Historically, scalding milk accomplished at least 2 purposes. One is that it killed any bacteria in the milk. (Pasteurization took care of that problem decades ago for most bakers.) The other is that it neutralized the protease enzymes in the milk, which can inhibit yeast growth. The experts are somewhat split on whether pasteurization disables the protease enzymes. I suspect ultra-pasteurization (which is what most dairies use these days, because it's faster) might do a better job neutralizing the protease enzymes than the old pasteurization process did, because it uses a somewhat higher temperature for a shorter time, but I haven't looked to see if there are any scientific tests of that hypothesis in the journals.
I think there's a third reason, though. Scalded milk smells and tastes different than milk straight out of the carton, and I think that has an impact on the bread flavor and possibly on texture.
I always let scalded milk cool back down to the point where it's no more than warm (say, 110 degrees) before using it to make bread. Yeast dies at 138 degrees, so you certainly don't want to pour HOT milk in with yeast.
The tangzhong method (pouring boiling hot water/milk on the flour or cooking it on the stove to gelatinize it) is another option for bread recipes that call for milk, but that might be a separate thread.
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