What purpose did boiling the meat broth serve in both the spallanzani and pasteurexperiments?

In both the Spallanzani and Pasteur experiments boiling the meat broth served the purpose of killing off existing microorganisms in the broth, this includes any airborne microorganisms. This prevents any contamination as the unwanted bacteria die at high temperatures, meaning that the experiments can be fairly conducted to have reproducible and reliable results.