"Gay-Lussac found that food treated by Appert's method was quite stable, but as soon as it was exposed to air, fermentation and/or putrefaction set in. Further experiments enabled Gay-Lussac to prove that air was the causative agent, and that if liquid foods were actually boiled, and then exposed to air, then the onset of the two processes was delayed. He also found that if brewer's yeast was heated, it was incapable of initiating fermentations."