Whisky Magazine Issue 13
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the final instalment of Philip Hills' abridged extract from his fascinatin new book Appreciating Whisky.
The chemistry of maturation is a pretty tough subject which, if you don't already know a lot of chemistry, will require a good many years of study. However, there are certain basic things which happen to alcohol which you ought to know about and can be outlined fairly briefly.
When an alcohol is exposed to air, oxygen combines with the hydrogen of the alcohol leaving it short on a couple of hydrogen atoms per alcohol molecule. In that state it may be described as having been dehydrogenated. Hence the name given to the resulting class of compounds – aldehydes. The compound produced by the dehydrogenation of ethanol is called acetaldehyde and its molecular formula is C2H4O. Acetaldehyde occurs naturally in beers and wines through exposure to oxygen and by the action of yeasts.
Acetaldehyde is one of the main causes of hangovers. An enzyme called alcohol dehydrogenase, which is found in the liver, converts ethanol in the blood to acetaldehyde and makes us feel bad the morning after.
There is a common belief that whisky does not go off in bottle and nothing need be done to keep bottled stocks in good condition. This is not the case: whisky left in the presence of a relatively large volume of air for a long time will form acetaldehyde.
Ethanol is not the only alcohol to undergo dehydrogenation. Formaldehyde, which comes from the oxidation of methanol, is perfectly ghastly. It's used to preserve dead flesh and gives a dissecting room its stench.
Fortunately, the ...