Only warm what you are going to use for a single feeding, then discard what isn't eaten. Just like you would for a human infant. Also, if you can, don't warm the milk directly in the microwave. Either zap some water to make it hot, then rest the bottle in until it comes up to temperature, or use the stovetop. It really doesn't take long on the stovetop. I know when I was bottle feeding, I'd come into the kitchen, fill a pan with water, and set it to heat (without the bottles), then fed and pottied my dogs. Once the dogs were food five minutes later, the water was quite warm. Only a couple minutes in the water, and the bottles of was at a proper feeding temperature, not too hot or cold.
Also, you can use strait up whole cows milk, it doesn't have to be a special mix. Goat kids thrive on whole cows milk if whole goats milk is not available. My bottle kids grew like weeds.
Buttermilk is a fermented dairy product, it has beneficial bacteria and is very acidic (more acidic than normal milk). It IS turning the remaining milk into cheese, the whey and solids are separating. The higher the acidity goes, the more caesin (milk protein) separates.
