The Biochemistry of Fermentation Monitoring
In the world of homebrewing, Alcohol By Volume (ABV) is the primary metric of success for a fermentation. Since we cannot easily measure ethanol concentrations directly in a home setting, we rely on the relationship between sugar concentration and liquid density. As yeast consumes heavy sugar molecules and converts them into ethanol (which is less dense than water), the total density of the liquid drops.
Specific Gravity (SG)
Brewers use a hydrometer to measure Specific Gravity—the ratio of the density of the wort to the density of pure water.
- Original Gravity (OG): The density before yeast is pitched (high sugar).
- Final Gravity (FG): The density after fermentation finishes (low sugar).
The Formula
The standard ABV formula uses a derived constant (131.25) to estimate the percentage of alcohol based on the change in gravity.
ABV % = (OG - FG) * 131.25
The Limits of Accuracy
The standard 131.25 formula is a linear approximation. For standard beers (3% to 7% ABV), it is extremely accurate. However, for high-gravity styles like Imperial Stouts or Triple IPAs (10%+ ABV), the relationship becomes non-linear. In these cases, professional brewers use a more complex alternate formula: ABV = [76.08 * (OG - FG) / (1.775 - OG)] * (FG / 0.794).