So what is “mechanical” leavening? What is the “mechanism” that results in rising vis-à-vis a microbe or some chemical reaction? In a word: steam. Though to be fair steam is the chief rising agent in any type of leavening. Microbes or chemicals may create the initial bubbles in, say, a loaf of sandwich or banana bread, but it’s steam that’s ultimately responsible for blowing them up. What distinguishes pure mechanical leavening from these other types of leavening is that where mechical leavening is concerned, there is no instigator. The dough must be prepared in such a way that it rises on steam power alone.
Which is no small amount of force, as the steam locomotive amply demonstrates. Sure, if you heat CO2 bubbles up in an oven you’ll get some expansion (maybe a few times their original volume). Heat water though, and the steam that’s created will occupy 1400 times the amount of space that the water originally did. That’s a lot. Yet for all that pure mechanical leavening is one of the least employed of all leavening techniques, at least by home bakers. Probably because the dough preparation for mechanically leavened pastries can be a little…involved.
I’m thinking of course or layered, also known as laminated doughs.