Is reheating coffee bad for you? A scientist reveals the pros and cons

I’m the type of person who prides myself on juggling tasks and checking off to-do lists, but if there’s one ball I’m consistently dropping it's remembering to drink my morning coffee before it goes ice cold.

Dotted throughout my apartment on bookshelves, desks, and countertops are sad, half-drunk cups that got lost in my morning workflow. I could brew another cup. Instead, I often indulge in a shameful habit: sticking it in the microwave.

But is that the right choice? Internet and coffee connoisseur wisdom online will tell you to never reheat your coffee for fear that it will change not only the drink’s taste profile but even potentially reduce its caffeine content (shudder).

Referred to by colleagues and students as “Dr. Coffee,” Christopher Hendon is no stranger to the chemical inner workings of this caffeinated brew. Hendon is an assistant professor of computational materials chemistry at the University of Oregon. In this article, he helps Inverse lay to rest these rigid coffee rules once and for all.

“Coffee goes into the roaster with a finite amount of caffeine available,” explains Hendon. Once it’s extracted]in the water “caffeine isn’t going anywhere,” he says.

To get our bearings, it’s important to understand what chemistry is actually going on in your cup of coffee. According to Hendon, your brew is made up of a number of different acids extracted from the coffee beans including quinic acid which gives tonic water its bitter flavor.

In addition to these acids, coffee is also full of hundreds of volatile flavor compounds, including

• Carbonyl
• Sulfur alicyclic aromatic benzenoid
• Heterocyclic compounds.

These compounds may not sound appealing, but they’re the product of the roasting process which gives the beans their toasty flavor. Trace elements like calcium or chloride from your brew water may also make their way into the drink, Hendon says.

Finally, coffee is also full of caffeine — about 95 milligrams per eight-ounce cup on average — to be exact. This chemical compound is responsible for coffee’s stimulant effect and also adds a little to its bitterness.

Does reheating coffee reduce caffeine?

So how does this chemical cocktail react to being bombarded with microwaves or the slower touch of a stovetop? Luckily, you don’t need to worry about losing any caffeine.

“Caffeine is not really that reactive,” Hendon says. “It's a pretty stable organic molecule that is produced in the biological process of the maturing of a coffee seed.”

The only real way to lose caffeine from coffee would be through sublimation where the compound undergoes a direct phase change from solid to vapor.

This requires extreme heat, Hendon says — over 350 degrees Fahrenheit. So unless you’re warming your coffee in a preheating oven, you’re likely not exposing it to enough heat to affect the caffeine at all.

Despite its implausibility, however, Hendon says he can see where coffee drinkers might’ve gotten this misconception in their heads. He thinks it stems from a placebo effect or emotional context — more so than a coffee’s changing chemistry.

“Where the myth might come from is that when you drink a hot cup of coffee for the very first time through the buzz that you get from a hot cup of coffee — when you know it's freshly brewed and all that — affects you in ways that are intangible and not related necessarily to what the chemistry is that you're actually consuming,” Hendon explains.

He equates it to smelling bacon at dinner smacked on some cheeseburger versus the fresh smell of morning bacon. “That is a very different experience,” Hendon says.

In other words, your first cup just hits different.

As reported in the New York Times, the dwindling effect of your second or third coffee may also have to do with the paradoxical phenomenon that drinking more caffeinated coffee can actually sometimes make you sleepier.

“I've never had a cup of coffee in my life that I didn't get to the bottom of.”

But although your coffee’s caffeine levels remain stable, that doesn’t mean your coffee will go unchanged by its reheating, Hendon says.

Like any fresh food item, coffee is only headed towards tasting worse the further it strays from fresh, and reheating speeds up this process by shedding the volatile flavor compounds more quickly.

“Humans have extremely sensitive palates for certain compounds, like detections on the parts per billion scales,” Hendon says. “We have high fidelity assessment of differences in flavor, which makes us remarkable, but it also makes food chemistry in general very complex.”

It is possible that your coffee may taste worse after it's been nuked, Hendon says. But you can rest easy knowing that there’s likely nothing actually dangerous about this habit.

How to best reheat your coffee

If you’re on board with the slightly more bitter (or, slightly more caramel-y, according to Hendon’s palate) taste of reheated coffee, then you should continue reheating it.

However, he advises sticking with the microwave instead of the stovetop to shorten the amount of time those volatile compounds have to escape.

Even better yet, if you’re looking to keep your coffee warm for longer without reheating it, Hendon suggests trying out a temperature-regulating mug (like this one sold by Ember) or even a mini desktop warmer (which I use). Or you could even go old school and pour your coffee into an insulated thermos.

As for Hendon, he says he can barely remember a coffee that he didn’t gulp while it was still hot.

“I've never had a cup of coffee in my life that didn't get to the bottom of,” Hendon laughs. “I'd rather even just drink it at room temperature than heat it back up.”

CHECK, PLEASE is an Inverse series that uses biology, chemistry, and physics to debunk the biggest food myths and assumptions.

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