Boiling does the same thing as acid. It induces thermal hydrolysis which breaks down the covalent bonds. Only this time it's with water, and not acid. In gelatin, there is a relationship between hydrolysis, temperature, and time. The same would happen if you left it on lower heat over a long period of time. It's all about breaking bonds, whether microbial degradation or thermal, time is your enemy in this case. Kokopuffs wanna jump in here?