Standing a day or two, the milk sours. But you can make him sour and turn into cheese, not in two days, and in two seconds. To this we must add the milk a little vinegar. The cheese will immediately stand out.
Cottage cheese is casein, a milk protein. He in milk dissolved like sugar in water. But it is necessary to add to milk acid to casein stood out, taking with him and fat.
But in milk none don't pour acid. Why did it still sours?
Blame the tiny microorganisms that always float in the air, " lactic acid bacteria. Once in the milk, they are taken for work - turn the milk sugar into lactic acid. But on acid milk and stvorazhivaetsya.
The milk is not sour, it is necessary to boil. From boiling the bacteria die.
It happens that milk stvorazhivaetsya during boiling. This is because the bacteria has already managed to work and to prepare acid.