My method for sour-worting a Berliner Weisse:
My BK has a 10gal volume, so I made a batch size of 9.5gal, preboil volume of 9.75gal.
Grain bill was:
7# pilsner malt
5# wheat malt
Mashed at 149F for ~90 min (until fully converted). Sparge to get a Preboil volume of 9.75gal. Make sure when you drain your tun, your BK is already on your burner. You don't want to move a kettle with almost 10gals of liquid!
At this point, I wrapped my heatwrap around my kettle and put the probe to my temp controller on the outside of my BK to get a pretty decent reading of the wort's temperature. When the temp dropped to 115F, I tossed in a pound of unmilled 2-row. The outside of the grain is covered in lacto bacteria, or so I'm told.
After tossing in the grain, I covered the entire surface of the liquid with Al foil, making sure I push out as much oxygen as possible. You want zero air at the surface of the wort, so try to get as much out as possible. Put the lid on my kettle, set the temp controller to maintain a temp of 110-120F, wrap the kettle in blankets, winter coats, whatever it takes to help insulate and maintain the temp.
I left it for 48 hours at this temperature. After the desired duration (the longer you wait, the more sour and tart it'll get) I unwrapped all the insulation, removed the foil, and started fishing out the grain. I used 1# this time. There was a TON of grain in the wort.... I will probably use just 0.5# next time. Try to get out as much as possible.
When you're finished fishing out the grain (there will be a lot at the bottom, fyi), bring your wort to a boil. I boiled for 15 minutes, killing all the bacteria that soured the wort. That's the best part about this method, NO CHANCE OF CONTAMINATION!
At the start of the boil I added just 5 IBU of Hersbrucker. At flameout, I turned on my IC and chilled it down. When it got to ~70, I racked 5.5gal to a carboy and pitched WLP029. I fermented at ~62-65.
My OG was 1.032 and FG was 1.001. The beer is super tart, fruity, slightly sweet and very dry. It's quite delicious!
For the other 4 gallons, I kept those in the boil kettle, covered the opening with cheesecloth and left out overnight. Then I racked it to a carboy. Within a few days, fermentation occurred. Now it's got a slight pellicle. I'm going to leave it until about May. It should be quite interesting!!!