When fermenting a batch of beer we often times find ourselves wondering when fermentation will be finished. We have expectations of how our yeast will perform based on an attenuation range for our selected yeast strain but there are many other variables at play. Your wort composition and yeast health will vary from batch to batch and estimations of final gravity are exactly that. A quick and easy test that can be performed at the beginning of fermentation is called the forced ferment test also known as the limit of attenuation test or the fast ferment test.

The theory behind the forced ferment test is simple. Take a small sample of the pitched wort you created and place it on a stir plate in a warm location. Only enough wort is needed to take a hydrometer reading. The constant oxygenation and warm temperature significantly speed up the fermentation of this small sample of wort. Fermentation can be done is as little as one day but will most likely be completed well before the primary batch is finished. The final gravity that is measured at the end of this test can be considered to be the lowest your primary batch will reach. Usually the actual final gravity of the primary batch is a point or two higher.

Performing the fast ferment test will ease any worries you may have about wort fermentability. The FFT can also help you make decisions in the fermentation process. Planning on dry hopping near the end of fermentation or beginning a diacetyl rest? The forced ferment test will tell you where that end is. It is also a good indicator of contamination in the primary batch. If the FG ends up under the FFT then you can be pretty sure that some wild yeast or bacteria got into the fermentor.

I try to perform this test for every batch and it really helps alleviate some of the guess work. For example, I recently brewed a stout that I expected to finish with a final gravity of around 1.012. After performing the FFT it only actually finished at 1.017. I then knew that I created a wort that was not as fermentable as I had hoped, likely due to an excessive amount of flaked barley. If I did not have this information I would have been worried when my primary batch finished at 1.018.

Below is a series of photos with instructions under each explaining the process I use and variations for other brewing equipment.

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Clean and sanitize a small flask. I like to use a 250mL Erlenmeyer Flask with a small stir bar. Ideally you would run this through an autoclave or a pressure cooker to sterilize but a good soaking in starsan will suffice. Cover the lid with a small piece of sanitized aluminum foil. You could also use any other small container that will work on a stir plate. I suppose a mason jar might even do the trick. Just make sure it is clean and sanitized.

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Here is the step where I am a bit spoiled. Pulling a small sample from the conical racking arm is very easy. I sanitize the triclover barb, flame the tip with a small torch, and use the foil from the flask to cover the opening.

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Pull a small sample. You only need enough to be able to obtain a gravity reading once the FFT is completed. I generally shoot for 150mL. If you do not have a conical, you can simply divert this small amount of wort into the flask when you are filling from your kettle. You could also use a wine thief similar to how you would take normal readings from a carboy or bucket. Just take precautions to be as sterile as possible. Any small amount of wild yeast or bacteria can skew the results.

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Here you can see the small sample pulled from the conical on the left. The 2000mL flask on the right contains my pitch of yeast for the primary batch which has already had the starter beer decanted off. This is a key point where my approach differs from published methods which instruct pitching the yeast and THEN pulling your sample. I like to pull my sample from my oxygenated wort and then pitch yeast into my sample. I’ll explain this process below.

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Flame the opening of your sample. If you do not have a bunsen burner, a small alcohol lamp or even a candle can work. This is the best method to attempt to make the transfer without picking up any wild yeast or bacteria. If you do not have any of these flame sources, use a lighter to flame the opening and make the transfer in a draft free room. Notice that the 2000mL flask is empty as it has been pitched into the primary batch.

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Even after pitching into the primary batch, there are always some dregs leftover that do not pour out. There is an amazing amount of yeast in those dregs, enough that you will be overpitching your small 150mL sample. Pour the sample into the dregs from your yeast starter after flaming the lip. If you are pitching directly from a vial or smack pack, you can pour you sample into that as well.

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Give the small sample in the large flask a good stir and then pour back into your small flask. You will notice the frothy head that transfers back. That is all of your yeasty goodness. Briefly flame the opening of the small flask and the aluminium foil and cap loosely.

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Place your sample on a stir plate and let it rip. You do not need a huge vortex for the FFT (or any starter for that matter), just a small dimple on the surface. This is plenty to dissolve the needed oxygen into solution. Ideally you want to place this in a warm location or even an incubator at 80°F if you have access to one. In the winter I usually put my starters on top of my refrigerator.

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After a day or so you will notice significant yeast growth and the FFT will start to turn a milky color.

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Once fermentation has completed, shut off you stir plate and let the yeast settle to the bottom. Look at all that yeast! If you are making another beer with this same strain, you could use this to step into another starter, especially if you took precautions to be as sterile as possible.

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Pour you FFT into you hydrometer test tube and take a gravity reading. Note that 150mL is just enough to get an accurate reading. If you expect the FG to be well below 1.010 you may want to take 200mL at the beginning to be safe.

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And finally a shot of the reading. Go ahead and taste the FFT beer but do not be alarmed when it tastes oxidized/papery/yeasty/harsh. This pale ale ended up finishing just a touch over 1.011. This means I can expect the primary beer to at least hit an FG of 1.013. I plan on dry hopping directly in the fermentor with only 4 gravity points (1°P) left to ferment so I now know that when I get a reading of around 1.017 I should be good to go.

So there is my method for performing the FFT. I think if you can find a way to work this into your brewing routine you will be amazed at the information you can gather from this simple test. Feel free to comment below with questions/comments or lay out a method you may use for the FFT. Cheers!

1 Comment

  1. Good stuff, I have been reading about FFT all night and came across your method. I like the idea of using the dregs of the yeast starter to perform the test. I will be trying a FFT with my next batch. Thanks for the good writeup

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