I can't remember where I read it........maybe Palmer's.......but another reason for extended mash times are when mashing thin. What I read was......a thick mash concentrates the enzymes and allows for a more rapid and efficient conversion.......while a thin mash dillutes them, thus requiring more time for a complete conversion. It seems the author felt that mashing at 1.7 to 2.2 quarts per lb should mash up to 90 minutes for complete conversion.
I tried the thin mash process proposed on this board this past saturday, with a hybrid batch/fly sparge.......efficiency was excellent, and the quality of the wort was noticeably better in the last gallon of runnings than I have been used to.
My technique was: 1.7 quarts per lb. Mashed at 152 for 90 minutes (end temp was 146). After vorlauf, drained MLT (got about 2.5 gallons of wort). Fly sparged with 185 degree water, raised grain bed temp to about 158. No stirring at all. Fly sparged until I had about 1 inch of water above the grain bed, then shut it off. Waited 10 minutes, then drained. Collected another 2.5 or so. Repeated with water heated to 190, stopped with water level about level with grain bed. Waited 10 minutes, then drained.......ended up with about 7.2 gallons total wort. Efficiency was 82%, but most importantly, I think it's a process that I can easily reproduce each time without worrying about flow rates, or stirring, or messing with the grain bed, or repeating the vorlauf between water additions.
I am going to attempt to repeat this my next batch and see what happens.
Of course, the real test will be when my oatmeal stout is finished.....I can't wait to compare it to my last oatmeal stout which was traditionally fly sparged.