The quantification of red wine phenolics using fluorescence spectroscopy with chemometrics
Date
2021-03
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
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.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Description
Thesis (MScAgric)--Stellenbosch University, 2021.
Keywords
Red wines, Phenolic compounds, Fluorescence spectroscopy, Wine and wine making -- Analysis, Chemometrics, Phenols -- Analysis, UCTD