The Forced Ferment Test
A forced ferment is a test used by many professional brewers and brewery labs to determine the minimum final gravity possible from a freshly-made batch of wort.
By diverting a sample of wort from the main batch and causing it to undergo a rapid fermentation — through some combination of overpitching, elevated temperature, and/or agitation or stirring — brewers can anticipate the final gravity (FG) well ahead of time and plan for the late stages of fermentation and beyond with greater certainty, all while the main batch is still approaching high kräusen. Obviously this is important for a commercial brewery, since speedy and efficient turnaround in the brewhouse and cellar is desirable. It is used to check consistency in commercial brewhouse operations, specifically enzymatic action in the mash. It’s also important because by knowing the final gravity, the brewer can tell if the fermentation is finished, or if it has stalled prematurely. Chilling a tank when it is stalled, but when there is still fermentable sugar left in the beer, is bad for a packaging brewery and can lead to spoilage or overcarbonation in the bottle.
But what about us homebrewers? Many brewing software programs calculate an anticipated FG for homebrew batches based on the apparent attenuation range for yeast strain, typical fermentability of grains and sugars, and the mash regimen used for each batch. But as we know, a lot can happen between grain and glass.
For all-grain brewers, a forced ferment test can provide deeper insights into your system and process: An unexpectedly high FG in a forced ferment test indicates too many unfermentables in the wort, either as a result of mash temperature or incomplete saccharifcation, and a hint that it may be time to recalibrate your thermometers. A forced ferment that finishes with a lower FG than the main batch may be a sign of fermentation issues in the main batch, such as underpitching or O2 deficiency.
With a forced ferment test, extract brewers can get a grip on the maximum fermentability of their chosen liquid or dried malt extract on their equipment, which can help improve consistency and repeatability.
For homebrewers of all methods, a forced ferment can answer questions about — and help remedy — stuck fermentation issues, as well as possibly saving some time and final yield by eliminating the need for some hydrometer samples late in the fermentation.
A forced ferment started on brew day can help a homebrewer anticipate any stutters or stops with the main batch; for example, when using a saison yeast strain that is known for taking breaks mid-fermentation. If a fermentation stalls, pulling a sample from the fermenter and performing a forced ferment test will determine if the fermentation is truly stuck, or if the main batch just needs a little encouragement — perhaps in the form of warmer temperatures, additional yeast, a bit of O2, or just more time.
Knowing what FG reading to watch for means knowing that fermentation is done, and not having to wait for a hydrometer reading to stabilize in real time over the course of days, or trying to guess by watching kräusen or airlock bubbles. That means a lot less sanitizing and cleaning of a beer thief and/or stopcock, and — ideally — less volume lost to hydrometer samples. It also means that you’ll know exactly the point when your batch is ready for its next step, whether secondary, lagering, or packaging.
Should I Do a Forced Ferment Test?
You’ll have to answer that for yourself. If, like me, an FBI profiler would say of you that your airlock-sniffing habit is likely to escalate, you are probably a likely candidate. Although beer geeks, large-batch brewers, and the technically minded might find a lot to love, conducting a forced ferment test is not necessary in order to successfully brew, ferment, and enjoy a batch of your own beer. Homebrewing only needs to be as simple or complicated enough to give you enjoyment.
Performing a Forced Ferment at Home
Please read through all steps before starting! Maintaining excellent sanitation practices are a must, since contamination of the sample with wild yeast or spoilage bacteria can skew the readings. Here are the steps:
- Collect a sample of fresh wort from the main batch.
Because the end goal of a forced ferment is to measure the gravity, the sample you pull from the main batch of wort has to be big enough for a hydrometer reading (see top photo on page 68) — say, approximately 1 to 2 cups (250 to 500 mL).
After the boil is finished and the wort is cooled, decant the wort sample into a sanitized container with roughly 50% headspace — a flask, Mason jar, or similar vessel will work fine — and cover the container loosely with sanitized foil or lid.
- Inoculate the wort sample.
Here you’ll need to make a choice — inoculate the forced ferment sample at the same rate as the main batch, or inoculate at a much higher rate (5–10x) than the main batch.
A higher pitch rate will yield the fastest test result in the shortest amount of time; this approach is called for by many forced ferment protocols (like this one by Wyeast Laboratories: www.wyeastlab.com/he-brew-qc.cfm). To do this on a homebrew scale, add 15 mL (approx. 1 tablespoon) of yeast slurry from a starter culture, or 3 grams (approximately 1 teaspoon) of dried yeast per cup of sample wort. The liquid and/or fermented starter wort from a liquid slurry may lower the SG of the sample by 0.001–0.002 or so.
Pitching at the same rate as the main batch is probably more manageable for most homebrewers, since there is less potential for wort dilution and less handling required, therefore less opportunity for contamination of the sample; this simpler method is also used successfully by the lab staff in at least some commercial breweries. With this approach, just wait to collect the forced ferment sample until after the main batch is aerated/oxygenated and the yeast pitched.
- Ferment the sample.
Move the inoculated sample to a warm spot (75–80 °F/24–27 °C) to encourage rapid fermentation. If you have a stir plate, this is a great time to use it — set to low speed and let it whirl (see the bottom photo on page 68). If you don’t have a stir plate, agitate the flask or jar frequently by shaking or swirling.
- Measure the gravity of the sample.
After 24 to 48 hours, the forced ferment sample will have reached final gravity. Allow the yeast to settle, then measure gravity with a hydrometer (see top photo on this page). Once you have recorded the target FG for your batch, the forced ferment sample can be discarded.
