Interesting wine experiment: how to discolour a glass of wine

The grape harvesting season has arrived and along with it all sorts of questions about wine and the science behind it. Much like its flavour and its fragrance, the colour of wine is the result of a subtle chemical balance, a fruit of generational know-how.

Contrary to a very common misconception, it isn’t the colour of grapes alone that determines the colour of wine. But this can be very confusing since, if a ‘chardonnay blanc’ grape will always produce a white wine, how is it that a pinot noir could be used to produce both a red or a white wine? Effectively, it’s about the presence (or not) of the red grape skin during the maceration process which will determine the colour of a wine. However this is not the only factor determining the colour of a wine and the colour can even be controlled by natural additives.

Interesting wine experiment #10: the casein (a type of protein) in milk allows the colour of white wine to be corrected but can also partially discolour red wine.

EXPLANATION: When we pour milk into a glass of wine, the two liquids will first mix together but the acid in the wine will soon disrupt the milk’s structure. Consequently, the caseins (the proteins in milk) will precipitate by carrying pigments with them. After being left to settle for half an hour to an hour, we will find the precipitated casein at the bottom of the glass and that in the upper part of the glass, the wine is discoloured.

This partial discolouring of the wine is not actually linked to the white colour of the milk, but to the chemical structures of the two liquids.

To check the results, we can repeat this experiment with a mix of water and corn flour. Upon adding this solution, we immediately get a similar result. However, after half an hour or an hour left to settle, the corn flour has fallen to the bottom of the glass and we see that in the upper half of the glass, the wine has hardly changed colour.

So how does the red wine change colour?

Though there are still some mysteries left to be solved regarding the science of wine, we do know that the colour of red wine is linked to the processes of molecules called ‘anthocyanins’. These molecules are the red, blue and black pigments which are mostly found in the skin of red grape. White grapes don’t have these anthocyanins which is why a wine made only with white grape will always produce a white wine. But, since anthocyanins are usually found in the skin of red grapes, depending on the maceration process and the contact time of the skin with the juice, a red grape could produce a white, a rosé or a red wine.

Anthocyanins are quite delicate but during fermentation, they join with the tannins of the wine to create bigger and much more stable complexes. Tannins are compounds that bind to and precipitate proteins. The tannins can be trapped by the casein (a type of protein) in milk which results in reactions such as those seen in this experiment. All of these complex molecules mentioned above contain antioxidants which is why red wine is said to be good for the heart.