San Fran's Might Microbes
Unlike over-the-counter baker's yeast, the microbe culture used in creating San Francisco's famous sourdough bread includes a mix of yeast and bacteria species. As the bacteria in this mix breakdown sugar, they release lactic and acetic acid. These acids sour the mixture. Yeast strains that are resistant to this acidic environment survive the pH shift and convert remaining sugars to carbon dioxide and ethanol. The ensuing expansion of carbon dioxide gas causes the bread to rise.