I've mix my post-workout shakes the night before, and leave them in the fridge overnight (about 9 hours). Then I take them with me to the gym in the morning, add my creatine, shake shake shake, and drink them down.

I've noticed that when I take them out of the fridge, there's about an inch or two of frothy foamy goo on top. It goes away after the mixing. It got me wondering though... the foam is growing in there overnight. What is it? Is it bad, or can I just carry on the way I have been?