Reader Jayne wants to know how puff pastry can rise without any yeast, which is present in other laminated doughs like Danish and croissant. Jayne, I love your question and I thank you for asking it. The answer is: steam. Puff pastry needs no leavening agent because it’s comprised of hundreds and hundreds of individual layers of dough, all of them separated by layers of butter. When the pastry is inserted in the oven the butter melts, freeing and lubricating the dough sheets so they can separate from one another.
Meanwhile, water that’s been trapped inside the dough and the butter begins to transform to steam, which expands to occupy something on the order of 1600 times as much space. The steam pushes out in all directions, and indeed most of it escapes out the sides of the pastry. That’s a good thing, otherwise you’d have a vol au vent the size of a bath tub. Yet its effects can be clearly seen in the finished pastry, which can be up to six or seven times the dough’s original thickness.
The natural question at this point is: if puff pastry rises so well without any yeast, why bother to include it in Danish and croissant dough? The answer is texture. Puff pastry, as you know, is rigid stuff. All those ultra-thin sheets of dough get quite brittle as they dry out in the oven. Bite down on them and they go crunch. Danish pastries and croissants, by contrast, are bread doughs. When they’re laminated they aren’t folded as many times so the layers are thicker. Only the outer few layers of a Danish or a croissant dry out in the oven, so while you do get a slight crunch when you bite into one, the inner layers remain soft and moist. Now you’re making me hungry, Jayne. Thanks again for the question!