First, I wanted to see if pressure cooking the decoc was feasible. I had also wanted to address an issue that George De Piro had brought up recently regarding whether cooking a fraction with more simple sugars was better than a more dextrinous fraction. I disqualified my simpler sugar part of this experiment though, because I don't feel I'd gotten as good a conversion as possible. My conversion temperature kept falling from 149F (65C) to 140F (65C). I brought it back to temp and repeated this twice more before quitting it in disgust (femto-brewing, Feh!). I don't think as much starch was made available for conversion in this portion as with the dextrinous sample since starch gelatinizes at 148F (65C) and it kept falling below this. The dextrinous portion was very well converted, however. Here's what I did.
1) Measured 8 oz. crushed (250g) Belgian Pilsner into a two 1-quart (1L) mason jars and added 21oz (620ml) water to each to simulate a moderate 1.33:1 (qt/lb) mash. This is thinner than any decoc fraction I'd normally boil, but I wanted to develop a good conversion before boiling to also test the dextrinous vs simple sugar question. I also expect that this would cook (brown) poorer than a stiffer decoc fraction so it could stand as a torture case. In a grain bill with poor diastatic power, I would pull a decoc this thin so I could get as much work out of the enzymes during the mini-rest as possible, before they were denatured in the boil.
2) Doughed in at 105F (40C) and in a water bath I brought both jars' contents to 135F (57C) and rested for twenty minutes to allow the enzyme systems to go into solution, the protein benefits of this rest were irrelevent for this test, though. pH 5.2, Temps were measured in the mash, not the waterbath.
3) Heated the waterbath with both jars to saccarification temperatures. I removed the simple-sugar jar at 149F (65C) and put it inside an insulator (now see why it kept falling?) took the remaining jar to 156F (69C) and rested for thirty minutes. It was very sweet. The other, after two more tries was sweet but still had some starchy flavor remaining.
4) Covered the jars with aluminum foil to prevent spluttering grains from clogging the pressure relief valve of my pressure canner, and placed them inside the canner along with about 1/2" of water to produce steam in the vessel. I cranked up the heat until the canner vented steam for about five minutes (this evacuates the air inside) and then brought the canner to 250F (121C) at 15 lbs (6.8K) and reduced the flame to a tiny, temp-maintenace size for twenty minutes. Reduced pressure and tasted.
Both jars were the same degree of dark tan. Both jars were tasty but still had a slight, raw starch flavor. The simple sugar jar more so, most likely because it still had more starch remaining after sugar conversion. I wouldn't sweat this too much since in practice the starches would be given back to a viable enzyme pool on returning the decoc to the mash. Both produced a whiff of DMS on lifting their foil lids but that dissipated quickly and didn't show in their taste after stirring up.
I specifically tasted for tannin flavors and found none. This was a particulary crummy grind too. The brewstore (not my usual) used a coffee grinder and I had to put it through at "Perc" and then at "Drip" grind before getting a reasonable crush. The husks were torn to shreds by that time. If any batch would yield husk flavors this one had the opportunity.
I returned them for another twenty minutes at the same temp and pressure, and this produced a truly great decoc. They were now both darker still, between the color of corrugated cardboard and brown wood. Both tasted fully cooked, with the dextrinous jar tasting much better than the simpler sugar jar, but remember that that comparison may not be completely valid. However it does show that a dextrinous fraction (my practice) produces a really great result when cooked this way. I subscribe to the school of thinking that says decocs produce "better" malt flavor not "more" malt flavor.
To use this in brewing I would pull the stiffest practical fraction of grain into my three-gallon cheapo canning pot and put it, covered, into the canner and cook for at least forty minutes at the above temp and pressure. After cooling it with an immersion chiller back to 135F (57C), the temperature that the rest of my mash is resting at, I would add it back and take the whole mash to whatever sugar conversion temp is appropriate for the beer.
The double-boiler, pot within the canner, approach is pretty important to keep from scorching the grains. Their pot rests on a stand a little above the bottom of the canner, and effectively doesn't go above 250F (121C) but if you were to put them directly into a pressure cooker the direct firing would certainly burn them on the bottom. It's the nature of pressure cooking that food doesn't scorch if not in contact with the heating surface. The whole volume of liquid water and vapor acts like a cooking surface at 250F so it cooks evenly. There's also no risk of hotside aeration while cooking since there's no air in the vessel after steam purging. I'm sold.