What is the unique leavening agent for popovers?

The unique leavening agent for popovers is steam. Popovers are a type of quick bread that relies primarily on the rapid expansion of steam for their rise. The batter is made with a combination of flour, eggs, and liquid, which is similar to a crepe batter. When the batter is baked in a hot oven, the liquid in the batter turns to steam. As the steam expands, it creates pockets of air, causing the popovers to puff up and form their characteristic hollow centers.

While yeast, baking soda, and egg whites can serve as leavening agents in other types of baked goods, they are not the primary leavening method in popovers. Yeast is used for fermented breads, baking soda is typically used in conjunction with an acid for certain quick breads, and egg whites can add some volume when whipped but do not provide the same mechanism of leavening that steam does in this context. The reliance on steam gives popovers their light and airy texture, making it the standout leavening agent for this particular pastry.