Let me extend the excellent info and discussion from
@dmtaylor and
@Miraculix with some recent personal experience and cogitating.
Here's the interesting thing for me: I've been wondering for a while about doughing in at strike water temps that are from 162 to 168, typical values for anyone doing either BIAB or a traditional mash tun system. In my system, I'm underletting the grain, and given the grain bill and full infusino, been doing that about 162 or so. That's a fairly high temp for Beta Amylase, and as it takes about 7 minutes for the complete underlet,
that means the grain at the bottom of the mash tun is being continually bathed in 162-degree water.
That can't be doing anything for the Beta Amylase in that layer of grain. So I've been cogitating on that--am I destroying a lot of the enzymes I need by the time I get the temp down to, say, 152 for the remainder of the mash?
It dawned on me--if I'm doing a step-mash, the grain I'm underletting never sees temps higher than 149--and in the early stages of the step mash, they're lower than that. Further, the starch in the grain is going through a range of temps which (given Miraculix' comments above) should enhance gelatization, and ultimately, conversion.
But if I'm doughing in using 162-degree strike water, or, as is the case with limited strike water in anticipation of using sparge water, doughing in at 168 degrees....I have to be denaturing some, or much, of the Beta Amylase.
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I've recently been diddling with step mashing with interesting results. My son had done some step mashing with a kolsch, starting at 125 and then going up to 149 degrees. I decided I'd play with it too, as i have a RIMS system that allows for increasing the mash temps. My first attempt ended up with the mash a little warm, at 132 degrees; I was pumping 139-degree water into the mash. Don't you just love empirical knowledge?
But it turned out fine.
I've done two Kolsch's doing the 132 to 149 step mash. I hold at 132 for 10 minutes, then it rises to 149 over the next 15 minutes. I'm reading temps at the output side of the RIMS tube, so temps are really not rising beyond 149.
Both of those Kolsch's had the lowest final gravity I've ever seen in a beer, the one using WLP-029 at 1.007, and the one using Wyeast 2565 at 1.006. They were/are delicious, still with terrific flavor (I do a weird recipe, so it's a "flavorful" Kolsch). Though I prefer the WLP-029.
Clearly the Beta Amylase is chopping down those starch chains. I don't know what the Alpha Amylase is doing except giving the Beta Amylase more starch molecule ends to chop sugars from.
I just brewed my dark lager using the same type of step mash, though I started at 136 and then ramped to 149. It's done fermenting, and the Tilt-indicated gravity is 1.010. I use WLP-940 for this, and I've never been below 1.013 for final gravity. I was looking for a bit drier, crisper beer, and I'm hopeful that's exactly what I'm going to get.