Percents and Alcohol
Posted Mar 04th 2012 | By:
Sugar's volume relates to the space it takes up, not to how many molecules it has. Sugar's volume in aqueous solution is nearly zero. The fermentation reaction is this: C12H22O11 +H2O + invertase →2 C6H12O6 C6H12O6 + Zymase → 2C2H5OH + 2CO2 [Wikipedia] Which is to say, for every equivalent molecular number of glucose molecules, yeast create two molecules of ethanol and two molecules of CO2. That is only the approximate conversion rate - in reality it's not a perfectly efficient process. Instead of using %-sugar, we tend to speak in terms of "specific gravity". Remember I said above that sugar has nearly no volume in water? That's because when you add it, the sugar dissolves and the molecules intercalate with the hydronium ions of water. It's like dumping sand in a box of marbles -- the sand fills in the cracks, so the box isn't any more full. The box *is* heavier, however, because although the volume is the same, the mass contained in that volume is not. The "specific gravity" of the box of marbles (now sandy marbles) has changed by adding the sand. We call the density of water without solutes to be value 1.000 Adding sugar pours sand in the box of marbles, so the more sand, the higher the gravity. A gravity of 1.050, for instance, is a good starting point for beer or cider, whereas a gravity of 1.090 would be better for wine. As to percentages... I don't use them, so I don't know them. I do know this: 1 lb. of sugar into 1 gallon (~8 lbs.) of water, fermented, makes about 5% alcohol. lb:gal ~5%ABV. That's all you really need to know. But you should be using a hydrometer to check your gravity.