Thanks to the bread-baking craze spurred by stay-at-home orders this spring, yeast is a hot commodity. Home bakers may know they need yeast to make their bread dough rise, but they may not realize how yeast works. If you're squeamish, be warned: The dirty details of your bread recipe may not be something you want to read while eating.
Yeast is a single-celled organism, and it's an essential part of many foods and drinks, including baked goods and beer. Yeast lives in the air all around us, which means you can grow your own at home using just flour and water. (Yeast that's cultivated this way is called starter, and it's used to make sourdough bread). If you don't have a week to collect yeast from the environment, you can purchase the instant yeast or active dry yeast that comes in a packet. This yeast is dormant and can be activated by adding it to warm water.
Yeast alone isn't enough to make bread dough rise, though. To work its magic, it needs two additional ingredients: sugar and time. Yeast cells eat sugar, which is present in flour in the forms of sucrose, fructose, glucose, and maltose. As the microorganisms consume these sugars, they release carbon dioxide gas and ethyl alcohol through a process called fermentation. This is also why adding table sugar or honey to a bowl of yeast and warm water can help activate yeast that's slow to wake up. So when you see a bowl of yeast, warm water, and honey start to foam, you're basically watching yeast fart.
In dough, the carbon dioxide forms bubbles that allow it to rise. It's how a ball of bread dough containing yeast can double in size in just a few hours. The alcohol from yeast also contributes to bread's rise in the oven. In extremely hot temperatures, the liquid alcohol evaporates, resulting in gas bubbles that give the bread some extra height.
Yeast is a common ingredient, which makes it a great entry point into learning more about culinary science. If you want to further your yeast education, this genetic engineering kit lets you make your own fluorescent yeast at home using jellyfish genes.
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