Beginner FAQ: How to tell if my beer has finished fermenting?

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 in our wort - and as a byproduct of this consumption, carbon dioxide (CO2), alcohol and heat are generated - along with other flavours and compounds that are imparted into the beer to give it it's unique flavours.

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. For homebrewers, packaging will often be into bottles.

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.

So, how to know if fermentation is fully completed? The only way to know for sure is to take hydrometer readings to measure the specific gravity of the wort. If there are no changes to the specific gravity reading for 3 consecutive days, then fermentation is complete.

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.

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.

We get it though - taking a couple hundred mL for a floating hydrometer sample every day for 3 days adds up - and who likes wasting beer, right? There are other alternatives though such as the Rapt Pill floating hydrometer which 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 which 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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