We grew some barley at the farm associated with the college where I work and after having it malted we found out it is quite high in beta glucans. The maltster recommended a beta glucan rest at 113F and I'm planning to do one for 20 minutes. For the beer I'm brewing, a robust porter, I'd normally do a single sacc rest at 152F for 60 minutes. Should I simply decrease that to 40 minutes for a total of 60 minutes make other changes to account for the fact that alpha amylase will have been working, albeit at a much slower rate, during the initial rest plus the sacc rest and avoid too thin a beer?
The answer is a little complicated, and my take on things is a little bit different. If you're planning to do a beta glucan rest at 113F you'll be well below the nominal temperature where beta amylase becomes active ~130°F. Beta amylase activity peaks around 146°F. That will also be below the range where Alpha amylase becomes active. Chances are that 113F beta glucan rest temperature will not result in gelatinization sufficient to allow beta and alpha amylase enzymes to convert the starches to saccharins. So, maybe.
Now, with all that said, 153F is the perfect compromise temperature for a single temp mash since it maximizes in between both enzymes' activity ranges. That of course presumes that sufficient time at a high enough temperature has allowed for adequate gelatinization of the grains.
So really what you're asking is whether 40 minutes is long enough for conversation to occur.
The answer again is, maybe. Generally if you mash in at say 153F, the temperature will ensure gelatinization and beta and alpha amylase conversion after 60 minutes. Some data suggest that (mostly) complete conversion happens in as little as 20 minutes. So if you rest at 113F for beta glucan and then rest for 40 minutes at 153F, it "might" work, even "likely " work.
But the real question is, why not just leave the mash at 153F for 60 minutes which would almost guarantee conversion of the starches. If the maltster recommended a beta glucan rest, it's probably a reasonably good idea, since he knows his grain better than me. But conversion is not going to occur at 113F.
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