This is part 2 of our article series on how to tell when fermentation is complete. Click here to check out How to Know When Fermentation Has Finished - Part 1

Monitoring Fermentation - The Scientific Way

This article is for homebrewers and home distillers. In order to follow along, you will need a brewing hydrometer and a hydrometer test jar. This simple guide will go through each step of the process of using a hydrometer. We will discuss how to determine "potential alcohol," how to calculate actual "alcohol by volume" (ABV) and how to be absolutely certain that a fermentation has finished.

The specific gravity of water is 1.000. When a specific gravity reading of beer wort is taken, it will have a higher density (before fermentation) compared to water because of the sugars present. During fermentation these sugars will be consumed by yeast causing the density and therefore specific gravity to decrease. So, to re-cap, pre-fermentation liquid will have a high specific gravity relative to fermented liquid which will have a lower gravity. The difference between these two numbers will tell you the ABV of a fermented solution.

Equipment Needed to Use a Hydrometer

• Beer / Wine Hydrometer-  a spirit/Proof & Tralle Hydrometer cannot be used for to determine the specific gravity of beer, wine, or other low ABV solutions.

• Hydrometer Test Jar- a plastic or glass tube that can hold the liquid being measured

• Beer Sampler- Using a turkey baster or a wine thief can make it easier to take a sample of the liquid being measured

The Basic Process

• After mashing, boiling, etc, but before pitching yeast, take a hydrometer reading to determine specific gravity.
• Ferment, as normal.

• When you think fermentation has finished, i.e. when the airlock stops bubbling for a day or two, taken another reading.

• Wait 24 hours and take one more reading. If the number is the same, fermentation is likely complete. Additionally:

• If the post-fermentation gravity reading is at 1.000 or less, it is definitely done.
• If the post-fermentation gravity reading is 1.020 or higher, wait a few days then take another reading
• Keep taking readings (if needed) until the gravity stops dropping (which means that fermentation is complete).

To determine ABV, plug the original gravity (pre-fermentation) and the final gravity (post-fermentation) into an ABV calculator. This will determine the alcohol by volume of the solution. For more information on this, check out our article on "Alcohol Content."