The souring and curdling of milk is caused by bacteria.
slows growth of bacteria
Lactobacillus a bacillus bacteria used in curdling of milk.
Rennin deactivate the casein micelles in milk, causing a distinct separation of curd protein (solid) and whey protein (liquid).
Milk curdling is a chemical change.
Curdling.
Bacteria in the milk consume the lactose sugar and produce lactic acid as a byproduct. The increase in acidity caused by the lactic acid lowers the pH of the milk, leading to the sour taste and curdling of the milk proteins.
A sentence could be "I knew the milk was off because it was curdling"
No. Once milk has spoiled there is nothing you can do to fix it. Milk spoilage causes changes in proteins that cause curdling and the milk will likely be sour. Bacteria might be killed off with boiling, but the damage will already be done.
In your stomach.
If your milk is clumpy is it sour probably by setting out to long. This clumping is called curdling, and is something that happens naturally if milk has been left out in room temperature for too long or if it reacts to some enzyme or bacteria that causes it to curdle.
Because it can help with the purification, in the process of curdling milk etc.. It also helps you to convert Milk into other products, such as cheese and yoghurt. Enjoy! Andrew