The Chemistry Column

Changes in the barrel

Claire Gormley

Imagine a chemical reaction. What do you see? Is it fast or slow? What is changing, and how can you tell? Many of you are probably visualising two different liquids being mixed in a beaker— perhaps some bubbles form, a colour appears, a solid collects at the bottom, or maybe the beaker explodes, releasing a burst of heat. All of these are valid depictions of a chemical reaction, and they are all likely to have occurred in a chemistry lab. We often forget, though, that chemistry is happening all around us every day— in the baking of a cake, the lighting of a match, or the rusting of metal.

When a chemical reaction occurs, the atoms involved in the reaction rearrange to produce an entirely new chemical with different properties. This rearrangement is key, as it’s what separates a chemical change from a physical change. In a physical change, the chemical doesn’t make or break any bonds; it simply transitions between a solid, liquid or gas. The most famous example of this, of course, is ice, water, and steam. All have the chemical formula H2O, but each appears physically different. Another key difference to note is that a chemical change is often irreversible— a factor which winemakers, in particular, need to consider when they begin the process of turning grapes into wine. The flavour and feel of wine are determined by a complex mixture of hundreds of different molecules constantly in flux, as well as by outside variables such as the temperature, our mood, or the food we have just consumed (Waterhouse, Sacks and Jeffery, 2016). But one critical reaction for the development of a wine’s unique flavour is oxidation. This is a chemical reaction that takes place all around us, as virtually everything is exposed to oxygen. Put simply, during an oxidation reaction a substance reacts with oxygen and loses an electron.

In winemaking, the process is much more complex. Most chemists agree on the following mechanism of wine oxidation: First oxygen (O2) reacts with a transition metal catalyst, such as iron. This oxidation ultimately forms hydrogen peroxide (H2O2) and quinones, which are molecules that help to create unique flavours by reacting with wine nucleophiles, like bisulfite, thiols and flavan-3ols. One of two things happens next. Either H2O2 reacts with bisulfite to end the oxidation reactions or, if there isn’t enough bisulfite, H2O2 will react with ionised-iron to produce aldehydes and other oxidised compounds. These compounds continue to react with the wine nucleophiles, generating more of the distinguished grassy, nutty or apple-y ‘oxidised flavours.’ Finally, once the nucleophiles are exhausted, the aldehydes and oxidised species accumulate, and the wine is said to be oxidised (Waterhouse, Sacks and Jeffery, 2016).

This process starts as soon as the grape is removed from the vine. Winemakers will try to control the exposure of the grapes to oxygen, and thus the development of oxidised flavours. Once oxidised there’s no going back, so it’s important for winemakers to get the process right if they want to produce their signature flavour year after year. They may harvest at hotter or cooler times of the day. They may ferment the wine in a steel vat instead of a wooden barrel to allow for greater oxygen exchange. They may purposefully add oxygen, using methods like open-tank fermentation, pumping over, racking or bâttonage (Teclemariam, 2019). Even sealing the bottle with a screwcap instead of a cork can make a difference to how the flavours will develop— a cork will let through trace amounts of oxygen, allowing the wine to age after being bottled (Masterclass staff, 2021). So, next time you raise a glass, consider the chemical reactions that have brought that taste to your lips. Indeed, the chemical reactions that are still going on all around you…

References:

Teclemariam, T. (2019) What is Oxidation Doing to My Wine? Wine Enthusiast. [Accessed on: 02 May 2022]. www.winemag.com/2019/04/09/wineoxidation/ Masterclass

Staff (2021) Wine 101: What Are the Differences Between Corks and Screwtops? Masterclass. [Accessed on: 02 May 2022] www.masterclass.com/articles/wine101-what-are-the-differences-between-corksand-screw-tops#what-are-wine-screw-topsand-how-do-they-work

Waterhouse, A. L., Sacks, G. L. and Jeffery, D. W. (2016) ‘Wine Oxidation’, in Waterhouse, Sacks and Jeffery’s Understanding Wine Chemistry. Chichester: John Wiley & Sons: 278