My recent article on reiterated mashing received some interesting comments from readers. Several commented that adapting the idea to a brew-in-a-bag (BIAB) approach might work. I’m not set up to do BIAB brews, so I haven’t tried it. But, I suspect it would probably basically work, although I would wonder if the extract efficiency would be high enough to be worthwhile. Hopefully someone will try it and take good notes. At minimum, you should be able to make a BIAB beer at almost double the maximum specific gravity as is normally obtainable (for the same length boil). Besides the grain bill, the amount of water in the kettle before and after each mash, the specific gravity after the first mash and the progress of specific gravity increase during the second mash would all be interesting.
Step Down Mash
Eric Branchaud mentioned that, with a BIAB approach, you could do the reiterated mash as a “step down” mash — the first mash could be at 162 °F (72 °C), favoring alpha amylase. (At the high conversion temperature, this would also convert quickly.) When the second bag of grains is dunked, it would lower the temperature, maybe down to optimal beta amylase range. This combination — if the second mash was rested for enough time — would make for a very fermentable wort, which would be exactly what you would want with this big beer. (You could try this idea following the basic outline I presented.) I thought this was an interesting idea and hope somebody tries it.
Two Brews — For High Gravity Plus High Extract Efficiency
To me, the most intriguing comment to me came from “jeffjm.” He suggested collecting the first wort only (no sparging) from both grain beds for the main beer, but then sparging both grain beds and combining the worts to make a second beer — brewing the highest gravity beer possible using the technique, but also getting decent extract efficiency because the sugars left in the grain beds are rinsed out and used for a second, moderate-gravity beer. (You could even add some specialty grains to the grain bed(s) before sparging to recover the second wort.)
This would make for a very busy brew day, but two or three homebrewers, using two homebrew rigs could pull this off. Plus, you would get two beers out of the deal and have a hell of a story to tell your homebrew club buddies. I definitely want to try this sometime this year.