The chemistry of cookies - Stephanie Warren

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You stick cookie dough into an oven, and magically, you get a plate of warm, gooey cookies. Except it's not magic; it's science. Stephanie Warren explains via basic chemistry principles how the dough spreads out, at what temperature we can kill salmonella, and why that intoxicating smell wafting from your oven indicates that the cookies are ready for eating.

Lesson by Stephanie Warren, animation by Augenblick Studios.

In a time-lapse video,

it looks like a monster coming alive.

For a moment, it sits there innocuously.

Then, ripples move across its surface.

It bulges outwards,

bursting with weird boils.

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 apron deceive you!

Bakers are mad scientists.

When you slide the pan into the oven,

you're setting off a series of chemical reactions

that transform one substance, dough,

into another, cookies.

When the dough reaches 92 degrees Fahrenheit,

the butter inside melts,

causing the dough to start spreading out.

Butter is an emulsion,

or mixture of two substances

that don't want to stay together,

in this case, water and fat,

along with some dairy solids

that help hold them together.

As the butter melts,

its trapped water is released.

And as the cookie get hotter,

the water expands into steam.

It pushes against the dough from inside,

trying to escape through the cookie walls

like Ridley Scott's chest-bursting alien.

Your eggs may have been home

to squirming salmonella bacteria.

An estimated 142,000 Americans

are infected this way each year.

Though salmonella can live for weeks

outside a living body

and even survive freezing,

136 degrees is too hot for them.

When your dough reaches that temperature,

they die off.

You'll live to test your fate

with a bite of raw dough

you sneak from your next batch.

At 144 degrees, changes begin in the 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.

This linked structure makes

the runny egg nearly solid,

giving substance to squishy dough.

Water boils away at 212 degrees,

so like mud baking in the sun,

your cookie gets dried out and it stiffens.

Cracks spread across its surface.

The steam that was bubbling inside evaporates,

leaving behind airy pockets

that make the cookie light and flaky.

Helping this along is your leavening agent,

sodium bicarbonate,

or baking soda.

The sodium bicarbonate reacts

with acids in the dough

to create carbon dioxide gas,

which makes airy pockets in your cookie.

Now, it's nearly ready for a refreshing dunk

in a cool glass of milk.

One of science's tastiest reactions

occurs at 310 degrees.

This is the temperature for Maillard reactions.

Maillard reactions result

when proteins and sugars break down

and rearrange themselves,

forming ring-like structures,

which reflect light in a way

that gives foods like Thanksgiving turkey

and hamburgers

their distinctive, rich brown color.

As this reaction occurs,

it produces a range of flavor and aroma compounds,

which also react with each other,

forming even more complex tastes and smells.

Caramelization is the last reaction

to take place inside your cookie.

Caramelization is what happens

when sugar molecules break down under high heat,

forming the sweet, nutty,

and slightly bitter flavor compounds

that define, well, caramel.

And in fact, if your recipe calls for a 350 degree oven,

it'll never happen

since caramelization starts at 356 degrees.

If your ideal cookie is barely browned,

like a Northeasterner on a beach vacation,

you could have set your oven to 310 degrees.

If you like your cookies to have a nice tan,

crank up the heat.

Caramelization continues up to 390 degrees.

And here's another trick:

you don't need that kitchen timer;

your nose is a sensitive scientific instrument.

When you smell the nutty, toasty aromas

of the Maillard reaction and caramelization,

your cookies are ready.

Grab your glass of milk,

put your feet up,

and reflect that science can be pretty sweet.