Nice one - thanks for the tip Manticle.
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 (5-10x). 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. I generally find when performing the fast ferment test with the pitched yeast and bread yeast in parallel, that bread yeast shows about the same final extract as ale yeasts and a final extract that is 0.2 – 0.3 Plato higher than a lager yeast. ½ tsp dry bread yeast to 240 ml (8 oz) of wort is plenty. Since slight differences between bread yeast and the actually used yeast seem to exist, it is best to perform the fast ferment test with the yeast that was pitched. Bread yeast however will also show you the final extract as well, but possibly with an error of up to 0.5 Plato or 2 specific gravity points.
Cap the bottle or flask with tin foil or airlock. An airlock is beneficial 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 even on a stir plate the amount of aerobic metabolism should be marginal due to the Crabtree effect. Yeast will not metabolize sugars aerobically if their concentration is high enough (> 0.5-1%). I rarely use an airlock these days out of convenience.
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 may 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 lies completely flat and no CO2 escapes when I shake it. When dealing with high gravity worts (18+ Plato, 1.072+ SG), I may also add some air/O2 to the headspace and dissolve it into the sample through shaking just to kick start any stalled fermentation.
Now you can measure the extract (gravity) of the sample beer. This will give you the lower limit of the final extract (final gravity) that you can expect from this wort. Depending on yeast characteristics and fermentation conditions, the actual beer final extract (final gravity) may be a higher than that. 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.