What is Forced Fermentation?
How do you know when fermentation is complete?
One way is to measure the specific gravity, SG, several days in a row to see if there is a change. If the gravity reading is steady, fermentation is most likely complete. (Unless, of course, you have a stuck fermentation.)
Another way is to perform a forced fermentation test. What is a forced fermentation test?
A forced fermentation test (FFT) is a way to “determine the fermentability of wort and determine what the terminal gravity (or finishing gravity, FG) of the wort should be if fermentation goes to completion” per Imperial Yeast. I’ve included several references below on how to perform the test, but I’ll summarize the procedure here.
How to Perform a Forced Fermentation Test
To perform a forced fermentation test, here are some of the equipment you’ll need:
- Flask or other suitable container that can hold 250–500 mL of wort, including headspace room for krausen
- Aluminum foil or foam stopper
- You want oxygen to be able to enter the solution during the test, but keep out other nasties, as well as allow carbon dioxide produced during fermentation to escape
- A stir plate and magnetic stir bar (optional)
- This is optional. You can use the SNS (shaken, not stirred) method, as promoted by Denny Conn for yeast starters
- Just shake/swirl the flask as often as you can (e.g., every time you walk by it, give it a shake)
- This is optional. You can use the SNS (shaken, not stirred) method, as promoted by Denny Conn for yeast starters
- Hydrometer
Procedure
- Take 500 mL of cooled wort from your kettle/fermenter after cooling but before yeast pitch into a sanitized flask or container
- Add 15 mL yeast slurry (1 tablespoon) or 3 g dry yeast to the wort
- Cover with foil (or foam stopper) and place in a warm (24–27 °C/75–80 °F) area to promote rapid fermentation
- Gently agitate as often as possible
- E.g., use the SNS method mentioned above, or use a stir plate, creating a gentle vortex
- Observe fermentation. Once fermentation is judged to be complete (usually after 24–48 hours), de-gas the sample and take a hydrometer reading
This will provide you with an idea of what the final gravity should be in your main fermentation. It’s okay if the finished beer’s gravity doesn’t match the FFT results, but the gravities should be similar—perhaps off by just a few points (e.g. 1.014 for the beer versus 1.012 for the FFT).
Example Forced Fermentation Test
I recently brewed an Irish Red ale.
For the forced fermentation test, I put approximately 700 mL of wort into a 1 L Erlenmeyer flask and added 5 g SafAle S-04 yeast. The FFT wort was fermented at 26 °C (79 °F) with constant stirring on a magnetic stir plate. Here are the results:
| Irish Red | FFT | Bulk Fermentation |
|---|---|---|
| Fermentation Temperature | 26 °C | 17 °C |
| OG | 1.059 | 1.059 |
| FG | 1.013 | 1.016 |
| ABV | 6.0% max | 5.6% |
I performed a similar procedure for two lagers I brewed: Munich Helles and Doppelbock.
| Munich Helles | FFT | Bulk Fermentation |
|---|---|---|
| Fermentation Temperature | 20 °C | 13 °C |
| OG | 1.052 | 1.052 |
| FG | 1.010 | 1.010 |
| ABV | 5.4% max | 5.4% |
| Doppelbock | FFT | Bulk Fermentation |
|---|---|---|
| Fermentation Temperature | 20 °C | 10 °C |
| OG | 1.090 | 1.090 |
| FG | 1.020 | 1.022 |
| ABV | 9.25% max | 8.9% |
Sources
Brewer, The Phantom. “Forced Ferment Test.” Maltose Falcons, Maltose Falcons, 28 Aug. 2009, www.maltosefalcons.com/blogs/brewing-techniques-tips/forced-ferment-test. Accessed 07 November 2023.
“Forced Fermentation Test – What Is It and Why Do It?” Grainfather Community, 27 July 2021, grainfather.com/forced-fermentation-test-what-is-it-and-why-do-it/. Accessed 07 November 2023.
Dawson, Michael. “The Forced Ferment Test.” Brew Your Own, 1 Nov. 2014, byo.com/article/forced-ferment-test/. Accessed 07 November 2023.
“Forced Fermentation Test.” Imperial Yeast. https://www.imperialyeast.com/resources/document-library/forced-fermentation-test-proceedure. Access 28 November 2023.