I do all my braising in a heavy duty restaurant style pot. I braise everything at 285 degrees for long periods of time in the oven. Durning the process of prepping the roast I cut/trim off all the excess fat. My cows are pasture raised corn finished with a lot of marbling. I think what your seeing is the braising liquid emulsifying with some of the fat. When you take out the roast you will see a separation of the liquids. When a stock is being produced in a correct manner it's call degreasing. Skimming the top of the stock to make a more clear and perfect tasting liquid. I don't skim the fat off the top of my braising liquid until it's finished cooking. I then skim off as much as I can.......