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I do BIAB - 20l batches in a 50 litre pot. So I start with just over 30l of water.

Can I do a double batch of grain with about 40l of water. When finished split the wort equally in to 2 cubes and top up with water.

Would the reduction in water to grain reduce the efficency out of the grain??

Just woudl be nice to mash once and get two brews out of the process?


firmitas, utilitas, venustas
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Yep double batch BIAB can be done in a 50L pot. I've done maybe a dozen double batch BIABs. It's best to be done by adding top up water to the fermenter.

You do hit a bit of a ceiling as to how big you can make your beers for a double batch. I found that as the grain bill become larger (say over 9kg) efficiency drops off pretty sharpish once you have a grain bill over 9kg. At 70% you can still make 2 kegs of 1050ish beer with that so it's not too bad.

For double BIABs, to get a reasonable efficiency you'll need to do a bit of sparging. I used an esky. Anything that's big enough to take your grain bag and enough water for the grain to rinse in is fine.

Here's what i used to do;
- Add 50L to the keggle and bring to 85C,
- Drain off 20L into my 44L esky, (eskys are good cause you need to hold the water at that temp till the mash is finished)
- Top up/adjust the keggle water to hit mash in temps, say 70C. (bit of cold water will get it right)
- Add grain then mash for 60 mins
- direct fire to hit mash out temp of 76C while stirring and sit for 10 mins
- I then pull the bag out (9kg is about as much as I can pull out of the keggle without too much hassle) dump it into the esky full of now 80C water for a dunk sparge.
- Give it a stir, leave for a bout 5-10 mins.
- Suspend the bag over the esky (by hand is ok... never found a skyhook very useful)
- Squeeze it till it pretty much stops dripping (no need to go overboard, just to stop dripping)
- dump the bag wherever
- Drain the contents of esky back into the keggle to hit approx 42L. It's touch and go for a little while, at this volume, just watch for boil overs
- then do your hop additions as per usual

If you're over your preboil volume, just boil off till you hit your desired mark.
Pretty easy really. The extra bit of faffing about is worth it, when you get to fill 2 kegs.

Employing this method I easily extract 34L to fill the 2 ex Fresh Wort Kit cubes (any cubes are fine as long as you squeeze the air out) This leaves behind about 2-3 litres of trub. I also calculate a top-up of 4 litres each to get 21L into the fermenter, gives me a pretty much spot on 19L into the keg, after polyclar and filtering.

Good luck with it... nothing like 2 kegs from on brewday

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