The curdling of milk by lactic acid bacteria. As the bacteria ferment lactose and produce lactic acid, the increasingly acid conditions cause the normal bundled micelles of casein proteins (left) to fall apart into separate casein molecules, and then rebond to each other (right). This general rebonding forms a continuous meshwork of protein molecules that traps the liquid and fat globules in small pockets, and turns the fluid milk into a fragile solid