Yeast, when added to warm water, is reactivated and begins to grow by consuming available sugars, releasing carbon dioxide, ethanol, and other byproducts.
Here's a more detailed explanation:
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Reactivation: Dry yeast is dormant. Warm water reactivates it, "waking it up" from its dormant state. The optimal temperature range for this reactivation is typically between 100-115°F (38-46°C).
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Feeding: Once reactivated, the yeast cells begin to consume sugars (simple carbohydrates) present in their environment. If the water doesn't contain sugar, the yeast will not multiply much. However, if sugar or flour (which contains starch that can be broken down into sugars) is added to the water along with the yeast, the yeast cells will have a food source.
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Multiplication: As yeast consumes sugar, it multiplies through a process called budding. A new yeast cell forms as an outgrowth of the parent cell.
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Byproducts: During sugar consumption (fermentation), yeast produces carbon dioxide (which causes dough to rise) and ethanol (alcohol). It also releases other compounds that contribute to flavor.
In summary, yeast doesn't "grow" in just water. It needs sugar to fuel its growth and reproduction. Water simply provides the necessary moisture and, if warm, optimal temperature for the yeast to become active and start the fermentation process.
