"I have found that if you cook the brats on the grill first, then put them into the beer they don't shrink as much. Here's how I do it: I brown the brats lightly on both sides, then I place them in 'warm' beer for a few minutes. Repeat several times until brats are cooked thoroughly. Now you can put the brats into 'hot' beer to hold for individual serving. I catered several parties where I cooked brats for up to 100 people and this always was a hit. When I said brown the brats lightly, I meant about 20 seconds per side on hot coals then into a 180 degree beer bath. Any hotter and they will shrink. I continue alternating the grill and beer bath until they are fully cooked. It's a lot of work but my brats weigh almost as much cooked as when they went on the grill. Another thing you might try is adding chopped onion to your beer bath."
" Cook your brats by lightly browning over medium coals. This is an important step. You need to “Sear” the sausage first being sure not to allow the heat to split the casings. Have a beer bath on the side at a temp of 180 degrees. Alternate the brats from the coals to the beer bath. This will take about 10 to 12 repetitions starting with about 25 to 30 seconds per side for the first grilling and then increasing in grill time as the casings on your sausage get accustomed to the heat. This is the most important part of cooking sausage. Unless you are steaming or boiling sausage, you need to cook and hydrate, without allowing the casings to split, over and over again until done."