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Is My Beer Done Fermenting?

Updated: Jan 22, 2022



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How do you know when your beer is done fermenting? Is it time to transfer to a keg? Or bottles? When is it time for the next step?

What if you could predict, with great accuracy, the specific day to transfer the beer from the yeast, before the batch has even been brewed?

Rely on experience but use data as a guide. So often, I read recipes that have instructions like, "let beer ferment for two weeks, then take a final gravity and transfer." What if your beer hits final gravity on day 5? What if it is still fermenting after two weeks (like a lager)? Why wait some indeterminant amount of time before checking gravities?

Our rule of thumb for checking gravities:

  • Ales - Every day!

  • Lagers - Every two days

Why put so much energy into checking gravities? Because numbers don't lie. Below is a few gravity curve charts from real batches - no fake data here. You tell me when the beer was done fermenting...

What day was this beer done fermenting?

Or this one...

I can do this all day!

Data...and more specifically, data visualizations - make determining the end of fermentation mindless. There's no math involved here. When the curve levels out, it's done.

There's a caveat to this though. Just because the gravity curve levels out does NOT mean you have to transfer to a secondary fermenter, keg, bottle, or whatever. It just means that the fermentable sugars are all gone. Depending on the yeast strain, style of beer, and other factors - you may still need/want to do a diacetyl rest or some other fancy thing.

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Knowing when to do things can be a little confusing for beginner brewers. I know that when I first started, I felt like the timing was critical - even transferring a single day too soon or too late would have dire results for the finished product. Fortunately, brewing is both an art and a science, and the rules are not always so rigid.

Knowing when to transfer your beer off of the yeast does have its benefits. Too little time on the yeast and your beer will be "green" - swamped by compounds and flavors that have yet to be processed by the still active yeast. Too much time on the yeast and you run the risk of off flavors due to autolysis. This is a commonly quoted drawback of extended fermentation - and honestly one that I do not concern myself with too much. The time it would take to reach that autolysis point is measured in weeks, not days.

yeast cells seen through a microscope
Dead yeast cells (blue)

While autolysis and other negative effects may be motivators for you to transfer on time - a motivator for me as a homebrewer (and I would assume this goes for professional brewers as well), is simply - efficiency.

Knowing down to the day when your tank or carboy will be free for the next batch is not only convenient, but for brewer's that have limited equipment this knowledge could take your production to the next level by allowing more efficient brewing schedules.

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At Arithmech, we're creating software that helps you predict and analyze fermentation patterns. AleProof has the functionality to give brewers and cellarman a predicted window for when your beer will be done fermenting. This is done by combining simple statistics about past batches using the same yeast strain. Why does the specific strain matter? Because each strain ferments at different rates and to different attenuations.

The image below is taken from AleProof and depicts results from real data.

The math behind this is elementary but this post won't dive into it. The ranges in the image above were derived from the data of three past batches using the same yeast strain (White Labs WLP001). For each batch, we watched the gravity curves and "marked" the End of Fermentation (EOF) when the gravity was the same two days in a row. For example: if day 4 was 1.015, day 5 was 1.013, and day 6 was 1.013, the EOF day is day 5. The vertical green range is the earliest to latest (min to max) EOF. The solid green line is the average.

The horizontal blue range is derived in a similar fashion. We take the final attenuation values for each previous batch and calculate final gravities for the current batch. This gives a minimum, maximum, and average predicted final gravity based on real data.

The intersection of these two ranges gives you a window for not only WHEN the beer will finish, but at what gravity it will finish. Based on this, if we have an OG of 1.045, we can confidently predict that this beer will be done fermenting in 5.3 days with a final gravity of 1.009. Think this could give you an advantage? We do.

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