Brewing with the Aurochs: The Importance of Gravity

This is the first in a series of posts covering different elements and components of the brewing process, written by Aurochs team members Dave Rasmussen, Doug Foster, and Ryan Bove. For a quick introduction to the brewing process, check out out our previous post on the topic.

Brewing beer reduces gravity. We don’t mean it decreases the force that warps space-time as described by Einstein’s elegant equations. We’re referring to the effect that brewing has on alcohol content and beer flavor. In the context of brewing, and more generally in the context of hydrodynamics, “gravity” describes the density of a particular fluid relative to a reference fluid. The reference, or benchmark fluid, is usually water. This is primarily, and somewhat arbitrarily, because water is the most abundant liquid on the planet.

In short, gravity is a measurement of the density of a liquid to compared water. It is commonly measured using a floating hydrometer (or, less commonly, a refractometer), which is submerged in a graduated cylinder filled with the subject liquid. The density of water is indexed to a value of 1.000, meaning a liquid with a density greater than water has a gravity more than 1.000.

Gravity is a very important metric in the brewing process. It’s what enables brewers to determine when fermentation is complete, as well as the alcohol content of beers.

Gravity is measured after boiling the sugary wort, periodically during fermentation and immediately following fermentation. The first measurement is the original gravity (abbreviated OG).  This is the density of wort, relative to water, before the yeast is pitched and the fermentation process has begun. Since wort necessarily contains sugar, it is denser than water, and thus has a gravity measurement greater than 1.000, usually in the ballpark of 1.050. Brewers can reach a higher OG by adding more fermentables, or grains, into the recipe.

During fermentation, the yeast consumes the sugar and converts it into carbon dioxide, alcohol and various flavor compounds. As alcohol, which is less dense than water, replaces the sugar, the gravity of the wort approaches 1.000. Gravity measurements measures must be taken every few days during this fermentation process to track and record the declining density.

After one to two weeks in the case of ale and six weeks in the case of lager, the fermentation process is complete. The yeast is no longer active, and clump together as it falls to the bottom of the fermentation tank. To confirm that this process is complete, the brewer will check the specific gravity log to see if the gravity over the past several days has stayed the same. This means that the density of the beer, and therefore the alcohol content, has remained constant, indicating a complete fermentation. The final measurement is called the final gravity (abbreviated FG). The difference between the OG and the FG allows the brewer to closely estimate the percentage of alcohol by volume. The higher the FG, the more sugar that is left over in after fermentation, producing a sweeter beer.

The process of declining density during fermentation is known as attenuation, which is measured in percentage terms. Pardon the math in a beer blog, but attenuation is measured as [(OG-FG)/(OG-1)] x 100. Brewer’s yeast typically has an attenuation range of 65-85%.  A brewer can forecast the fermentation completion date by using the particular attenuation characteristics of their yeast and the sugar content of their wort.

OG plays a critical role at Aurochs Brewing Co, as it does at every other brewery. In our next post in this series, we’ll investigate other critical metrics and components of the brewing process.