Practical investigations
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I am doing an investigation on wines, and have found that when I exposed wines to air, their pH levels increased i.e. they became less acidic. Through research on the internet I have found that this is due to oxidation, which results in flat (less acidic) wines. However I could not find out how this oxidation occurs. Could you explain and give the reaction?
Igloo writes
The chemical processes occurring during the production and storage of wine are extremely complex, and a very large number of different reactions are taking place during its oxidation.
Perhaps the key to your answer is sulphur dioxide.
As you probably know sulphur dioxide is always present in wine. Some is produced naturally as a by-product of the fermentation process itself, and extra amounts are usually added by wine makers in the form of potassium metabisulphite, K2S2O5. This compound dissolves in water, and, in the presence of hydrogen ions, forms sulphur dioxide:
S2O52- + 2H+ -> 2SO2 + H2O
This sulphur dioxide serves several functions which help to prevent wine spoilage. First it is a bactericide, secondly it helps to inactivate spoilage enzymes such as polyphenol oxidase, and thirdly it is a reducing agent in its own right and therefore acts as an antioxidant. Some of this sulphur dioxide is “free”, i.e. present in its molecular form dissolved in the wine, whilst the rest is “bound”, e.g. in the ions HSO3-, SO32-, and is attached to certain types of organic molecules, notably aldehydes and sugars.
Perhaps you have already carried out experiments to determine the “free” and the “bound” sulphur dioxide present in wine samples – see our tutorial on these procedures on React.
With regard to your question, I suspect that there are several competing processes occurring.
Some will undoubtedly lower the pH:
e.g. oxidation of ethanol and ethanal to ethanoic acid
Others will cause the pH to increase:
e.g. the oxidation of ethanol (CH3CH2OH) to ethanal (CH3CHO) causes “free” (acidic) sulphur dioxide to be removed from the system by becoming bound to the product, which is, of course, an aldehyde; the conversion of (acidic) phenols to quinones; and perhaps the escape of (acidic) SO2 to the atmosphere, particularly if the wine has been splashed about.
If you have found that the pH increases, then maybe the latter set of processes outweighs the effects of the former.
Incidentally there is an excellent web article on the function which sulphur dioxide carries out in wine-making.
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updated: 16 February 2005
