In a time-lapse video, it looks like a monster coming alive. For a moment, it sits there innocuously. Then it pulsates. Ripples move across its surface. It begins to bulge outwards, bursting with weird boils. In mere seconds, it triples in volume, its color darkens ominously, and its surface hardens into an alien topography of peaks and craters. Then, the kitchen timer dings. Your cookie is ready.
What happened inside that oven?
Don’t let the sugar and spice deceive you, for the baker is a mad scientist. In an act of aproned alchemy, she uses her oven to transform one substance into another. As her sugary creation heats up, the chemical compounds in the dough undergo a series of reactions that change them into different ones.
When the dough reaches 92° Fahrenheit, the butter melts, causing the dough to start spreading out. As it melts, it releases trapped water, and as the cookie gets hotter, the water expands into steam. It pushes against the dough from the inside, trying to escape through the cookie walls like Ridley Scott’s chestbursting alien.
When the temperature reaches 136°, it’s too hot for any salmonella that may have been squirming around in the eggs. They die off. You’ll live to test your fate with the raw dough from your next batch.
At 144°, change begins to occur in proteins, which come mostly from the eggs in your dough. Eggs are composed of dozens of different kinds of proteins, each sensitive to a different temperature. In an egg fresh from the hen, these proteins look like coiled-up balls of string. When they’re exposed to heat energy, the protein strings unfold and get tangled up with their neighbors until they form a linked structure. This structure is what transforms an egg from watery to almost solid, and gives substance to squishy dough.
Water boils away at 212°, so like mud baking in the sun, your cookie dries out and stiffens. Cracks spread across its surface. The steam that was bubbling inside evaporates, leaving behind airy pockets that make the cookie light and flaky. Now it’s nearly ready for a refreshing dunk in a cool glass of milk.
One of science’s tastiest transformations occurs at 310°. This is the temperature for Maillard reactions, which turn food Thanksgiving turkey-brown and emit the rich, nutty aromas that we associate with things like burgers and bacon. They result when proteins and sugars bind together and then break down, forming brand-new compounds that human taste buds are partial to.
Caramelization is the last reaction to take place inside your cookie. And in fact, if your recipe calls for a 350° oven, it will never happen, since caramelization starts at 356°. If your ideal cookie is barely browned, like a Northeasterner on a beach vacation, you could have set your oven to 310°. If you like your cookies to have a nice tan, crank up the heat (caramelization continues up to 390°).
And here’s another trick: you don’t even need that kitchen timer. Your nose is a readily accessible, sensitive, scientific instrument. When you smell the nutty flavors of the Maillard reaction or the toasty ones that signal caramelization, your cookies are ready. Grab your glass of milk, put your feet up, and reflect that science can be pretty sweet.
Get more food science at Stephanie’s blog, testtubekitchen.com.