lucky peach


The Science of Stale Coffee

The science behind your daily cup.

By Harold McGee January 8, 2016
Illustration by Jason Polan

It was a dazzlingly bright and blue morning—the kind of day that makes me think San Francisco has better weather than it does—when I met up with Harold McGee. McGee had agreed to let me poke and prod him to see what sort of breakfast-y food-science facts I could squeeze out of the giant brain in his remarkably normal-sized head.

We attempted to convene at a diner, but the place we’d picked had a line around the corner. We retreated to Swan Oyster Depot, where we figured we could conjure thoughts about eggs while slurping melon-sweet Kusshis and amber glasses of Anchor Steam. But the line there was twice as long, stretching ever so gently uphill, toward Oregon. So we walked a few blocks and took a table at the Belcampo Butcher Shop & Restaurant on Polk Street, where our conversation turned to coffee, which we were not drinking, as we chatted through half an hour, waiting for lunch service to start. —Peter Meehan

My taste in coffee has changed a lot over my lifetime. I began with basic percolator coffee after graduating from tea, which is what my parents drank. I discovered dark roasts in college, bought my first grinder, and then moved to the West Coast, where I discovered Peet’s. Years later, Peet’s gave way to Blue Bottle, Four Barrel, and so on. And for years, those coffees were what I really loved, but lately I’ve found them to be too acidic, too tart. I’ve moved back in the direction of the darker beans. There’s a coffee roaster here in San Francisco named Andrew Barnett, who has a place called Linea Caffe, where he roasts darker than the other guys, and that’s where I usually go for coffee these days.

Sometimes, when I come back from a trip, I’ll find that I didn’t think ahead and put my coffee in an airtight container in the freezer. I’ll open my kitchen cabinet, and find that all I have to work with is a bag of maybe two-cups-of-coffee’s worth of beans that have been sitting there for three weeks. I’ll make a cup of coffee with it, and you know what? I’ve come to enjoy that cup of coffee, that “bad” cup of coffee.

So now I do it on purpose. I don’t bother making a special point of putting coffee in a jar to put in the freezer to preserve the beans, because I’ve come to appreciate stale coffee. It’s a flavor I recognize from the not-so-good old percolator days that gives me a certain amount of pleasure.

While I’ve had eye-opening revelations with coffee—an Ethiopian coffee from Blue Bottle that tasted like blueberries comes to mind—I’ve never really become a nerd, which I guess is another way of saying I’m a dilettante about coffee.

It reminds me of a conversation I once had with Jeffrey Steingarten about a melon I’d tasted. It was wonderful, I told him, and really interesting because there was a hint of the squash family to its taste, a pumpkin-y note. (Melons and cucumbers and squashes are all in the same family.) Jeff said, “So what you’re saying is that it was a bad melon, McGee. Have you never heard of connoisseurship?”

I’m not a connoisseur; I dabble. To the extent that I can make connections for myself that create bridges between my experiences, I do. I think there’s a lot of “geeking out” on the details that go beyond what we can actually taste. I am most interested in the actual experience of consuming coffee. Sometimes you have the connoisseurs’ perfect melon, and sometimes you have a cup of stale coffee.

And because I know a little bit about what’s going on in food and why you end up with different flavors, I find that that cup of coffee is a good chance to refresh the circuits in my brain that ask, Why does it have that stale flavor?

Well, the more volatile and unstable molecules in the coffee have dispersed and disappeared, and some of the not-so-volatile things have stayed behind and reacted with each other and with oxygen. It’s a more complicated version of what happens to old cooking oil. You’ll know what I’m talking about if you’ve ever bought a too-big bottle of canola oil that ends up in the back of your pantry for a few months. When you dig it out to cook with it, it smells different. Instead of being neutral, it has this very distinct oily flavor and aroma.

What’s happened is that the oil has oxidized from the action of air and light—two sources of energy that transform the oil. They break the fat molecules apart into smaller molecules and then break those fragments into even smaller ones. The oils themselves are so large that they don’t volatilize—they cling onto each other, because they’re long and they can’t escape. What you’re smelling are the pieces that are light enough to float away. That’s a big part of why storing your coffee in a vacuum bag in the freezer keeps it freshest: you’re preventing its more volatile aromas from escaping, and you’re keeping heat, light, and oxygen away from the beans, so they can’t start breaking up their more sensitive molecules.

Now, the exact chemicals volatizing from your staling beans are different from what comes out of the Mazola—coffee beans have many hundreds of molecules in them that can present a wide spectrum of smells. But food chemists (and people like me who try to understand what the food chemists catalog) try to simplify things. We get the major categories right, understand the processes at work, and then try to construct a plausible story for why things smell like they do. The story of why you taste and smell the things you do is always a story that someone made up based on the information available to them.

Other aspects of the flavor experience of coffee can be more easily quantified. Take acidity—you can actually notice the difference from one coffee to the next, not only by taste, but also by what it does to the milk you pour into it. Elli, my companion, likes to put soy milk or soy creamer into her coffee, and with Four Barrel and Blue Bottle, the soy milk curdles as it goes into the cup, due to the higher acidity in those brews.

Acids are readily available in all coffee beans. Coffee beans are, reductively, made of two different kinds of material. One part is nourishment for the seed. The coffee plant normally grows in the understory of the forest with a lot of competition for light, so it has to have enough energy stored up in its seed to push out its leaves and make chlorophyll. The other part is defensive chemicals, so when an insect or a fungus begins to nibble on the outside of the bean, it gets a hint of astringency or bitterness, and it leaves the bean alone.

The sugars in the nourishment part of the package break down into familiar food-like acids during roasting—acetic acid, for example, which is the main acid in vinegar, or lactic acid, which might add a buttery note. They are liberated in the early stages of the roasting process, so when you halt a coffee at a light or medium roast, you maximize its acidity.

Later in the roasting process, when the beans go shiny and dark, you begin to destroy those acids, and that’s why darker roasts can seem milder, even though they’ve actually been heated in a more extreme way.

Sorting all these flavors and how they come to the cup is what interests me, and why it can be just as interesting to untangle the hows and whys of a stale cup as it is to explore the floral and fruity notes of a really fine cup. Staleness is a part of life and part of what happens to all foods. I’m catching the coffee at a different stage in its lifetime, and I think it’s good to have the whole spectrum available to think about and experience…

A waitress interrupts Harold to take our order.

Actually, I haven’t looked at the menu. But I’d like the bone broth. I’ve never had bone broth. Have you?

This interview was edited and condensed, but we did just talk about coffee until the waitress arrived. We never even got to eggs. Maybe in the future!