Why does milk sour in the absence of refrigeration?
The bacteria which lay dormant in a chilled environment (4 - 1 degree c) reproduce at a high rate when the milks ambient temperature increases. This bacteria makes the milk sour. In recent years improved methods of destroying these trace bacteria have been undertaken which has allowed milk to stay fresher longer under correct refrigeration
Milk that requires refrigeration still has bacteria in it. The pasteurization process does not sterilize the product, but is meant to destroy targeted pathogens. Some bacteria remain and that number depends upon the original bacterial load of the raw milk. Those remaining bacteria still grow under refrigeration and will eventually sour the milk.
There is no proof that Cleopatra ever bathed in milk. Perhaps milk was used in a bath oil or mixed in a skin lotion. You have to remember that Egypt is a hot country and in Cleo's day there was no refrigeration. If she bathed in milk as some suggest, it would be stinky sour milk hardly a bath for a queen.