@moniker7: Bacteria inoculation (here, via buttermilk containing live and active cultures) and rennet is used to make most kinds of cheeses. Curdling directly with acidic liquids produces a similar whey, but there are differences, and the curds can only be used for some kinds of fresh cheeses.
@zach: The bacteria from cultured buttermilk are meant form a civilization; this acidifies the milk and allows it to curdle. Good news is that it’s practically impossible for harmful strains of bacteria to compete in this environment, so your milk is automatically protected from spoiling.
There’s a lot of speculation here, so I thought I’d link a reference for the chemical makeup of dried whey (spoiler alert: it’s over 70% sugar):
The micronutrient content of dried sweet (bacteria) and acid whey is given on nutritiondata.self.com, but I imagine these numbers vary wildly depending on the pH, temperature, etc. when curdling the milk.
Whey protein powder has undergone processing to separate it from the lactose, and heat will denature the proteins. I don’t recommend using cooked whey as a protein source because is it mostly caramelized/burned sugar. Bad luck!