Homebrew School: How to Tell if Fermentation is Finished?

This is a common question for beginner homebrewers and it's a very important one to understand. As we well know, fermentation is the process undertaken by yeast that consumes sugars within the wort - and as a byproduct of this, carbon dioxide (CO2), alcohol and heat are generated, along with other flavours and compounds that give beer it's unique taste.

Why Do We Need to Know When Fermentation is Complete?

It's important that brewers understand when the fermentation period is completed so they know when they can proceed with the next step of the brewing process, usually packaging, but in some cases it could be dry hopping, or cold crashing the fermenter. For homebrewers, packaging will often be into bottles.

The Dangers of Incomplete Fermentations

Packaging before fermentation is completed can be dangerous - especially when glass bottles are involved. If the beer is packaged into a bottle before fermentation is fully completed, the remaining sugar in the wort will be consumed by the yeast, along with the priming sugar that would have also been added to the bottles. As a result of this, the CO2 that is produced from the mini-fermentation that occurs in the bottle will create an excessive build up of pressure which can lead to exploding bottles - aka. "bottle bombs". Definitely not something you want to happen.

Methods NOT to Use to Check Fermentation is Complete

Let's dispel a few myths and bad advice that we often see floating around about how you can check if fermentation is complete.

Air Lock Activity

Bubbles in the airlock are not a reliable way to measure fermentation activity

Relying on air lock bubbling/activity (or lack thereof) is not a reliable method for determining the completion of fermentation. As fermentation slows down towards the end, air lock activity will dramatically drop but fermentation can still be slowly occurring with no visible signs in the air lock.

Recipe Final Gravity

Relying on reaching your recipes expected final gravity is also not a reliable method. Yeast are living organisms and as a result are somewhat unpredictable and don't always behave the same. For any given batch, the yeast may consume more or less sugar than expected which is known as over attenuation or under attentuation respectively.

The Best Way to Check Fermentation is Complete

The only way to know for sure that your fermentation is complete is to take hydrometer readings to measure the specific gravity (SG) of the wort. If there are no changes to the specific gravity reading for 3 consecutive days, then fermentation is complete.

Specific Gravity refers to how much sugar is present within a liquid, and represents the density of the fluid in comparison to water. If the gravity does not change for several days, it is safe to say that the yeast are no longer actively consuming the sugars in the wort and therefore fermentation is finished.

A floating hydrometer is used to measure specific gravity

Gravity is measured using a hydrometer, with the most common being a floating hydrometer. Taking a sample can require a couple hundred millilitre's, which can add up when you're taking samples over multiple days, and who wants to waste precious beer, right?

There are other alternatives though such as the Rapt Pill floating hydrometer, which remain submerged in the wort throughout fermentation and can give real time readings on the specific gravity of the wort and send them to an internet dashboard (ie. web page) or mobile app where you can view them. This saves on the wastage of taking small samples for floating hydrometer readings.

Gravity readings are an important part of brewing - they also allow you to calculate how much alcohol has been generated by the fermentation and imparted into your beer!

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