[SciShow intro plays]   Michael: When we think of sourdough bread, lots of us think San Francisco, or maybe Alaska. But the history of these tangy loaves goes further back than the California Gold Rush. We think ancient Egyptians were eating sourdough too, making it the oldest leavened bread – or, bread that has something in it to make the dough rise.   For sourdough, that “something” is a mix of wild yeast and bacteria, which break down sugars and make chemicals that give the bread its chewy, sour taste. Generally speaking, bakers make bread dough rise using yeast, but the source of the yeast is what makes sourdough special. Nowadays, you can buy little foil packages with Saccharomyces cerevisiae yeast – a species that we use in baking and brewing.   But before grocery stores, people had to culture their own wild yeasts, keeping them alive in something called a starter. You can make sourdough starter by mixing some flour and water and letting it sit out, because the air around us is full of microbes like yeast and bacteria. After some time, different species will make their home in the goop, and keep multiplying as long as you keep feeding them. Microorganisms like yeast and bacteria break down organic compounds in a process called fermentation – producing other chemicals like gases or alcohols. So when yeasts digest the sugars in flour, they make bubbles of carbon dioxide gas, which make bread dough rise.   The bacteria that end up in the mix are usually part of the Lactobacillus genus – which includes species that we use to make yogurt and cheese. These bacteria break down sugars into chemicals like lactic acid and acetic acid, giving sourdough its signature tartness. And different species produce slightly different flavors.   For example, in 1971, microbiologists Leo Kline and T. Frank Sugihara were researching San Francisco sourdough starters. And they found that the most common yeast species were Candida milleri and Saccharomyces exiguus, while the main bacterium was the never-before-seen Lactobacillus sanfranciscensis.   These yeasts and bacterium have a special relationship: Unlike the yeast S. cerevisiae, these yeasts are really good at surviving in the acid that the bacterium produces and they don’t digest the sugar maltose. But L. sanfranciscensis needs maltose to stay alive, so these microbes can live and grow without fighting over resources.   As long you keep adding flour and water, a starter can live for years. And different flours, temperatures, humidity, and feeding schedules can change how your bread turns out. So next time you’re biting into a delicious sourdough loaf, be sure to thank the microbes that make those flavors possible.   Thanks for asking, and thanks especially to all of our patrons on Patreon who keep these answers coming. If you’d like to submit questions to be answered, or get some videos a few days early, go to Patreon.com/SciShow, and don’t forget to go to YouTube.com/SciShow and subscribe!