You could be forgiven for thinking there’s not a great deal that’s interesting about the chemistry of vodka. After all, isn’t it essentially just a mix of two compounds, ethanol and water? Though this is pretty much the case, there’s more to vodka than you might expect. Here we take a look at some of its chemical secrets.
First, let’s briefly summarise how vodka is made. The method is similar to that for most fermented spirits. Though the stereotypical image most people have of vodka is that it’s made from potatoes, in fact it’s much more common for it to be made from cereal grains, including corn, rye, and wheat. Fermentation of these grains using yeast produces alcohol (ethanol), but only up to around 16% – too low for vodka.
Further steps are necessary to arrive at the finished product, the key step being distillation. Distillation involves the boiling of the mixture; because ethanol boils at 78˚C, it boils off before the water does and it can therefore be concentrated. Unfortunately, a lot of the other compounds produced during fermentation boil off at lower temperatures than water too, so precise control of the distillation process is necessary to ensure that these aren’t present in the final product.
Often, it’s distilled more than once to ensure a minimal amount of impurities remain. To be even more certain of this, many manufacturers filter the vodka through activated charcoal, which helps pull out more of any impurities still present. More traditional manufacturers rely on precise control of the distillation process, however. After all of this the vodka’s alcohol percentage is around 96%. The final step in the process is diluting it with water to bring the percentage down to around 40%.
The final product has little other than ethanol and water present, so in theory all vodkas should be essentially identical in perception and flavour. However, if you’ve ever compared a high quality vodka with the cheapest stuff you can buy in your local supermarket, you’ll know that there’s often a slight but discernible difference. This can be due to a handful of reasons.
One suggestion is that differences in perception could be due to differences in they way ethanol and water interact in different vodkas. As well as existing as individual molecules, the water and ethanol molecules can form structures called hydrates. These are cage-like structures, with a number of the water molecules surrounding an ethanol molecule.
The most common of these hydrate structures in vodka has around 5 water molecules to each ethanol molecule. Researchers discovered that its concentration varied in different brands of vodka. Though their hypothesis has yet to be conclusively confirmed, they speculate that these structural differences in different vodkas could account for slight differences in taster perceptions.
Another factor is impurities. Though most of these are removed during distillation and filtration, small, milligram amounts will still remain. These impurities can include other alcohols, such as methanol and propanol, as well as compounds such as acetaldehyde. Cheaper vodkas contain higher levels of these impurities, which can negatively affect flavour perception, and lead to a vodka that’s less smooth.
The final factor is additives. Though we think of vodka as just ethanol and water, it’s actually permitted in a number countries to add small amounts of other additives. Mostly, these are to improve the smoothness of the vodka, so they’re likely to be found in higher amounts in cheaper vodkas containing more impurities. Compounds used for this purpose include citric acid, glycerol, and sugar.
Vodka isn’t always unadulterated of course, and flavoured vodkas are also possible by adding various compounds or extracts after the manufacturing process. One of the most well-known flavoured vodkas is Żubrówka, which is flavoured using bison grass. Interestingly, this was (and still is) banned by the FDA in the USA, as flavouring with bison grass also leads to the presence of the compound coumarin, which in much larger amounts has been shown to exhibit liver toxicity in rats. A form of Żubrówka is now sold in the US, but it’s one in which the coumarin content has been removed.
So, there you have it – there’s more to vodka than just ethanol and water after all!
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References & Further Reading
- Vodka: analysed, filtered and poured – L Jarvis, C&EN
- Vodka’s molecular cocktail – M Lalloo, Chemistry World
- What’s inside? Vodka – P Di Justo, Wired
- A collective measure of structural differences in vodkas (£) – N Hu and others
- Ion chromatography to detect adulteration of vodka and rum (£) – D Lachenmeier and others
- Identification of vodkas from ion and gas chromatography (£) – V Arbuzov & S Savchuk