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How yeast makes bread bubbly and delicious

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Bubbly pizza crust, fluffy doughnuts, and airy bread all rely on yeast to get their lift, but it does more than just help the dough rise.

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Yeast are living microorganisms in the fungi group. Dry active, instant, and brick yeast are grown commercially and available in grocery stores. Anyone can harvest wild yeast at home by making a sourdough starter.

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Sourdough starters are made from flour and water. A mature starter is filled with dozens of types of yeast, along with a handful of other bacteria.

To keep the yeast in a sourdough starter alive, it needs to be fed with a mixture of flour and water.

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Four to six hours after feeding, the starter will bubble up. This is known as the “active phase,” when the yeast cells are consuming nearby food and dividing rapidly.

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Active sourdough starters produce lactic acid, which is what gives sourdough its signature flavor.

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The bacteria that produce the best flavor are called “heterofermentative lactic acid bacteria.” They produce diacetyl, lactic acid, and other esters. They only grow up to 30 degrees celsius, so room temperature can affect the flavor of the rising bread.

Starters are used to make bread during the active phase. The rapidly diving cells continue to eat, divide, die, and expel gas. This is what causes bread dough to rise.

The high heat of the oven will kill the yeast, but by then, their work is done. The bacteria that grew during fermentation will still impart that rustic sour flavor.

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