Fermentation, redux.

Took my hydrometer readings.

We care about gravity (really, the density of the beer) because we’re trying to figure out how much sugar the yeast have eaten and turned into alcohol. The goal terminal gravity is the significant numbers in the original gravity (ignore the “1.0”) divided by 3.7. It estimates (for an average yeast) the change in gravity we would experience with “perfect” fermentation. After only four days of fermentation, I’m doing pretty well. I’m certainly well within measurement error, given that the hydrometer floats and bobs around, and bubbles from the beer attach themselves to it causing artificially high readings.

The alcohol content is a function of the gravity change. The rule of thumb is to (again) remove the meaningless “1.0” from the front of your gravity readings and then subtract them. That’s the amount of sugar that has been removed by the yeast. Multiply that difference by 0.1275 and you get your alcohol by volume. Interesting to note that wines can end up with a final gravity of *less than* 1, since alcohol is lighter than water, and the percentage is high enough to matter. When homebrewing, if I can’t do the math in my head, it doesn’t matter. If I ever make wine, I’ll get a calculator for the kitchen.