Monday, March 23, 2009
A little cooking chemistry
Ever wonder how flour becomes layers of pastry? How egg whites crispen into perfect angelic meringues? How a soufflé stays up? How caramel doesn’t burn or turn into rock (hopefully)?
Thanks to the blogger’s experience as an ex-chemistry major, these questions can be answered.
Let’s examine pastry to start. Pastry dough is, in its most primitive state, made of flour, butter, and a little water. The butter should be cold, so that the molecules that make it up will best retain their structure. Likewise, the water should also be cold so that the butter does not start to melt. Essentially, the kneading of the butter-flour-water mixture should not melt the butter, instead, the butter and water remain cold and just change shape – into a ball of dough.
The kneading has combined the water and the butter, however, they do not completely mix because of the chemistry of the two molecules. Water is a polar molecule, which means that it has a permanent charge in one of its molecules that is not cancelled out by another charge in the opposite direction. Fats are nonpolar; as they are much larger molecules than water, they have more charges in opposite directions that eventually cancel each other all out.
As everyone knows, mixing oil and water does not work – the two repel each other, because one is nonpolar and the other is polar. This same principle applies to baking. The water and the butter do mix inasmuch as they form a dough with the flour to clue everything together, but they do not actually combine – the butter and water rest as layers of sand, unmixed.
Then, when everything goes into a hot oven, the water evaporates into steam as it cooks, the steam expands into bubbles in the dough, leaving behind inflated layers of pastry with lots of air in between.
In the end, when the dough comes out of the oven, it takes the form of flaky, crispy, buttery pastry. Success!