Distillation 101

Pot stills at the Glenfiddich distillery
Pot stills at the Glenfiddich Distillery in Scotland, By Colin Smith, CC BY-SA 2.0, Link

If I’m making some cheese fondue and my recipe calls for a cup or two of Chablis, I can still give it to my kids without getting them drunk because the alcohol will all boil off in the cooking process, right? Not exactly…

A common misconception I hear often is the idea that if a mixture of two liquids with two different boiling points is held at a temperature above the lower point of the two, the more volatile liquid will be boiled off, leaving only the higher boiling point liquid behind. Or, more specifically, if a pot containing a mixture of ethanol (the alcohol some of us drink) and water is heated to somewhere just above 173.1ºF, the boiling point of ethanol, that in a short amount of time the ethanol will be driven off. Unfortunately, in reality you’ll be left with a hot pot of boozy water. It’ll stay that way until it slowly evaporates (not boils) away completely. At no point will you have a mixture which is alcohol-free. 

The boiling point of an ethanol-water mixture is determined by the ratio of the two. A mixture of 50% ethanol and 50% water (by volume) has a boiling point of about 180ºF. This is higher than pure alcohol, and lower than pure water. If this mixture is heated to 175ºF, it’s hot, but not boiling. Because it’s not boiling, there isn’t a considerable amount of anything being driven off. If we heat it further, it will reach its boiling point, but will not get any hotter. Unless we change the ratio of water and ethanol, the temperature will never rise above 180ºF. 

The interesting thing, though, is that when the mixture is boiling, and we do have a considerable amount of vapor being driven off, that vapor has a concentration of alcohol greater than 50%. It will in fact be somewhere close to 80% ethanol by volume. If we were to take that vapor and condense it back into a liquid, we’d have a mixture that’s still 80% ethanol by volume. And that, in essence, is exactly how distillation works!

If we look back at our boiling pot of alco-water, since the vapor being driven off is higher in alcohol concentration, it has the effect of reducing the ethanol concentration in the pot. Thus, it slowly becomes less alcoholic. And, in turn, its boiling point increases. The temperature of the boiling mixture will asymptotically approach the boiling point of water, but it will never quite reach it. Similarly, the concentration of alcohol will asymptotically approach 0%, but won’t quite reach it before the mixture is completely boiled away.

Pretty neat, huh? So what about that cheese fondue? First off, kudos for being the kind of parent that shares their fondue with their children. But be careful– even though you’re probably boiling, say, two cups of wine down to one cup, some of the alcohol will have been driven off but there will still be a non-negligible amount left.

A note to “cover my butt”: Distilling alcohol without a license is illegal in the US, as it is in most countries. Learning about distillation is perfectly legal, but practicing is not. I help legally licensed commercial distilleries measure and control their equipment, and I find it interesting, which is why I share distillation-related information here.

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