The science of alcohol

May 13, 2004 11:43 AM

I know that there are no carbs in hard liquor (gin, vodka, etc), but why? I've been told that the carbs are burnt off in the distilling process, but I'd like to know exactly how that occurs. Weblinks are helpful.
posted by Juicylicious to science & nature (6 comments total)

Making Alcohol by Fermentation

"Enzymes in the yeast first convert carbohydrates like maltose or sucrose into even simpler ones like glucose and fructose, both C6H12O6, and then convert these in turn into ethanol and carbon dioxide."

The whole process of fermentation is designed to take high amounts of carbohydrates and convert them by the power of yeast into alcohol. I have a friend who tried to create vodka from potatoes. It apparently tasted like nail polish, more so then vodka usually tastes like. Distillation at home i shard because it requires exact levels and such.

Oh uh so "burnt off" isn't right. Converted would be the better term I would say.
posted by geoff. at 11:59 AM on May 13, 2004

An interesting and seemingly quite comprehensive discussion of alcohol metabolism explains a bit more about the amount of calories in alcohol.
posted by daver at 12:32 PM on May 13, 2004

What geoff said. To expand, a bit on carbohydrate biochemistry, without (hopefully) getting too technical.

A carbohydrate is any molecule made of sugar units. (A "sugar unit" has a certain defined chemical structure which I won't go into.) There are many different types of sugar units, and they can link to each other in a bunch of different ways. Sucrose (common table sugar) consists of one glucose unit, and one fructose unit, connected in a certain way. Since a sucrose molecule consists of two sugar units, it's called a disaccharide.

Carbohydrates can be a single sugar unit on their own (monosaccharides), two sugar units (disaccharides), a handful of units (oligosaccharides), or many many units, potentially into the thousands (polysaccharides). This last category includes things like starches, cellulose, and glycogen. (Things start to get a bit fuzzy when you talk about cellulose: the human body can't break it down, and it passes through undigested, so cellulose is a carbohydrate to biochemists, but maybe not considered one by nutritionists.)

Anyway, as Geoff said, in the fermentation process, the yeast break down complex carbohydrates into monosaccharides, and then break those down into carbon dioxide and alcohol. Once that's happened, they no longer have the defining chemical structure of carbohydrates.

If left alone during fermentation, yeast will convert virtually all the carbohydrates to CO2 and alcohol, leaving a nutritionally negligible amount. (e.g., dry wines). Alcoholic beverages with some residual sugar may be achieved by a number of methods: physically removing the yeast before fermentation is complete (e.g., beer, I think, at least the mass-produced ones); adding excess alcohol before fermentation is complete, killing the yeast (e.g., Port); starting with a liquid which is so high in sugar that the yeast effectively poison themselves with the alcohol they produce before all of the sugar is converted (e.g., Sauternes).

However, even if you did distill one of these beverages with residual carbohydrates, the resulting liquid will still have no carbohydrates, because carbohydrates are essentially non-volatile, so they don't evaporate with the alcohol, water, and limited other flavor-producing compounds during the distillation process. (If you try distilling sugar water, you're just going to get water out the other end.)

One other point to take away from this: it's important not to confuse compounds which contain calories (i.e., anything the body can use as an energy source) with carbohydrates. Protein, fat, alcohol, and (most) carbohydrates can be used by the body for energy, and thus all four are said to have calories, but protein, fat, and alcohol are not made up of sugar units, and thus are not carbohydrates.
posted by DevilsAdvocate at 1:06 PM on May 13, 2004

Great writing, DA!
posted by five fresh fish at 6:35 PM on May 13, 2004

I seem to remember this exact question being asked here. Hm.
posted by abcde at 2:06 AM on May 14, 2004

Great answer which deserves a sidebar.
posted by dash_slot- at 3:59 AM on May 14, 2004