I am making sourdough, and as per my recipe, I have left a combination of flour and milk out to try to attract surrounding yeast and bacteria. The recipe states that the mixture should bubble and smell sour, but if it starts to smell bad or turn pink, it should be thrown out. Well it's been almost two days now, and it has bubbled, but I'm not sure about the smell. It smells like expired milk (which is no surprise, given that the milk was a day away from expiration when I used it) which I would definitely call a sour smell. Except, I would also call it a BAD smell, which is where my confusion lies. Is this expired milk smell bad within the definition outlined in the recipe, or is this the correct sour smell? The recipe also states that if it is turning pink, it should be thrown out. I may be imagining things, but the starter has an ever so subtle pinkish hue; it is still basically white, but I see just a hint of pink. So what do I do? Is my sourdough starter a bust, or is it doing exactly what it should be doing?