Final answer:
The primary leavening agent in puff pastry is steam, not yeast. Steam is produced from the water content in the dough, which expands and separates the layers of dough at high temperatures, leading to the characteristic flaky texture.
Step-by-step explanation:
The primary leavening agent in puff pastry is steam rather than yeast. Unlike in the production of typical bread, where yeast produces carbon dioxide to make the dough rise, puff pastry relies on the water content within the dough turning into steam during baking.
The steam expands and separates the thin layers of dough, creating its characteristic flaky texture. This is a prime example of physical leavening, where high temperatures are vital, relating to Charles's Law, which describes how gases expand with increased temperature. Puff pastry is quite different from bread such as sourdough or other yeast-leavened breads, where the carbon dioxide generated by the yeast during fermentation creates the airy structure.