What gas helps croissants rise during yeast fermentation?

What gas helps croissants rise during yeast fermentation? Carbon dioxide. Croissant yeast produces carbon dioxide as it metabolizes sugars during fermentation, and that gas expands inside the dough’s existing air cells. The result is a larger, lighter dough before baking.

Croissants use Saccharomyces cerevisiae, the same species commonly used in breadmaking. When oxygen becomes limited, the yeast’s anaerobic fermentation converts sugars into ethanol and carbon dioxide. The gas dissolves into the dough’s watery phase, then diffuses into small bubbles formed during mixing. Gluten helps trap those bubbles, allowing the dough to expand during proofing.

Carbon dioxide is only part of a croissant’s lift. In the oven, water turns to steam, and steam pressure expands the alternating dough-and-fat layers. This is what strongly contributes to the pastry’s dramatic flakiness and height. Yeast fermentation and steam therefore perform different jobs: fermentation inflates the dough before baking, while steam drives much of the final pastry lift.

A common mistake is choosing oxygen because yeast needs oxygen for some growth. However, the gas that leavens the dough during fermentation is carbon dioxide, not oxygen.