Fast Ferment Test
The idea of the Fast Ferment Test, is to completely ferment all fermentable sugars in a sample of wort. The attenuation that can then be determined is the limit of attenuation of that wort and it only depends on the yeast species (ale vs. lager yeast) and the wort composition that has been established by the brewing process (mainly mashing) [Narziss, 2005]. Lager yeasts are able to ferment some types of sugar (trisaccharides like maltotriose, melibiose and raffinose) that ale yeasts can’t ferment and because of this, lagers yeasts will display a slightly higher limit of attenuation in worts that contain these sugars. When properly done, the final attenuation of the test is not affected by the yeast strain, yeast health, pitching rate, temperature and other fermentation parameters.
How to perform the fast ferment test?
To perform the test you need a sample of wort and yeast. The amount of wort should be enough to perform a hydrometer reading later (6 to 8 oz (120 to 200 ml)). The amount of yeast should result in a pitching rate for that sample that is well above the pitching you would use in a beer. Here is a quick and easy way to prepare the test when propagating yeast in a flask or large bottle (growler):
- decant the spent starter beer
- add fresh wort to resuspend the yeast
- pitch most of the yeast/wort mixture and leave a little in the flask or bottle
- add some more fresh wort to ensure there will be well enough for taking a hydrometer sample later.
If you can’t (or don’t want to) spare any yeast from the yeast that is pitched (pitching from a vial or pack of dry yeast), you can also use dry bread yeast instead. Bread yeast seems to behave like ale yeasts when it comes to types of sugars that are fermented. At least that’s what I found when using bread yeast for fast ferment tests that were done parallel to fast ferment tests of the same wort with lager yeast. ½ tsp to 240 ml (8 oz) of wort is plenty (For now I have to add a word of caution here. The few times I used both bread yeast and the yeast strain that was pitched for the fast ferment test, the bread yeast FFT showed an FG that was 0.2 – 0.5 plato higher than the one of the yeast that was actually used. I still have to run a few more experiments with various yeast strains to verify the claim that the FFT is largely insensitive to the yeast that is used for it.).
Cap with tin foil or airlock. I like to use an airlock as the absence of O2 will keep acetobacor bacteria down and the sample will not taste as sour later. It also makes sure that the sugar will be fermented. But the amount of aerob metabolism should be marginal unless you place the test on a stir plate.
Place the test at a warm place ( 20C / 70F and above, warmer for ale yeasts) and shake it occasionally to keep the yeast in suspension. You can also place it on a stir plate if you have one to spare for that. It should take 1 or 2 days for the fermentation to be over and I tend to give it another 2 days until I see that the sample lays completely flat and no CO2 escapes when I shake it. When dealing with high gravity worts, I also like to aerate it again after the fermentation seems over, just to kick start any stalled fermentation. But this is only a concern for high gravity worts (18+ Plato, 1.072+ SG).
Now you can measure the extract (gravity) of the beer. This will give you the limit of the final extract (final gravity) that you can expect from this wort. Depending on the yeast and fermentation conditions, the actual beer final extract (final gravity) will be a little higher. See Understanding Attenuation for details on that. Give it a taste as well. It generally tastes like bad home brew because of the higher fermentation temperatures.
How does this test help me with my brewing?
There are a number of benefits to knowing the limit of attenuation of the wort produced. So many in fact, that I now perform the test for every beer that I brew (I used to do the test only for lagers). It became as essential as taking a starting extract (starting gravity) reading or recording the mash temperature.
It will answer an important question when the final extract (final gravity) of the beer is not as expected: Is the problem mashing or fermentation? Based on the answer you can target your efforts of fixing the problem (see Understanding Attenuation for details)
It will also allow you to start lagering or cold conditioning earlier. Now you are not looking for the gravity readings to stabilize, you are looking for a targeted gravity to be reached. Note that your target FG is not the FG of the fast ferment test, but a little higher than that. How much higher depends on the yeast strain and style of beer.
Palmer [Palmer, 2006] was referring to brewing lagers as flying blind because you don't know when they are done fermenting. With a fast ferment test you will be able to determine the FG of your lager (or ale) before the actual batch has finished fermenting. This is especially helpful for all grain brewers because the final gravity of the beer can greatly be influenced by mashing.
Sometimes, the yeast will stall before reaching the targeted FG (based on the fast ferment test result). In this case you can accept it and fix it with the next batch (better aeration, other yeast strain, temperature) or try to kick start the fermentation by warming it up, rousing the yeast or pitching fresh yeast.
When cold conditioning before bottle conditioning, the process of cold conditioning can be started as soon as the extract (gravity) falls below a level that leaves less fermentable sugars in the beer than are needed for carbonation. Here is an example. I recently brewed a Weissbier with a wort that showed a final extract of 2.5 Plato (1.010 SG) in the fast ferment test. The actual fermentation was at 3.5 Plato (1.014 SG) after 5 days. Since I was in a rush to get this beer to cold conditioning, I felt comfortable moving it to the 10C (50F) fridge. Even though this stalled the fermentation and the FG won’t be getting closer to the targeted FG of 2.6 Plato (1.0105 SG), I know that I can account for the amount of fermentable sugars left in the beer when it comes to calculating the amount of sugar needed for bottle conditioning (see Accurately Calculating Sugar Additions for Carbonation for details on that). But when that level of precision is needed for the gravity readings, I suggest investing in a hydrometer for the range 0.980 – 1.020, which makes the final gravity readings more precise.
Sources
- [Narziss, 2005] Prof. Dr. agr. Ludwig Narziss, Prof. Dr.-Ing. habil. Werner Back, Technische Universitaet Muenchen (Fakultaet fuer Brauwesen, Weihenstephan), Abriss der Bierbrauerei. WILEY-VCH Verlags GmbH Weinheim Germany, 2005
- [Palmer, 2006] John J. Palmer, How to Brew, Brewers Publications, Boulder CO, 2006