What is the primary leavening agent in choux pastry?

The primary leavening agent in choux pastry is steam. When choux pastry is baked, the high moisture content within the dough evaporates, producing steam. This steam expands, causing the pastry to puff up and create a hollow structure, which is characteristic of choux pastry. The unique nature of choux pastry allows it to hold this steam, making it crucial for achieving the desired texture and volume.

Baking soda and baking powder are chemical leaveners that work by producing gas when they react with moisture and heat. However, they are not typically used in choux pastry, as the dough relies primarily on steam expansion rather than gas production from a chemical reaction. Yeast, on the other hand, is a biological leavener that ferments and produces carbon dioxide gas, but it is not compatible with the quick-cooking method used in making choux. Thus, steam is recognized as the pivotal element that allows choux pastry to rise effectively during baking.