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Yeast cake and trub question

iamjoeyjo

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If I fill my fermenter with 15L of beer, later on when racking time comes tge yeast cake can take up a litre or two of bottom space.

My question is, does this raise the beer up past the 15L mark so I can still harvest that beer or is the yeast cake now counted as part of that 15L I brewed.  Will I end up with a more concentrated beer in that case?  What is the best way to aboid the trub, filter it before it hits the fermenter?

 
Solids displace liquids. However, in this case there is a surprising amount of detail that goes into a complete and accurate answer to that question.

The trub is loose enough that there is a fair amount of liquid in it. Using finings like gelatin help coagulate yeast more densely than you might get without it. This is dependent on the yeast strain and pH of the beer. 

You can never expect to get a full volume of beer back after fermentation. The simplified answer is that part of your loss is CO2 from fermentation, yeast cells taking up water plus how water, sugar and alcohol molecules fit together more compactly than just water and sugar.

The third issue is temperature since water is at its densest at 4C and you're likely measuring it at 20C. To be fair, the difference is only 0.3% so maybe not measurable at 15 liters, but take that to 10+ hectoliters and it does start to show.
 
The best way  of dealing with trub is to remove it before it gets to the fermentor. Irish moss for the last 15 minutes of boil and a bazooka screen in your boil kettle and careful syphoning or pumping out will remove trub and keep it from getting into fermentation. Also making clear mash run-off by returning runnings to the mash until they are clear helps keep solids to a minimum.
With a clean fermentation you can easily re-use the yeastcake, without having to rinse it, and this helps increase yields by adding a quart or so to the new batch at pitching time, the same way adding a quart of starter does. 



 
I plan my recipes so that I end up with 5 gallons in the keg.  This generally means that I need to 5.5-- 6.5 gallons from the kettle for fermentation.  There are a lot of factors that contribute to the start amount.  You will lose wort at the kettle, boil trub & shrinkage.  In the fermenter how well the yeast attenuates, if you dry hop whole or pellet hops.  Do you transfer to secondary for any reason.  These factor all add up 
 
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