The Faults in Our Wines

To all the wine lovers out there, have you ever wondered how many chemicals are involved in conjuring the unique taste of a wine? The chemical composition of soil in a certain vineyard, the environment and climate that season, the carefully selected barrels the wines are aged in. These are just some of the factors that contribute to the thousands of chemicals that affect a wine’s taste. But with these processes tightly controlled, wine faults still occur, literally souring the taste of a good wine.

To be fair, the sheer number of compounds in wine that are able to activate your taste receptors is immense. And throughout the process of wine-making, one tiny change anywhere along the line can alter the products significantly enough for one to discern – and not necessarily in a good way. These are known as ‘wine faults’, some of the more common ones will be discussed below.

Sulfur-Based Wine Faults

Fermentation is a key aspect of winemaking, with bacteria in the form of yeast added to the grape ‘juice’ (or grape ‘must’, if you’re in the business). The sugar in the juice is converted by yeast to form ethanol, an alcohol. Some methanol is also produced during the fermentation process, albeit in tiny amounts.

If you think it is remarkable that wine (under the right conditions) can be stored relatively unchanged in a bottle for decades, you can thank the additives that are added before, during and after the fermentation process.

Sulfur derivatives are popular as they prevent oxidation as well as acting as an antimicrobial agent. Sulfur dioxide is the most common of the lot, with the gas dissolving in wine and producing sulfite ions:

H2O + SO2 → H+ + HSO3 → 2H+ + SO32-

The sulfite ion (SO32-) binds to precursors involved in oxidative reactions and prevents them from reacting with oxygen, which would spoil the taste of the wine. When it is added above a certain threshold however, it can give a perception of a mothball-like taste.

Other sources of sulfur compounds include hydrogen sulfide (think rotten eggs!) as a by-product of fermentation by yeast via the sulfate reduction pathway in a nitrogen-poor environment1.



Oxidation – Wine’s Biggest Enemy

As mentioned above, oxidation of the chemicals in wine leaves behind some very unsavory results. Oxidation of the components in wine requires just two ingredients – oxygen and a catalyst. No matter how airtight a bottle or barrel is, oxygen can slowly enter through diffusion, while yeast provides the catalyst in the form of enzymes.

If too little sulfur dioxide is added during the process, bacteria can thrive in wine and oxidize its components, especially ethanol as it is in plentiful supply. This oxidation of ethanol is one of the most common wine faults, a process that is accelerated by higher temperatures. That’s why wines should be stored in chillers.

alcoholoxidation
Simple, yet not so simple oxidation that is catalyzed by ethanol dehydrogenase in bacteria or yeasts.

The oxidation of ethanol in wine leads to the formation of acetaldehyde (also implicated in hangovers), which is further oxidized to acetic acid. Acetic acid is also known as vinegar, leaving you with an actual sour taste in your wine. Of course, there are other chemicals in wine that are also readily oxidized. Anthocyanins, catechins, epicatechins and phenols are among these, which leads to a loss of colour, flavour and aroma – or ‘flattened’ wine.

phenol oxidation
Two-step oxidation of the phenol hydroquinone to benzoquinone.
Those barrels better be airtight

The Importance of Storage

Storage conditions such as temperature, humidity, exposure to light as well as the bottling of wines can contribute to the introduction of unwanted chemicals in what would have been a fine wine – we’ll look at the formation of some of them here!

Wines that are stored at room temperature or higher will generally age at a faster rate, as heat can accelerate chemical reactions. When extremely high heat is introduced, it can cause thermal expansion of liquid and gas inside of a wine bottle.

If the cork is partially pushed out of the bottle, or wine is visible on the top of the cork while it is still in the bottle, it has most likely been heat damaged. Due to the prevalence of such heat-damaged wine, many consumers mistakenly attribute the taste to poor quality. Damn you, climate change!

Aside from heat, light is also responsible for the introduction of wine faults. Wines that are exposed to prolonged UV radiation (light with a wavelength of 325-450 nm) apparently taste of wet cardboard, in a wine fault known as ‘lightstruck’. Light-colored champagnes are the worst affected, with red wines largely protected due to the presence of UV absorbing phenols.

F1.large
Here are some phenolic compounds, which are able to absorb high energy UV radiation3

Also important are the corks that stopper a bottle of wine. Some of these corks are known to introduce a chemical known as 2,4,6-trichloroanisole (TCA) into the wine. This wine fault is known as ‘cork taint’, which introduces a medicine-like taste that our taste buds can detect even at very, very low levels. It also suppresses the effects of other, nicer tasting compounds on your taste receptors, replacing all of that good stuff with a moldy taste.



What Makes a Wine Fault?

In the end, there are simply too many compounds present in wine to properly identify them individually as wine ‘faults’. What’s fascinating is that many known ‘bad’ characteristics in a supposedly spoiled wine are actually also present in ‘good’ wine, albeit in differing concentrations. A minute difference in concentration of a chemical – we’re talking parts per billion – can influence our taste receptors in either a positive and negative way. Thank you for reading!

winegraphic
Just some of the compounds present in a glass of red wine – look at those fancy chemical structures4!

Reference

  1. Linderholm, A. L., Olineka, T. L., Hong, Y., & Bisson, L. F. (2006). Allele diversity among genes of the sulfate reduction pathway in wine strains of Saccharomyces cerevisiae. American journal of enology and viticulture57(4), 431-440.
  2. Takeuchi, H., Kato, H., & Kurahashi, T. (2013). 2, 4, 6-Trichloroanisole is a potent suppressor of olfactory signal transduction. Proceedings of the National Academy of Sciences110(40), 16235-16240.
  3. Kennedy, J. A., Saucier, C., & Glories, Y. (2006). Grape and wine phenolics: history and perspective. American Journal of Enology and Viticulture57(3), 239-248.
  4. Hufnagel, J. C., & Hofmann, T. (2008). Orosensory-directed identification of astringent mouthfeel and bitter-tasting compounds in red wine. Journal of agricultural and food chemistry56(4), 1376-1386.

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