Hi, yes I read about this recently in school, but cant remember the details lol. ANyways it goes somthing like this, there is a very small amt of bacteria in urine naturally, and bacteria always needs somthing to feed off of, so there is another component in your urine which the bacteria will use as food, causing the bacteria to multiply-I think of it like this, you have some food, its perectly healthful and beneficial food, but if you let it sit out it will go bad! I just assumed the expiration for urine used during urotherapy is much quicker than food! lol