I was doing some random reading about mash enzymes and realized I had the wrong impression about which enzyme did what during conversion. I had my thoughts backwards about alpha- and beta- amylase. So I’m posting my new found knowledge here:

Beta-amylase– only cleaves starch chains from the ends resulting the disaccharide maltose (2 glucoses). Optimum temp is 140-148F
Alpha-amylase– preferentially cleaves the brach points of starch molecules leaving unbranched chains, can also cleave the chain ends to generate maltose. Optimum temp is 158F-ish.

When we mash at 154 we get a good mix of both beta- and alpha-amylase activity. That “cooperativity” leads to a very fast conversion. Next time you do a single 154F mash, get some iodine and start testing it every 5 minutes. Seeing as it only costs a few drops of wort, it’s not a big deal. From what I have read within 20 minutes or so you’ll be probably be done converting. (I may have to try this at some point and report back as an experiment) The longer mash time just leads to a drier and drier wort. (FWIW, I still do 60 minute mashes. Although I did my ESB with a 40 minute 154F rest and it seemed fine).

Remember too that alpha-amylase (158F optimum temp), can also cleave the ends not just the branches. So you can get a very fermentable wort by mashing at 158F. It just takes a lot longer time. If you have ever heard of people doing overnight mashes, that is what they are doing. Essentially degrading all the beta with a 158 mash in, then its just alpha slowly working on all the branched then unbranched starches (amylose).
I think that is how things like Mich Ultra is made, long high temp mashes.