Subject: Why doesn't yogurt reproduce like other bacteria?

Hi! My name is Stephanie and I'm a sophomore zoology major at Iowa State 
University. In my molecular biology class we learned that bacteria (as well as 
other prokaryotes) reproduce at a rate of about 1 cycle per 20 minutes. That's 
a lot of bacteria that could be reproduced in a very short amount of time. It 
then crossed my mind that yogurt is composed of bacteria. Why is it that a cup 
of yogurt doesn't turn into 100 cups of yogurt after a very short while? Is 
there some component that prevents it from reproducing?
Thanks for your help!

Re: Why doesn't yogurt reproduce like other bacteria?