A recent article (1) in the Journal of Agriculture and Food Chemistry discusses the relationship between young red wine color and grape phenolics. On reading, the first question I asked is how is wine made? So, of course, I checked with Wikipedia.
The most important difference between red and white wines is the fact that red wine is made from red or black grape pulp together with the grape skins (must), whereas white wine is made from white grape juice. However, white wines can be made from red grape juice as long as the skins are not present. Grape skins contain anthocyanins and tannins, which contribute to both the color and the strong flavor of red wines. Merlot wine, for example, has a high tannin content which is why it is strongly astringent.
For red wine, the must undergoes a 5 – 14 day maceration during which the ethanol content increases due to natural (?) fermentation and anthocyanins and tannins are extracted from the grape skin. As the ethanol content increases, more of these polyphenols are extracted. The degree of extraction varies depending on time and temperature of maceration and fermentation, concentration of ethanol, and other winemaking conditions. During fermentation, when the yeast is converting the grape sugar to ethanol and carbon dioxide, and during maturation, these polyphenolic compounds further react.
The hypothesis of the article was:
It may be possible to predict the wine color from the levels and the profile of grape phenolics.
For this study they consider young red wines. Presumably they did not have enough time to mature the wines for years. Young wines were fermented for fourteen days at 25 oC ; they do not appear to have been matured in any way.
So how do you measure wine color?
Color should be measured so that the wines all have the same pH and should be filtered to remove any solid particles. Boulton’s color assay (also) can determine the total wine color and the color caused by copigmentation, anthocyanins and polymeric pigments. Color intensity and tonality can be measured using a UV/Visible spectrophotometer at wavelengths 420 and 520 nm.
When the polyphenolic content of grapes and wine were compared to the total wine color, there was good positive correlation for both grape and wine anthocyanins, despite the fact that anthocyanins were only responsible for ~50% of the color of the wine. It is possible that this correlation would decrease with wine maturity as polymeric pigments and copigmentation would increase.
All in all, the level of anthocyanins in the grapes could be used to predict red wine color.
1) Jensen, J.S., Demiray, S., Egebo, M., Meyer, A.S. (2008). Prediction of Wine Color Attributes from the Phenolic Profiles of Red Grapes (Vitis vinifera). Journal of Agricultural and Food Chemistry DOI: 10.1021/jf072541e