Just desserts: Analysing wine phenolics

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  • Published: Jan 15, 2014
  • Author: David Bradley
  • Channels: Chemometrics & Informatics
thumbnail image: Just desserts: Analysing wine phenolics

Grape expectations

Unripened Muscat grapes - Photo by David Bradley

Cheminformatics can extract antioxidant capacity and phenolic content from sweet Muscatel wines using their Fourier transform infrared spectra (FTIR) with attenuated total reflectance (ATR), according to research published in the journal Food Chemistry.

Sandra Silva, Rodrigo Feliciano, Luís Boas and Maria Bronze of the Instituto de Biologia Experimental e Tecnológica, in Oeiras, Portugal, point out that wines are considered to be an important source of dietary phenolic compounds. Dessert wines contain high levels of unfermented sugars, which endows them with their sweet taste. Moreover, the production of dessert wines, such as Moscatel de Setúbal usually involves maintaining the fermenting liquid in contact with the grape skins for longer than is common for other types of wine. This in turn leads to a higher concentration of phenolics in the red varieties than in the white dessert wines, the team suggests.

Antioxidant wine

It was previously reported that the phenolic content of white wines ranges from 165 to 1425 milligrams per litre, whereas red wine concentrations can be 1018 to 4059 mg/l expressed as gallic acid equivalents (GAE), they add. They point out that earlier research has looked at the GAE of various fortified wines, including sherry, port, Madeira and muscatel. They explain that reversed phase high performance liquid chromatography (HPLC) is often hyphenated with UV–Vis detection, electrochemical, fluorescence or mass spectrometry to analyse individual phenolic compounds. Additionally, the Folin- Ciocalteu colorimetric technique is commonly used to obtain total phenolic concentration and there are many techniques for obtaining a total antioxidant reading for a sample. But, all of these methods are time-consuming and usually involved sample preparation involving chemical reactions to "develop" the colour.

Desserted

The team points out that infrared spectroscopy has many advantages, precluding complex sample pre-treatment for instance. The researchers have now used Fourier transform infrared spectroscopy (FTIR) attenuated total reflectance (ATR) to successfully determine the total phenolic and flavonoid content as well as the antioxidant capacity (DPPH and FRAP assays) of 56 Muscatel dessert wine samples.

They developed a prediction model for the parameters using Partial Least Squares (PLS) across the spectroscopic region 1800 to 900 per cm and carried out a cross validation by ignoring data from one technique at a time. They were able to demonstrate a minimum error of prediction for total flavonoid content of just 0.2% and suggest that this approach could be used for rapid screening of Muscatel dessert wines. The same methodologies also provide antioxidant approximations (maximum error for those of 22%). The researchers describe the results as preliminary for the time being and suggest that it should be possible to refine the model for more effective assessment by feeding in cross-validated information from a wider range of dessert wine samples.

Related Links

Food Chem, 2014, 150, 489-493: "Application of FTIR-ATR to Moscatel dessert wines for prediction of total phenolic and flavonoid contents and antioxidant capacity"

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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