I was troubled by this node, so I asked my friend Moira. She spent a large part of her final year at school investigating what happens when good milk turns bad.
She did this by placing vessels containing milk at various places around her house, leaving them for a while and seeing what changes occurred. The reaction of her parents to milk sitting in their home at 37 degrees celsius for a week was not initially one of delight, but they understood in the interests of science. Although one wonders if people used her airing cupboard for drying clothes ever again.
The souring of milk is due to the breakdown of proteins and the formation of lactic acid. Lactic acid, interestingly enough, was formerly used as a bleach, and also functions as a preservative, although once it has formed in your milk, you don't have anything worth preserving. A similar principle is involved in silage.
My friend found that if you leave it long enough, milk settles out into seven distinct layers, including a clear, almost colorless fluid, and the cottage-cheese-like curds to which this node title alludes. However, the formation of true cheese requires the addition of enzymes either produced by genetically-modified bacteria or extracted from cattle, plus considerable pressure and years of cheese-making experience.
She informs me that the final stage in the evolution of milk is mold. That is what happens to milk after cheese, what happens to cheese after cheese, and indeed what happens to people after cheese.
Incidentally, the project was a success, and Moira is now production manager at a small chocolate factory.
While I am grateful to Moira for her help, I acknowledge full responsibility for any errors in this piece, since I don't understand all this science stuff.