How To Do A Forced Fermentation Test
What is a forced fermentation test for beer?
Have you ever had a stuck or stalled fermentation and wondered how you can troubleshoot it?
A forced fermentation test (FFT) helps you predict the ability of your yeast to consume sugars in your wort. It gives you a likely end point for your fermentation so that you know ahead of time where it will finish.
This is really useful because by doing a forced fermentation test, you can tell whether a stuck or stalled fermentation is a result of a yeast issue or a result of a brewhouse issue such as incomplete starch conversion.
The forced fermentation test works by the principles of over-pitching a wort sample and stirring it at room temperature, both of which will accelerate the fermentation process and give you an endpoint faster than your actual main batch of beer.
Why should I do a forced fermentation test?
A forced fermentation test lets you determine whether a stuck or stalled fermentation is caused by a yeast issue or a brewhouse issue such as incomplete starch conversion.
For example, if your beer has an original gravity of 15ºP (1.060 SG) and your recipe predicted a final gravity of 2.5ºP (1.010), but your fermentation stopped at 4ºP (1.016). But, your forced fermentation showed a final gravity of 2.5ºP (1.010). This would indicate a yeast issue such as under-pitching or nutrient deficiency.
On the other hand, if the forced ferment showed a final gravity of 4ºP (1.016), that would indicate that the wort produced in the brewhouse does not contain sufficient fermentable sugars to produce the final gravity calculated in the recipe.
How to do a forced fermentation test?
The following instructions are adapted from the ASBC method Beer-16 to be more friendly to available resources in the typical craft brewery.
Materials
- 1x 1L Erlenmeyer Flask
- 1x Magnetic Stir Bar
- 1x Magnetic Stir Plate
- 500mL Wort
- 10 mL Yeast Slurry or 2g Dry Yeast
- Device for measuring specific gravity (hydrometer or densitometer)
Method
1. Sanitize or sterilize Erlenmeyer flask and magnetic stir bar.
2. Collect wort from brewhouse or fermentation vessel into flask in an aseptic manner. Warm to room temperature if cooler than room temperature.
3. Pitch 10mL of yeast slurry into the flask and mix well.
4. Affix an airlock onto the top of the flask and set the stir plate to medium speed. You should be able to see a small "cone" or indent at the surface of the wort.
5. Measure specific gravity at 48 hours (2 days) and 72 hours (3 days) or until there is no further change in specific gravity. For most strains, 72 hours is sufficient.
This method can be scaled down by a factor of 10 (50 mL in 100mL flask) if you have a handheld densitometer such as Anton Paar DMA35.
Can I do a forced fermentation test on slow/stalled beer?
Yes you can! This is a useful test to see if a stuck/stalled beer will continue to ferment or whether it requires intervention such as a new yeast or enzymes.