Wondering where the colour of red wine comes from? Think one can “see” the age of a red wine? Then read further….
The colour of red wine is important to many wine drinkers. We all know that young red wines have a reddish colour, while that of older red wines changes to a brick red colour and very old red wines to more brownish with a hint of red. The compounds that causes the red colour in young red wines are called anthocyanins. Anthocyanins occur in many different types of fruit such as red apples, cherries and obviously red grapes. In most red grape cultivars the anthocyanins are only found only in the skins (one can thus make a white wine from red grapes), with the exception of a few so-called Teinturier cultivars where it occurs in the flesh as well. Winemakers thus have to regularly mix the skins with the juice during fermentation to extract the anthocyanins from the skins into the liquid to produce a red wine.
Anthocyanins in young red wines can occur in different forms. The pH of the wine influences this to a large extend. The anthocyanins that give the red colour (called the flavilium ion) is actually a small % of the total anthocyanins, with a large part of the anthocyanins being colourless, with a small fraction being blue at normal wine pH (3-4). The flavilium ion has a positive charge, which leads to it absorbing the green spectrum of light, while reflecting the red spectrum, leading to our eyes observing a red colour in the wine.
You can easily test the effect of the pH on red wine’s colour at home: pour some red wine (preferably not your Petrus) into 3 wine glasses. In the first glass add some strong acid, such as sulfuric acid to decrease the pH, while you add a strong base, such as sodium hydroxide, into the third glass (thinking of it again, rather not try it at home, leave it to us crazy scientists, as strong acid and bases can be dangerous, but you can take my word for it. If you really want to do this at home, obviously discard the wine in glasses 1 and 3 afterwards, while it’s safe to drink number 2 and reflect on your high school chemistry marks…). You will notice that the wine’s colour in the first glass will have a more intense red colour after a few minutes, that in the second glass a normal red wine colour, while that in the third glass will have blueish/greenish colour. This is because in the wine with the strong acid, the pH decreased to around 1, where the red anthocyanins form dominates, while the blue/green form dominates at the high pH (7 and higher). This is one of the reasons why winemakers likes a lower pH in their red wines and some red wines stains your mouth a blueish colour, as the pH in your mouth is obviously higher than that in wine (a neat party trick for first year wine students). You can see these different colours, especially in the meniscus of the following Cinsaut wines, where I adjusted the pH to different values.
Now these anthocyanins are in a young red wine not bound to other compounds and are called free anthocyanins. However, as the wines ages in a barrel small amounts of oxygen comes into contact with the wine through the staves. This process called micro-oxygenation is actually beneficial for the wine, as it changes the colourless anthocyanins into the red form. One of the other reactions enhanced by this micro-oxygenation is the binding of the free anthocyanins to tannins in the red wine. This binding process leads to a product called a wine pigment. This pigmented colour is more stable in the wine, in other words, it does not drop out to the bottom of the barrel or bottle as easily during the ageing process. The colour of this pigmented form is also more brick coloured and even brownish, depending on the type of pigment and the wine itself, thereby leading to a colour change during ageing of the wine. So next time you open a bottle of red, look at the colour, think of the wine’s age, and tell your friends that seeing is believing…