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Rousing Yeast (aka Swirling)


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#1 MtnBrewer

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Posted 05 April 2009 - 12:28 PM

Rousing Yeast (aka Swirling) FAQContributed by AeneasMany times people report that their fermentation has completed but their OG is still too high. Often times rousing the yeast is recommended to help get the last 6–15 points of gravity out of the fermentation. This FAQ will tell you how and why.Q: My beer stopped fermenting, the krausen has fallen, but my OG is still too high. What can I do?A: This is a prime example of where swirling can help you get your beer closer to its target FG. What has happened is that your yeast have flocculated and dropped to the bottom of the fermenter, but left some sugars uneaten. The yeast are not going to eat anything at the bottom of the fermenter, so it is essential to get the yeast back in suspension to finish eating the sugar; then you can reach your FG.Q: OK, how do I get the yeast back into suspension?A: In order to resuspend your yeast, you need to create some motion in your fermenter, either by gently rocking it back and forth or by picking it up and swirling it. For carboys and buckets, the best way to do this is to grasp near the top, either the bucket handle or the neck of the carboy with one hand and the bottom of the vessel with the other. With a firm grasp of the vessel, start moving the vessel in rapid small counter-clockwise circles, creating a mild vortex effect in the fermenter. You will often notice airlock activity immediately. This is due to the evolution (dissolved gas escaping) of CO2 that is in solution, not active fermentation by the yeast. Continue for 10-15 seconds. This motion will resuspend the yeast into the beer, encouraging them to eat some more sugar.Q: How often should I rouse my yeast?A: In difficult fermentations, yeast should be roused 2-3 times a day until the FG is reached. I recommend the "wake up, return from work and before bed" routine. Continue as necessary until the desired FG is reached.Q: How will swirling effect my beer?A: Hopefully your gravity will drop to the desired FG. A word of caution though -- you need to be careful not to make many waves in the surface of the beer when swirling as this is a chance to oxidize your beer. This may be minor factor as there is not much oxygen left after active fermentation, but it can happen and damage your beer. Also, too much mechanical stress can damage yeast cells thereby resulting in autolyzed flavors entering the beer. This is why the swirling method is preferred over a harsher method of resuspension.Q: I swirled for days, but my gravity is still too high. What's going on?A: There are a couple problems that could cause this. You may have mashed too warm causing there to be a lot of unfermentable sugars in the wort, or your extract was very unfermentable. You may not have provided enough nutrients, including oxygen, in your wort for the yeast to grow properly or your yeast culture was poor. Alternatively, you may have reached the alcohol tolerance limit of your yeast, and no amount of swirling is going to help.


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