Ohio
ANCIENT Greeks and Romans packed their pies with honey and figs. The
coronation banquet of Henry VII in 1485 featured a crusted monstrosity from
which 15 live pigeons and one hunchbacked dwarf emerged. Today鈥檚 apple and
mincemeat offerings look tame by comparison. But no matter what the festive
filling, the best pies have one thing in common: light, flaky pastry shells.
Novice cooks shiver at the thought of their soggy or rock-hard failures. The art
of pastry-making must surely require a master鈥檚 touch.
Wrong. It鈥檚 merely a matter of applied biochemistry: the interaction of
proteins, sugars and fat. Ideal pastry is defined by the physics of flaky
textures, the way food breaks apart in our mouths, and the flow of molecules as
they tumble over our tongues. View pastry as a science rather than an art and a
light, flaky crust should be within anyone鈥檚 reach.
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To master pastry, you must understand the interaction of three ingredients:
flour, fat and water. Flour is mostly starch, but also contains two crucial
proteins that react with water to form a mesh-like network called gluten, which
gives bread dough its springy elasticity. But diners prefer flaky pastry to
springy. So pie makers try to minimise the meshing of gluten鈥攋ust enough
to hold the pie dough together and no more.
To control the gluten, you first cut cold butter or lard into the dry flour.
The cold does two things: it slows down the protein-protein interactions that
form gluten, and it keeps the fat in solid bits interspersed with the flour.
Next, you add ice water. This moistens the flour and makes its gluten proteins
mesh just a bit鈥攂ut since fat repels water, no gluten forms where fat
chunks have settled in. You can also minimise gluten by choosing 鈥渟oft鈥� pastry
flour, which contains less gluten than 鈥渉ard鈥� bread flour. Another must is
handling the dough as little as possible. Kneading develops the gluten, and
warm, sweaty hands raise the temperature and moisture content of the pie dough.
Some bakers also throw a bit of sugar or cornstarch into the dough to suck up
water and keep it from activating the gluten.
The new dough contains globs of fat surrounded by pockets of gluten mesh.
When you roll out the dough, these bits of protein-cloaked fat flatten into
flakes, so the dough consists of alternate layers of gluten and fat. Not very
appetising so far. But in the oven the fat melts and seeps into the gluten
layers, leaving behind air cavities that make a crispy, flaky pastry. If you
prefer to use oil instead of lard or butter, the dough will contain tinier fat
droplets and the finished crust will have a more crumbly, rather than flaky,
texture.
Oven magic
If all goes well, as the pie bakes much of the water in the pastry escapes as
steam through the air spaces left by melting fat, leaving the gluten proteins
and starch granules behind. Pastry dough that is 30 per cent water to start with
might end up 8 to 10 per cent water after baking. 鈥淭he material becomes dry,
like pieces of spaghetti,鈥� says molecular biophysicist Vic Morris of the
Institute of Food Research in Norwich. By comparison, bread dough starts at 65
per cent water and ends up at 40 to 55 per cent. The drying process is crucial
to the texture of the final pastry. If the pastry has the right crispy texture,
the dry flakes will snap in the mouth like dry spaghetti, says Morris.
At the first bite, an eater鈥檚 teeth shear the piecrust鈥檚 alternating layers
of gluten and air. As each layer breaks, you hear a snap鈥攅ssentially a
microscopic sonic boom, as the breaking fragments move at the speed of sound,
says Peter Lillford, a physical chemist at Unilever Research in Bedfordshire.
How the texture seems to you, though, depends on whether or not the crust yields
many snaps or just a few. A small number of fragile cracks makes for a desirable
crispy texture rather than an annoying crunchy one. 鈥淭he difference is how many
noises you make,鈥� says Lillford.
That difference is easy for tasters to recognise, but hard for scientists to
quantify. Physicist Athene Donald at Cambridge University uses machines that
apply weighted loads and measure how much force a piece of food can withstand
before it breaks. But these numbers are hard to relate to the words a consumer
uses. 鈥淵ou cannot say that if this measurement has a number greater than so and
so than it will be seen as crisp,鈥� she explains.
Lillford moves force measurements from the lab bench into the body鈥檚 own
chewing machine, the mouth. He does all kinds of unorthodox experiments such as
strapping electrode sensors to subjects鈥� jaws and connecting transducers to
false teeth to measure the force chewing exerts on food. People who bite harder
tend to release more flavour compounds at once, leading to a more intense taste
experience, he finds. They may prefer subtler seasonings than those who chew
more gently.
As well as mouth movement there鈥檚 鈥渕outh feel鈥�. This is largely the result of
fat droplets spreading over the surface of the tongue to stimulate sense
receptors. People have a strong preference for foods that evoke such a
melt-in-your-mouth feel, chocolate being a prime example. Piecrusts fit the bill
with their crackling disintegration of fat-rich layers. In fact, people are so
sharp at distinguishing fat from fat-substitutes that you can鈥檛 short the
shortening without compromising the mouth-feel, says Peter Wilde, a biochemist
at the Institute of Food Research.
Of course the best thing about the flaky, buttery crust is that it
complements the filling to provide the most pleasurable chewing, says Lillford.
Chewing has two goals: wetting the food and reducing its size to something easy
to swallow. A dry food item鈥攁 biscuit, say鈥攖akes a lot of chewing to
get it wet enough. And while a bite of meat is already moist, you have to chew
it a lot before it鈥檚 small enough to swallow. But pies fall into the happy
middle ground. 鈥淎 pie is a composite,鈥� says Lillford. 鈥淭he crust is the thing
you have to do the work on, but the filling moistens it.鈥� So it takes less work
to get a morsel of pie into a form worth swallowing.
At least, as long as you don鈥檛 fill the pie with live pigeons and a
dwarf.