v: 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.
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.
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.
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
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