Why is the yeast left in bottle of wine to make sparkling or champagne?

Yeast is used in the production of sparkling wine through a process called secondary fermentation, which is necessary to achieve two properties of these wines: carbonation and complexity of flavor. Here's a more detailed explanation:

1. Carbonation: The yeast plays a crucial role in creating the bubbles in sparkling wine. After the primary fermentation, which converts the grape sugars into alcohol, a second fermentation occurs inside the bottle. During this secondary fermentation, the yeast consumes the residual sugar in the wine and produces carbon dioxide gas as a byproduct. This gas remains trapped in the bottle, resulting in the effervescence characteristic of sparkling wine.

2. Complexity of Flavor: The yeast also contributes to the flavor profile of sparkling wine. During secondary fermentation, the yeast cells undergo autolysis, a process of self-destruction. This releases various compounds that add depth and complexity to the wine's flavor and aroma, including bready, yeasty, toasty, and nutty notes.

3. Extended Aging: In the production of high-quality sparkling wines, the bottles may be left on the lees (yeast sediment) for an extended period, sometimes for years. This extended contact with the yeast allows for further development of flavor and complexity.

It's important to note that the yeast in sparkling wine is inactive and does not continue to ferment indefinitely. The winemaker carefully controls the conditions, such as temperature and pressure, to ensure that the secondary fermentation produces the desired level of carbonation and flavor without compromising the wine's quality.