What makes milk sour is bacteria. Bacteria multiply by dividing and turn sour when they reach a certain level.
As the bacteria rate in our milk is 7000 to 10.000 / 1lt and as long as the cold chain (4-8 degrees) is not broken, it has a durability of 3 to 4 days. Since hygiene standards cannot be met in street milk, 1.000.000 to 10.000.000 / 1lt. bacteria formation occurs and durability is lower.