What Happens to Your Coffee As It Sits, and the Science Behind It

I am not a morning person. Despite spending most of my 20s working opening shifts at coffee shops—waking up with just enough time to brush my teeth and throw on some clothes before my usual 6 a.m. shifts—being a barista was an ongoing fight against my natural state. I am a morning curmudgeon who can’t be bothered to stir from bed before hitting the snooze button thousands of times. Once I’m up, I tend to move at a snail’s speed getting my day started. 

Being a slow starter in the morning also means I often pour myself a mug of coffee and completely forget about it, which I’m sure has happened to us all. Maybe you get a phone call, or maybe a kid needs something. Or maybe you just walk into a room, set your mug down, forget about it and walk away, only to discover your frigid cup hours later. 

So, what should you do with that forgotten mug? Should you throw out your cup of coffee, even if you just abandoned it for a few minutes? Is there a hard time stamp on how long a cup of coffee retains its freshly brewed flavor before its past saving and might as well be thrown away? (Or just slammed down if you really need that caffeine and don’t have time to make a new cup?)

We wanted to see how coffee flavor changes over time as it cools, so we put it to the test. I brewed coffee and tasted it at set intervals over two hours to analyze its flavor and to look into the science of what is happening to the coffee’s flavor as its temperature drops. I wanted to find out if there is an ideal time frame during which a cup of coffee tastes its best. Just how bad does coffee really get as it cools? I also performed side by side taste tests in which I compared the flavor of coffee that had been reheated in the microwave to the same batch of coffee left sitting for the same time, without reheating, and to fresh coffee to see how zapping it affects the quality. Finally, I applied my knowledge of flavor science to fully understand coffee’s flavor journey. Here’s what I found.

The Science Behind What Happen’s to Coffee’s Flavor as It Cools

Two significant factors impact how we perceive the flavor of coffee over time. Strangely enough, they almost fight one another. First, volatile components in coffee dissipate over time, meaning the sooner we can drink our coffee, the more flavor components are available to us. But the second component, temperature, inhibits our ability to experience the full range of flavors because humans are bad at tasting at temperature extremes (either hot or cold), meaning the closer to body temperature you can drink your coffee, the more flavor and complexity you can taste. 

How volatile compounds affect flavor: First, volatile components in coffee dissipate as coffee cools. This includes things like lipids, amino acids, and chlorogenic acid—but the specific type and amount of volatile components in a bean are significantly affected by the variety of bean, roast profile, grind size, brewing method, and even the kind of water you use.

Dr. Christopher Hendon, an associate professor of chemistry at the University of Oregon and co-author of the book Water for Coffee: Science Story Manual*says that the specific compounds you lose over time depend on the coffee itself, but you’re generally losing aromatics (the things that make coffee smell good). Since flavor comes from both our sense of taste and smell, this significantly impacts what we perceive in the cup. Hendon notes that “If you like the smell of the coffee when you grind it, that’s what you’re losing [when you allow it to cool].”

*(He’s currently working on a second edition of the book, the first edition is nearly impossible to find. I had to check out a copy from a friend of mine like it was a library book, and when visiting my house for dinner weeks later, he stealthily took it back).

What we describe as the flavor of coffee is a combination of what we taste and what we smell, and because many of the things that contribute to what we smell dissipate over time, we want to enjoy them as quickly as possible. “Most of coffee’s flavor is retro nasal, which is lost with the cup sitting over time,” says Hendon. 

How temperature affects flavor: The second major factor that impacts our ability to perceive specific flavors is the coffee’s temperature. “As you move further away from your body temperature, the thing you’re tasting and smelling becomes more difficult to perceive,” says Hendon. He notes that we’re bad at perceiving flavor at the polar opposite ends of temperature—very hot or cold. If you’ve ever heard you should drink crappy beer ice cold, this is why: When a beer is very cold, it’s hard to taste the off or negative flavors. The same is true with coffee, just on the other end of the temperature spectrum. 

Hendon cites a study published in 2000 in the journal Nature “Thermal stimulation of taste,” by Alberto Cruz and Barry G. Green, which tracked flavor sensations and their perceptibility over different temperatures. “Not every smell and taste comes into your perception at the same rate,” Hendon explains. Hendon says this is biological: “Humans can taste sweetness more as things progress towards body temperature… because you need to know if the plant you’re eating is going to kill you (bitter) at more temperatures than if the plant you’re eating tastes sweet.” 

Coffee has bitter components called chlorogenic acids, which you can pretty much taste at most temperatures. It’s other more pleasant notes that generally come out as coffee cools. “You need coffee to be dropping as close to body temperature as possible to be able to taste everything,” says Hendon. 

But let’s go back to the crappy beer analogy. Like beer, coffee can vary in flavor based on quality. And quality isn’t just impacted by growing conditions, but rather all along the supply chain, from how it’s stored at various stages to how it’s roasted. As coffee cools, quality issues will really jump out. “For some coffees, you might not want to taste everything,” Hendon says. High quality coffee that’s brewed well will probably taste pretty good as it cools, but low quality coffee or coffee that’s brewed poorly will show all of its bad flavors. 

It feels like the loss of volatile components is competing with our ability to taste more fully as items get closer to body temperature, so what’s the ideal temperature to enjoy coffee? Because every coffee is made of different compounds, it really depends. A coffee from Colombia might have different volatile components than one from Ethiopia; a darker roasted coffee will have different volatile components than a lighter roasted coffee, so that ideal point can be hard to find. “So the tipping point [is] between being able to taste everything, and not everything being available to taste,” says Hendon. With those caveats, I forged ahead and tested one of my own favorite brews at various temperatures to see how it changed over time. 

The Temperature and Time Taste Test

There’s a lot of science and research dedicated to exploring how flavor changes in coffee, from how it’s grown to how it’s brewed. But theory is different from practice, and I wanted to see if I, an experienced but realistic coffee taster, could tell the difference in flavor as a coffee cooled. 

The Test Set-Up

I designed an experiment in which I brewed a cup of coffee on my everyday coffee brewer (a Ratio 6, which I picked as my favorite brewer when I tested coffee makers for Serious Eats) and tasted and temperature tested (using an instant-read thermometer) every ten minutes for the first hour, then again at the one-hour and two-hour mark. Precision was key to this experiment: I took the temperature of the room (it stayed consistently 69℉), and I didn’t open any doors or use other appliances that might change the temperature of the room (Hendon notes that this could impact how the coffee cools and changes). 

I brewed a blend from Kin Kin Coffee called Weirdland at a 1:16 ratio of ground coffee to water (for this experiment, I used 60 grams of coffee to 960g of water). After the coffee was brewed, I immediately poured exactly eight ounces of coffee into the most standard diner-style mug I own, since volume affects the rate of temperature change. (Side note: Finding a standard sized coffee mug was difficult as I live in a house full of coffee lovers and we have an interesting collection of funky mugs, few of which were the right size for my tests. My only diner mug features an illustration of David Letterman, made by Variety Coffee Roasters back when I worked there in the mid-2010s.)

The Results

Here is a breakdown of the coffee’s temperature, my tasting notes, and a brief explanation of what’s happening in the cup at every interval of tasting.