Baking a crisp, flaky pastry shell is something of an art. Pie fillings tend to weigh down the dough, resulting a soggy base and overcooked rims. If we could somehow cook the dough faster, the pie shell would be crisper, but how?
A few years ago Cooks' Illustrated popularized the idea of adding vodka to the dough in lieu of water, claiming in their words:
For a pie dough recipe that baked up tender and flaky and rolled out easily every time, we found a magic ingredient: vodka. Using vodka, which is just 60 percent water, gave us an easy-to-roll crust with less gluten and no alcohol flavor, since the alcohol vaporizes in the oven.
To test this hypothesis, two flour spheres were created by adding water or vodka (80 proof) to 1/4 cups of all-purpose flour, and kneading for a mere 30 seconds. The spheres were the size of golf balls and weighed around 1.75 oz. No butter or fat was added, as we were simply testing the effect of vodka vs water.
Thermocouples were inserted into the center of each sphere, and baked in a 300F convection oven until they attained an internal temperature of 300F- that is, until all the moisture was driven off and evaporative cooling ended. Baking, until crispy, took over two hours.
Both spheres appeared identical, at least externally:
But after cracking open with a hammer, they couldn't be more different in appearance:
Under magnification, the water sphere is brown and filled with bubbles and strands typical of baked bread. On the other hand, the vodka sphere was lighter in color, and flakier in appearance- like a pastry shell. How is this possible?
The key is gluten and miscibility. Flour contains a long-chain polymer protein called gluten. When lubricated with water, the gluten strands can be straightened during kneading (rather like combing wet hair), forming a tough elastic microstructure of fibers and sheets which capture gases like a balloon. Water is weakly retained by the gluten structure, but not strongly chemically bonded, so it will boil off when heated.
Proteins, sugars and other chemical are needed to promote browning in the Maillard reaction, which becomes important just around 300F.1 By straightening out the gluten molecule, enough protein was exposed in the "water sphere" to brown during the experiment.
Conversely, the vodka side almost looks like pastry dough where a fat, such as butter or lard, was kneaded into the flour. Fat (like oil) does not mix with water, so the pastry becomes separated into distinctive, floating layers of moist flour lubricated by sheets of fat. Less exposed gluten protein is developed, which means less browning. Resulting in a lighter and flakier cooked dough.
But we did not add any fat to these spheres, at least not directly. Never the less, flour, even after the oily wheat germ is removed, still contains a small percentage of fatty lipids. The vodka (which is 40% ethanol alcohol) can dissolve out these lipids (and a bit of the gliadin, weakening the gluten sheets), which then lubricate and separate the flour into layers, just as if we had introduced cold butter.
So adding vodka to flour performs two functions. First, on a volume basis it contains 40% less moisture than water. Less water develops less gluten, softening the dough, and letting it dry out faster. Secondly, the alcohol-extracted lipids modifies the kneaded structure, creating a flaky microstructure.
We can discern these two effects very clearly by monitoring the internal temperature of each sphere during baking. Like barbecued brisket, the internal temperature "stalls" for hours as the evaporating moisture cools off the heating dough. Note how vodka, with its lower boiling temperature, evaporates more rapidly than water and so more effectively cools the sphere. The long flaky microstructure also makes it easier for ethanol to move rapidly from the interior to the exterior, compared to the more tightly intertwined water sphere.
This more porous structure also explains why the vodka sphere stalls at a lower temperature than the water sphere- its more porous so water cooling is more effective. Finally, notice how the vodka sphere dries out a half hour earlier- after all, it contains 40% less water.
Amazingly, the vodka pastry technique does makes sense. It dries out more rapidly than pure water, and accentuates the flaky microstructure. Plus it stalls like a barbecued brisket. What more can you ask for an afternoon's experiment in the kitchen?