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In a time-lapse video,[br]it looks like a monster coming alive.

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For a moment, it sits there innocuously.

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Then, ripples move across its surface.

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It bulges outwards,[br]bursting with weird boils.

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It triples in volume.

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Its color darkens ominously,[br]and its surface hardens

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into an alien topography[br]of peaks and craters.

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Then, the kitchen timer dings.

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Your cookie is ready.

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What happened inside that oven?

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Don't let the apron deceive you!

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Bakers are mad scientists.

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When you slide the pan into the oven,

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you're setting off a series[br]of chemical reactions

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that transform one substance, dough,[br]into another, cookies.

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When the dough reaches[br]92 degrees Fahrenheit,

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the butter inside melts,

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causing the dough to start spreading out.

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Butter is an emulsion,

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or mixture of two substances

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that don't want to stay together,

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in this case, water and fat,

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along with some dairy solids[br]that help hold them together.

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As the butter melts,[br]its trapped water is released.

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And as the cookie gets hotter,[br]the water expands into steam.

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It pushes against the dough[br]from the inside,

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trying to escape through the cookie walls

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like Ridley Scott's chest-bursting alien.

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Your eggs may have been home[br]to squirming salmonella bacteria.

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An estimated 142,000 Americans[br]are infected this way each year.

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Though salmonella can live for weeks[br]outside a living body

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and even survive freezing,

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136 degrees is too hot for them.

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When your dough reaches[br]that temperature, they die off.

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You'll live to test your fate[br]with a bite of raw dough

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you sneak from your next batch.

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At 144 degrees,[br]changes begin in the proteins,

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which come mostly[br]from the eggs in your dough.

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Eggs are composed of dozens[br]of different kinds of proteins,

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each sensitive to a different temperature.

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In an egg fresh from the hen,

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these proteins look[br]like coiled up balls of string.

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When they're exposed to heat energy,

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the protein strings unfold[br]and get tangled up with their neighbors.

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This linked structure

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makes the runny egg nearly solid,

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giving substance to squishy dough.

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Water boils away at 212 degrees,

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so like mud baking in the sun,

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your cookie gets dried out[br]and it stiffens.

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Cracks spread across its surface.

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The steam that was bubbling[br]inside evaporates,

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leaving behind airy pockets[br]that make the cookie light and flaky.

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Helping this along[br]is your leavening agent,

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sodium bicarbonate,

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or baking soda.

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The sodium bicarbonate reacts[br]with acids in the dough

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to create carbon dioxide gas,[br]which makes airy pockets in your cookie.

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Now, it's nearly ready[br]for a refreshing dunk

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in a cool glass of milk.

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One of science's tastiest reactions

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occurs at 310 degrees.

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This is the temperature[br]for Maillard reactions.

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Maillard reactions result

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when proteins and sugars break down[br]and rearrange themselves,

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forming ring-like structures,

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which reflect light in a way

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that gives foods like Thanksgiving turkey

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and hamburgers

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their distinctive, rich brown color.

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As this reaction occurs,

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it produces a range of flavor[br]and aroma compounds,

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which also react with each other,

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forming even more complex[br]tastes and smells.

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Caramelization is the last reaction

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to take place inside your cookie.

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Caramelization is what happens

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when sugar molecules[br]break down under high heat,

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forming the sweet, nutty,

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and slightly bitter flavor compounds[br]that define, well, caramel.

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

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it'll never happen,

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since caramelization[br]starts at 356 degrees.

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If your ideal cookie is barely browned,

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like a Northeasterner on a beach vacation,

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you could have set[br]your oven to 310 degrees.

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If you like your cookies[br]to have a nice tan,

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crank up the heat.

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Caramelization continues[br]up to 390 degrees.

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And here's another trick:

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you don't need that kitchen timer;

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your nose is a sensitive[br]scientific instrument.

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When you smell the nutty, toasty aromas

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of the Maillard reaction[br]and caramelization,

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your cookies are ready.

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Grab your glass of milk,

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put your feet up,

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and reflect that science[br]can be pretty sweet.