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Speaker 1: Welcome to brain stuff from how stuff works, Hey, brain stuff,

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Speaker 1: Lauren vocal bomb here. Hot coffee is supposed to be hot.

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Speaker 1: Cold coffee is supposed to be cold, and that's the deal.

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Speaker 1: It's pretty simple. The problem, then, with hot coffee is

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Speaker 1: that you can't drink the whole cup while it's at

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Speaker 1: the optimal temperature unless you're willing to really guzzle it.

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Speaker 1: So when it gets cold, you've got a few choices.

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Speaker 1: Drink it cold, reheat it, or just pour that cup

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Speaker 1: down the drain and start over with a fresh pot.

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Speaker 1: All three of these approaches are either perfectly fine or

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Speaker 1: completely barbaric, depending on whom you ask to Some hot

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Speaker 1: coffee that's gone cold is revolting to others. The reheating

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Speaker 1: process renders it completely undrinkable, but that doesn't stop us

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Speaker 1: from occasionally popping a cup of nine hour old office

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Speaker 1: Joe into the microwave at four pm to power through

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Speaker 1: until bedtime out of some combination of desperation and mild masochism.

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Speaker 1: Research into why heating this humble drink is pretty much

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Speaker 1: non existent, but it most likely has to do with

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Speaker 1: our sense of smell. Humans aren't great at separating our

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Speaker 1: gustatory that's taste and olfactory that smell responses, and coffee

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Speaker 1: has aromas and flavors that hit all five of the

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Speaker 1: tastes that can be picked up by your tongue and

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Speaker 1: other mouth and nasal passage nerves. Sweet, salty, bitter, sour,

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Speaker 1: and savory. So your personal sense of smell has a

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Speaker 1: lot to do with how coffee tastes to you, whatever

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Speaker 1: it's temperature, and however that temperature was achieved. That's because

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Speaker 1: the chemical makeup of coffee is astoundingly complex. Even though

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Speaker 1: its reputation relies heavily on the presence of caffeine, coffee

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Speaker 1: gets its flavor from around a thousand different chemical compounds. Also,

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Speaker 1: the final flavor of the coffee you enjoyed this morning

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Speaker 1: was the product of a dizzy ng array of variables,

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Speaker 1: including the temperature and weather conditions the beans grew under,

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Speaker 1: when the beans were harvested, how they were dried, stored,

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Speaker 1: and roasted, and how they were ground and brewed. So

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Speaker 1: while the compound three methyl mutinol might make your cup

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Speaker 1: of joe taste a little caramel like and ethyl no,

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Speaker 1: it may give it some fruity or notes. Each step

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Speaker 1: of the process either brings out or suppresses any one

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Speaker 1: of these aromatic compounds differently, which brings us to reheating.

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Speaker 1: We spoke with Christopher Hendon, a postdoctoral fellow at the

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Speaker 1: Chemistry Department at m I T and author of Water

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Speaker 1: for Coffee, a book about how coffee interacts with water.

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Speaker 1: He said, reheating coffee in principle can be an absolutely

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Speaker 1: fine approach to achieving a tasty beverage. In practice, this

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Speaker 1: is not usually observed because people reheat in ways that

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Speaker 1: promote the loss of delicious volatile compounds, and so the

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Speaker 1: process of heating, cooling, and heating again drives smelling and

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Speaker 1: tasty compounds out of the cup. According to Hendon, coffee

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Speaker 1: experts seem to prefer a drink that has been brewed

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Speaker 1: within the previous five to twenty minutes. It turns out

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Speaker 1: most people for the coffee as it cools to around

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Speaker 1: a hundred and fifty degrees fahrenheit that's sixty five degrees celsius.

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Speaker 1: This has to do with the way our taste pathways

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Speaker 1: respond to temperature in our food. But when we put

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Speaker 1: something in our mouth that's too hot or too cold,

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Speaker 1: we can't detect all of the compounds that contribute to

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Speaker 1: its flavor. Since coffee has loads of these compounds, the

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Speaker 1: temperature of the drink can affect the taste. Brewing the

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Speaker 1: coffee brings out certain aromatic compounds, but whether the cooling

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Speaker 1: process changes the chemistry seems to be controversial. Pendon says

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Speaker 1: it is absolutely benign, while others claim it makes the

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Speaker 1: drink more acidic as the coffee is exposed to air

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Speaker 1: and oxidizes. Reheating coffee to the same temperature it was

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Speaker 1: when it was first brewed might help you reach that

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Speaker 1: sweet spot temperature wise, but it also has the potential

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Speaker 1: to cause additional chemical reactions that further alter the flavor.

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Speaker 1: And if you're reheating coffee that already has milk or

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Speaker 1: sugar in it, that's even more in the way of flavors, proteins, chemicals,

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Speaker 1: and compounds to contend with. So although many coffee connoisseurs

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Speaker 1: will tell you it's a lost cause once your coffee

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Speaker 1: goes cold, others say it's just important to reheat your

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Speaker 1: coffee as slowly as you can in order to prevent

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Speaker 1: additional chemical reactions. But if you're reheating in the microwave,

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Speaker 1: don't worry about ghosts of past foods heated in that

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Speaker 1: microwave coming back to haunt your coffee. And and said,

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Speaker 1: the concentration of the volatile chemical composition of say, splattered

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Speaker 1: pasta sauce, is pretty low, so I would be surprised

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Speaker 1: if we could attribute the bad taste of microwaved coffee

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Speaker 1: to only that. Our advice crank your microwave's power down

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Speaker 1: to or lower and sap your cup of coffee in

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Speaker 1: thirty second intervals to prevent disaster. But of course we

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Speaker 1: encourage you to run your own experiments. Today's episode was

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Speaker 1: written by Jesselyn Shields and produced by Tyler Clang. For

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Speaker 1: more on this and lots of other tasty topics, visit

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Speaker 1: our home planet, how stuff works dot com.