How can moonshine cause blindness?
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Fermentation produces a lot of chemicals, not just ethyl alcohol (ethanol). One of the more common products is methanol, which is a different kind of alcohol, one containing one fewer carbon atom and two fewer hydrogen atoms.
This is naturally occurring in all kinds of alcoholic beverages, but when we drink non-distilled spirits, we’ll have to drink **A LOT** of the stuff to experience methanol poisoning, and ordinary unconsciousness is likely to take hold long before you’ve ingested enough methanol to cause damage.
However, distillation is a much trickier process. When you distill a wine, beer, or wash (one of the trade’s term for the pre-distilled liquid), the heat is applied and various compounds start to evaporate off. The more volatile a chemical is, the sooner it starts to boil, and then condense out of the still. Those first parts of the distillate are called ‘foreshots’ or ‘heads’, which are followed by the ‘hearts’ which is the high quality distillate, and then the ‘tails’, which includes less volatile more oily compounds coming out of the wash.
A good, careful distiller will check the specific gravity of the distillate at various stages during the run, and discard the heads and tails to produce a safe, delicious product, which matches the character of the spirit they’re trying to make. When they’re done, they blend all the parts they decided to retain together, and either sell them immediately, or flavor and age them according to the type of product they want to manufacture.
If you want to see this process in action, I *highly* recommend the channel [Still It](https://www.youtube.com/c/StillIt).
If, however, you’re an unethical criminal who merely wants to make cheap booze for people you don’t know, or you’re a rank amateur who doesn’t understand the chemistry of spirit-making, then you may decide to just bottle those heads and sell them. And now we’re off to the races.
The unfortunate fellow who imbibes that bottle will introduce methanol into their bloodstream, which their body will try to metabolize. One of the metabolites produced in breaking down the methanol is formic acid. Your body can withstand acids above a certain pH, but if they get *too low* some of your more sensitive tissues can be damaged, and one of the most sensitive organs to acidity is your optic nerve.
Also, it was not uncommon to ‘de-nature’ alcohol with methanol, so that it could be used for common industrial purposes like cleaning or paint removal or solvent or such. But during Prohibition, there were, again, criminals, who would buy these denatured spirits and either directly re-bottle them, or roughly distill them to remove the methanol, but if they did a crappy job? Bam, blindness.
There are other chemicals which can be introduced into improperly distilled spirits, of course, but methanol is the *classic*.
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