important phenolic compounds extracted throughout fermentation from the grape berry to the final wine matrix. These complex secondary metabolites have resulted in numerous equally complex analysis methods, the implementation of which are yet to form part of routine phenolic analysis during winemaking. In this study, front-face fluorescence spectroscopy was investigated for its suitability in quantifying phenolic parameters of unaltered samples and the subsequent implications for non-invasive analysis throughout fermentation. A front-face accessory and fluorescence spectrophotometer were successfully optimised in order to analyse samples directly, eliminating the need for sample dilution as with conventional fluorescence spectroscopy. A diverse dataset comprising 289 fermenting musts and wine were analysed using the optimised fluorescence protocol and the most commonly used UV-Vis spectrophotometric methods for the following phenolic parameters; total phenolics, total condensed tannins, total anthocyanins, colour density and polymeric pigments. Different statistical analysis methods were explored for their suitability in model development, specifically Parallel Factor Analysis (PARAFAC) and a gradient boosting machine learning algorithm (XGBoost). Subsequent to the investigation of the most optimal chemometric method, a machine learning pipeline was generated to develop accurate regression models per phenolic parameter. Successful models were obtained for total phenolics, total condensed tannins and total anthocyanins while polymeric pigments and colour density require further investigation and refinement. Following model development and optimisation, an external validation experiment monitoring a Cabernet Sauvignon fermentation was used to examine prediction accuracy under fermentation conditions, specifically investigating the effect of carbon dioxide and must turbidity. No effect of sample preparation treatment was found and the potential for analysing unaltered samples directly during fermentation was possible. Fluorescent properties of fermenting musts and wines were explored and the responsible spectral regions of interest tentatively identified. Differences in fluorescence between musts and wines were found and upon closer inspection, unique changes were monitored and identified throughout fermentation using the Cabernet Sauvignon experiment. The unique fluorescent profiles of wines is widely accepted, and the classification of South African red wine cultivars was successfully conducted using Neighbourhood Component Analysis (NCA). These results may have beneficial implications for authentication and quality control by industry bodies. Overall, front-face fluorescence spectroscopy holds several advantages including it being non- invasive, user-friendly, relatively economical, rapid and accurate, and thus presents itself as a promising alternative to the current phenolic analysis methods with the added benefit of direct phenolic analysis throughout red wine fermentation. The potential for implementation within on- line automated systems or portable optical devices may be of interest to producers and allow for monitoring of phenolic content and extraction directly from the fermentation vessel throughout red wine production.AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.Master