Sweet red wine: Ultraviolet exposure

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  • Published: May 1, 2013
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Sweet red wine: Ultraviolet exposure

Raisin d'être

UV-Vis spectroscopy has been used for the first time to study phenolics and other compounds present in a type of sweet, red wine emerging from the North West of Spain. The approach could be used to study the components of other beverages too as well as offering quality analysis (QA) data on produce.

UV-Vis spectroscopy has been used for the first time to study phenolics and other compounds present in a type of sweet, red wine emerging from the North West of Spain. The approach could be used to study the components of other beverages too as well as offering quality analysis (QA) data on produce.

Maria Figueiredo-González, Beatriz Cancho-Grande, Jesus Simal-Gándara of the Nutrition and Bromatology Group, at the University of Vigo's Ourense Campus, in Spain, explains how wine producers in Valdeorras in Galicia have aspirations to market new sweet wines made with red grapes, Vitis vinifera L. Garnacha Tintorera, a teinturier cultivar. As such, the team points in the journal Food Chemistry that there is a need to understand the chemistry and phenolic components of these sweet red wines. The team has now looked at two different samples of such wines made with dried, or raisined, red using UV-Vis spectrophotometry. They were able to obtain data on chromatic properties and global phenolic composition using this technique.

Mellow yellows

The team explains that high molecular weight polymers with a brown colour are generated during the grape drying process and can be isolated from sweet wines by a process of dialysis; these are responsible for the brown colour of the sweet wines, the team reports. However, the yellow hues present in the sweet wines are also higher as confirmed by their colorimetric indexes the team adds. They also identified polymerised anthocyanins and tannins associated with aging at least for the second sample, which was a fortified sweet wine aged in oak barrels. The naturally sweet wine had the highest levels of phenolic compounds because their concentration is apparently increased during evaporation of water from the grapes whereas phenolics are low in the fortified sample because maceration-fermentation is halted before pigments have leeched out of the grape skins into the wine.

Galicia currently five wine-producing Dominations: Ribeiro, Rías Baixas, Valdeorras, Ribeira Sacra and Monterrey and as with any wine producer the aroma profile, colour and clarity of their wines are vital to good sales. Wine production using off-vine grape drying, raisining, and fortification are key to the production of new sweet wines as far as some producers are concerned. Chromatography and the skills of wine tasters have been used to profile the wines but precise understanding of the various processes involved and their effect on the chemistry of the wine can be extracted using UV-Vis, according to the Ourense team.

Whine for table six

There were hints that melanins (formed by enzymatic oxidation), melanoidins and caramels (generated by non-enzymatic browning reactions) give rise to brown colours in sweet wines but learning about the pigments formed from sweet red wines made with dried grapes is relatively novel. Ultimately, the team's intention is to give wine producers useful benchmarks to facilitate consistent development of local production of sweet red wines. The benchmarks go beyond the often highly subjective assessment of wine tasters and provide solid data based on spectroscopy.

"What we want to achieve is the production of high quality sweet red wines, mainly from the sensorial point of view, especially colour," Simal-Gándara told us. "Some of the necessary steps will be the characterization of all the colour-affecting compounds to go deeply into the understanding of the interactions amongst them with the ultimate intention of optimizing the processes of grape-drying and wine-making."

Related Links

Food Chem 2013, 140, 217-224: "Garnacha Tintorera-based sweet wines: Chromatic properties and global phenolic composition by means of UV-Vis spectrophotometry"

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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