Forced Fermentation Test, for accurate attenuation
I have noticed that one of the most frequently asked questions by new and old brewers is, is my beer done fermenting? Well, there are a few ways to answer that; varying from pitching more yeast to be sure, comparing the expected attenuation from the yeast manufacturer with the actual attenuation, or moving the beer to a warmer spot. I have never been a huge fan of any of these options because of loss of flavor from overpitching or the creation of esters from warming the whole batch.
The fast or forced fermentation test is a way to circumvent that process to analytically determine what you final gravity should be. The way that I like to go about this is to create a starter that is slightly larger than is needed, and to save about 8oz or so of unfermented wort. I then take a hydrometer reading (or brix), aerate, and then use the extra yeast to overpitch this small batch, and allow it to ferment in a slightly warmer area. You can also aerate this small batch throughout its fermentation because there is no worry about oxidation. After a few days the fermentation will be done and you can take a final gravity reading. This final gravity reading is what you would expect out of your larger batch based the yeast strain, particular batch, and optimal conditions for your yeast.
As a side note, you can also use a variation of this after fermentation has seemingly ended, but you are unsure if it is fully attenuated. All you need to do is take an aseptic sample from you wort, large enough for a hydrometer reading, and place it into a santized beer bottle with an airlock. I then place that sample (which was likely aerated from the transfer) in a warmer area and then check its reading about two days later. By doing this, you can tell whether your brew is finished fermenting.
P.S. This method is also useful in finding the expected attenuation of harvested yeast strains.
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The blogger states that you can continuously aerate the forced fermentation throughout the fermentation because there is no worry about oxidation. If you continuously add oxygen to the forced fermentation, then by definition this is no longer a fermentation since fermentation only takes place in an anaerobic environment. In the presence of oxygen yeast will switch over to respiration metabolism of sugars. I wonder if the blogger could comment on the potential differences in final gravity of the forced fermentation wort when yeast are either respiring or fermenting. Instead would continual agitation of the yeast under anaerobic conditions be a more accurate way to measure attenuation via forced fermentation?
Comment by 1n1m3g — July 31, 2009 @ 6:22 am
What is the primary consituent of your mix that’s indicated or presumed by measuring it’s specific gravity with a hydrometer? Is it co2? alcohol? something else?
If you put 1 tiny crystal of sugar or salt into a glass of pure water, it changes the specific gravity (SG) of the resulting mix. Fact is, if you introduce ANY impurity into the water (kool aid flavoring, drop of lime juice, shot of vodka, dissolved hydrogen, helium, nitrogen, co2, ethylene glycol, sulfuric acid, spit, etc). All of these impurities affect the weight of 1 cubic centimeter of the resulting mix. So, in a batch of homebrew you are checking the SG on using a hydrometer, is there a single one of the many ingredients contributing the greatest effect to the overall density of the mix?
I’m guessing it’s co2 – but I don’t know. Thought one of you might.
Comment by Mike Spike — December 18, 2009 @ 9:57 am
Continuously adding oxygen is exactly what you don’t want to do. Yeast needs oxygen during its early growth stage but after that stage, adding oxygen only adds an oxydized taste to the beer.
Comment by Tom Hargrave — March 29, 2010 @ 5:47 pm
I just previously made my first batch.i was a little drunk when the process was over, so i never checked the original gravity..how do i know when my beer is ready to be bottled?
Comment by presto — July 29, 2010 @ 12:36 pm