It is these bacteria that will cause milk to sour. Bacteria is also used to acidify the milk. The reason to acidify the milk is to lower the pH of the milk. The amount of acid has a direct affect on the coagulation, curd strength among other affects on the cheese process. Too much acid will cause the cheese to be crumbly, low acid will cause the cheese to have a pasty consistency.
Bacteria feed on the lactose in the milk and produce lactic acid.
As time goes, the lactic acid increases thereby achieving a lower pH in the milk. Lemon juice or vinegar can also be used instead of the bacteria. Dehydration is the removal of the water. As the pH is lowered, the structural nature of the proteins also change. The casein proteins that form the curds entraps water and fat.