Last Month's Most Accessed Feature: White wine: Spectroscopic phenolic
- Published: Feb 7, 2017
- Categories: UV/Vis Spectroscopy
UV-vis, laser induced fluorescence and Raman spectroscopic techniques can be used to investigate the phenolic acids present in white wines, according to research published in the journal Food Chemistry by researchers in France.
Coralie Martin, Jean-Luc Bruneel, Frédéric Castet, Alain Fritsch, Pierre-Louis Teissedre, Michael Jourdes, and François Guillaume of the Université de Bordeaux, France, point out that phenolic compounds are widely distributed in nature and found in a wide range of plants. These compounds have various physiological roles in the species in which they are found but are also known to have antioxidant activity which is if interest from the perspective of human health. As such, analytical techniques are needed to assess the various sources of these compounds with precision, including plant-derived beverages such as wine, coffee, beer and tea all of which are known to be rich in phenolic compounds. The astringent nature of phenolic compounds also plays a part in the organoleptic properties of wines.
The team points out that a wide range of techniques can be used to determine phenolic compounds in beverages including UV-Vis. Light emission spectroscopic techniques, and Raman scattering, in particular, have not yet been exploited to their full potential, however, and only a few studies utilising fluorescence studies have been published. Indeed, the team asserts at the time of writing, only a single, limited, Raman study of white wines had been reported.
This dearth of research presumably reflects the fact that white wines are essentially water and ethanol and contain very low concentrations of congeners. For instance, dry white wines originating from the south-west of France are made by fermenting the grape juice away from the grape solids with no subsequent aging in oak barrels and so, unlike red wines, there is little opportunity for flavonoids, tannins and stilbenes to add to the wine to any significant degree. As reported previously, hydroxycinnamic and hydroxybenzoic acids are essentially the primary phenolic compounds that one will find in white wines.
The team has now used UV-vis spectroscopy and quantum mechanical calculations to corroborate analyses based on Raman spectroscopy with a view to demonstrating that resonance Raman spectroscopy could be a useful tool in the determination of phenolics in white wines nevertheless and to demonstrate that several unusual phenolics are indeed present in such wine.
In their model experiments the team demonstrated that only hydroxynamic acids are in resonance with a laser excitation line with 325 nm wavelength and are therefore at the origin of the strong enhancement of the Raman light scattering. They add that for experiments with actual white wines, rather than "model" mixtures, they see the same resonance Raman scattering, which they suggest means that the white wine's hydroxycinnamic acid content might be determined precisely with this technique. The further analysis of the spectra of a real dry white wine reveals qualitative data on the preponderance of p-coumaric and caftaric acids.
The next step with this research is to apply the technique to a wide range of white wines from different regions with the goal of demonstrating that the analysis could be used to authenticate the products.
Food Chem 2017, 15, 568-575: "Spectroscopic and theoretical investigations of phenolic acids in white wines"
Article by David Bradley
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.
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