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Fast Ferment Testing

Posted by mchrispen on 5/23/2015 at 10:12:40 AM

Fast Ferment Tests (FFT), sometimes called Forced Ferment Tests, have lost some favor in homebrewing. FFT can be used to measure the maximum fermentability of wort giving you an idea of where your beer's FG will land, and more specific information about the yeast's attenuation.

With the availability of very fresh and viable yeast in both liquid and dry forms, FFT  has been largely replaced by the best practice of creating a starter. FFT’s still have a role in brewing, and many breweries continue to run these tests when getting a fresh pitch of yeast or brewing a new recipe. Most sources cite Prof. Dr. agr. Ludwig Narziss for documenting this test and proving its usefulness.

The Procedure

Gather a small portion of your beer’s wort after chilling and aeration. You can use your hydrometer sample plus a few additional ounces, or a separate sample collected in a clean and sanitary vessel. Note your OG. Reserve some of your starter (or prepare extra cells in your starter) and pitch the yeast into the FFT. Usually, you will be over-pitching the cell counts which is fine for this test. Place the vessel, covered with foil or with an airlock at room temperature and let it ferment out. Note the FG and determine the apparent attenuation. This is more or less the level of attenuation and FG you can expect with the larger batch of beer. Generally, the yeast will attenuate further than it will under a cooler ferment in the larger batch, but give you very specific behavior information to compare.

The Vessel

Any sanitary vessel will work. I use flasks, beer bottles or glass or PET juice bottles. Anything that can be cleaned and sanitized will work. I seldom use an airlock, as foil is simple and allows additional oxygen uptake during the lag phase. Flasks are particularly useful and I often will use my 2L flask for this process. This also allows me to use my stir plate to continually rouse the yeast, accelerating fermentation. If you cannot use a stir plate, like making starters, swirl the fermenting beer up a few times a day for the same effect.

The Pros and Cons

There are many benefits using this method, the primary being that you are using the specific wort for your beer, including all of the issues that affect attenuation. When I am using a new yeast strain, I am getting very specific feedback: attenuation, expected FG in my particular beer’s wort composition, flocculation level and confidence in a successful fermentation. While the FFT is conducted warm, one can also detect the ester and phenol production of that yeast strain. Of course, if you use a stir plate or shake too often, you will likely get a fairly oxidized result. Once fermentation is complete, you can cold crash the FFT and taste the clear beer. You can harvest the yeast cake from this test for another batch of beer as long as you were very careful with sanitation, or use it to krausen the big batch of beer to help clean up fermentation off flavors.

Keep a Yeast Strain log. Note the pertinent information from the test, including the wort composition, gravities, attenuation and any particular notes, like flocculation, ester and phenol character, mouth feel, etc. Make sure to note the dates on the yeast as well.

The downside is really the extra process, and tying up potentially useful flasks or vessels. There is a bit more cleaning and sanitizing, but no more so than building a starter.

  1. Brew your beer adding a small additional volume for your FFT. I like to use about 1-2 liters, but you need just enough for a hydro sample after fermenting.
  2. Gather your FFT beer after you have chilled and oxygenated your wort. Note the OG.
  3. Pitch your yeast, at the required cell count in the big batch, and over-pitch your FFT
  4. Hold the FFT at room temperatures, occasionally rousing the yeast during fermentation. Optionally, use a flat bottom vessel and a stir plate. Or allow this to ferment at fermentation temperatures with rousing to create a beer with similar fermentation characteristics to the big batch.
  5. Upon completed fermentation, measure the FG and calculate apparent attenuation. Note any fermentation characteristics in your log.
  6. Optionally - cold crash the FFT and decant some clear beer for sensory evaluation. Note your findings in the log.
  7. Optionally - harvest and store the yeast in a sanitary vessel. Flame the lip or wipe with ethanol before transferring to prevent contamination. Store the yeast under the beer it created.


Matt is the homebrewer and writer behind, which was started to support Brew-Magic users with tips and tricks, recipes and general brewing information. The site also hosts a good deal of water management information, along with tutorials on using M. Brungard's Bru'n Water Spreadsheet.

Tags for this post: yeast, attenuation, forced ferment test, fast ferment test, starter, harvest yeast

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Edited after some good feedback on Reddit. Hopefully more clear!

posted by mchrispen on 5/26/2015 at 10:09:24 AM

I love this. I am a huge proponent of forced ferment tests, and do one on almost every single batch. I find that using a canning jar with the ring loose works well, and swishing the wort through my smack pack, vial, or starter flask before pouring it into the FFT vessel gets me the pitching rate I want (see below). My typical FFT sample size is 7-8 oz., which is enough for me to do two hydrometer tests.

My rule of thumb for pitching rate on FFTs is approximately 10x the recommended pitching rate for your beer.

The FFT can also be done on fermenting beer, and is useful when you didn't do an initial FFT to determine whether your fermentation is stuck or truly finished. Doing a late-stage FFT is interesting because you may not have the primary fermentation strain available, nor may you wish to pitch the same primary fermentation strain if you suspect a stuck fermentation. So it may be useful to pitch a different attenuative strain to see if you can get the beer to attenuate more -- such as the second strain you might wish to pitch to unstick the fermentation (e.g., Wyeast 3711, WLP 090, Chico, etc.)

If you do a late-stage FFT, you will definitely want to do it on a stir plate, and ideally at warm temps (70-80°F). I find that the top of a stand-up refrigerator is a good place that stays warm enough for this purpose.

posted by chino_brews on 5/26/2015 at 03:21:19 PM