Thanks for the post, so educating. Although the milk can serve as habitat for bacteria( esp. Lactobacillus), after opening the milk, microbes are reintroduced to the milk and they consume the lactose and produce organic byproducts like lactic acid. Acidic things are sour- acetic acid (vinegar), citric acid (citrus) etc. So the sourness is caused by the decreasing pH of the milk. The acidity also denatures the proteins present. That’s what’s responsible for the lumpy curds present in milk that’s been neglected in the back of the fridge for weeks...