That said, the beer being produced at that time was probably not quite like the fine Weissbiers one can buy today. Besides the carbonation level being lower (wooden kegs and taps can only hold back so much pressure), the grain was different. They did not use the same water-injection drum roasters that modern maltsters use today, but used wood-fired kilns. As a result, the grain would be somewhat more highly kilned, if somewhat uneven. Also, the grain itself was not as highly converted. Finally, Warner says that the ratio of wheat to barley malt was much lower than today's 2:1.
All that said, my current guess is that the modern decoction mashing technique can be fairly similar to what was being done at that time. Consider a double decoction. First off, if one can get the initial temperature right, the remainder of the process can be done reliably without a thermometer. And the starting point is 105 degrees F., which is about decent hot bath temperature. With a bit of practice, I've been able to get within +/- 5 degrees by feeling the temperature with my hand, so I have little doubt that this is how the brewmaster started the process. Second, since the malts were less well converted, the initial protein rest would be very helpful for removing the protein, and getting more efficient starch to sugar conversion.
All that said, for this batch we tried to use a close approximation to the right ingredients. This recipe is a modification of Warner's recipe for Munich Weisse [Warner, pp 107-108], but with a much different grain ratio. The double-decoction technique is right out of Warner.
One word of warning: this is a long brewing process. It took us about 7 hours all together, starting to toast and grind grain at 5:30 PM, and finishing washing the dishes at 12:30 AM. This is better done on a week-end day.
A couple of days ahead, start the yeast package. Then put it in a starter made from about 1/2 cup of dried malt extract and a couple of compressed hop pellets (the little ones), boiled in a quart of water, and then cooled.
First, toast the 3/4 lb of Pale (Barley) malt for about half an hour at 350 F. Cool this, and grind all the grain. Start about 2 gallons of water boiling.
Once the grain is ready, mix up 8 quarts of water at 110 F. in the mash-lauter tun. (Start with about 5 quarts boiling, and 2 quarts cold; then adjust while adding the remaining 1 1/2 quart.) pour in the grain in while stirring well. It should be at 105 F. Put on the lid and wait 10 mins.
Add 2 quarts of boiling, and 1 of cold, and stir well. It should be at 120 F. Close and wait 25 mins. Start at least 4 more gallons of water boiling.
First decoction: Remove 40% of the total volume, mostly solids, from the mash tun. In this case, that should be about 5 1/2 quarts. Use a small pot to scoop from the bottom, so that you get lots of grain, with just enough liquid to barely cover it. Scoop it into a large (2+ gal.) pot, and put this on the stove. Cover the mash tun. Slowly raise the temperature of the pot; you are aiming to get it to 160 F. in 15 mins. time (no faster), stirring frequently. Hold the temp at 160 for 10 mins. (On my stove, I turn the gas down so there is just barely a flame still going.) Then over 15 mins. time slowly raise the pot to a boil. Simmer for 20 mins.
Add the first decoction back into the mash tun. Also add 1 1/2 quart of additional boiling water. Stir well. The whole mess should be at 150 F.
Second decoction: Remove about 30%, or 4 quarts, of mostly solids again. Bring these up to 160 (fairly quickly), and hold for 15 mins. Then bring up to a boil over 10 mins. time, and simmer for 15 mins.
Add the second decoction back, along with 2-3 quarts of boiling water, and stir well. It should now be at about 160 F. Make sure you have enough sparge water.
Start recirculating the liquor in the mash-lauter tun (drain half a gallon out slowly, pour carefully back into the top). It will be very cloudy at first. When it starts running much more clearly, start collecting into the boiling kettle, and pouring boiling sparge water in the top.
Collect about 6 1/2 gallons of liquor, putting in about 3 gallons of sparge water in the process. The more slowly you do this, the higher your extraction rate will be. It took us about 30 mins.
Start the wort boiling. Be very careful about the hot-break; it will try to boil right out of the kettle. This is a much bigger hot break than we get with all-barley batches.
Add half (1/2 oz) of the hops after the hot break has happened, and the wort is happily boiling away. Add rest of the hops 10 mins. before stopping the boil.
Stop the boil, cool, transfer into a fermenter, aerate well, and pitch in the yeast.
For this batch, we got a starting gravity of 1.038, and a finishing gravity of 1.006, for 4 1/4 % alcohol by volume.
Is it worth all the effort? Yes - this is a great beer. Not as wheaty as a modern weisse, but very good and very drinkable (particularly at Pennsic). I am not yet convinced that you can get even close to the right "double decoction" taste from extracts. The modern taste is especially different because modern grain has not been malted in the same way that grain was back in the late 15th C. (That is not to say that you can't get good taste from extracts, it's just not the right taste.)
Tofi Kerthjalfadsson, guildmaster, BMDL Brewers' Guild.