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Decoction calculation

Hey guys. Does anyone have a program do calculate how big a decoction should be?

Im mashing 12 lbs of grain with 21qts of water (1.75qt/lb) right now at 145F for 30 minutes. I want to pull a thick decoction, boil it and return it to the mash to get up to 165F.

I have no idea how big the decoction should be though.

Does anyone have a program or spreadsheet that can calculate it and get back to me?

Thanks!

I tend to brew german lagers 6-9 times a year and try to decoct most of them.
I usually pull 40% of the mash, using a colander to get mostly grain.

My cooler has qt measurements on the inside that help with this.

Brewing TV’s Michael Dawson (who learned from NB founder Chris Farley) says to put 1qt of the thick mash per pound of grist. So for you you’d pull 12qts and boil for 10-30 minutes, then add back as much as you need to get to 165F. Let whatever is left cool to about that, then add back to the mash.

Here’s the awesome episode from which I learned this: http://www.youtube.com/watch?v=IIQPQmELWPo

Having said that, you may not need to pull quite that much of the mash since the temperature rise is smaller. I think he says to pull about half that to get from sac rest to mash out.

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