The laws of chemistry conspire against a better cup

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Baristas around the world have long been plagued by a mysterious phenomenon. Every morning they arrive in their cafes, select their beans, set their grinders, and get their coffee tasting exactly the way they want it. Then the place gets busy, and, for reasons no one has been able to explain, the coffee starts to taste worse.

“When you open the cafe, you’re alone. You pull the first few shots and they taste great,” says Sal Persico of George Howell coffee in Newtonville. “In the middle of a rush, you taste a shot and it definitely doesn’t taste like it did in the morning.”

Now, a new study proposes an explanation. The paper, a collaboration between a research chemist with a strong amateur interest in coffee and some of the top baristas in the world, finds that grinders in continuous use generate enough heat to change the chemistry of coffee beans and cause them to break into less uniform pieces.


“As beans go into the grinder there’s a lot of friction that heats up the burrs. The next shot is sitting just before the burrs every time, soaking in the energy,” says Matt Perger, a world champion coffee brewer from Melbourne and coauthor of the paper.

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In order to make a good cup of coffee, it’s important to grind the beans evenly. If the grind is uneven, with some big pieces and some small pieces, the rate at which flavor is extracted from the grinds will be off — either the big pieces won’t have enough exposure to the water, which means flavor is lost, or the small pieces will have too much exposure, which contributes a bitter taste to the final cup.

With materials in general, one good way to assure they grind into even pieces is to chill them. When materials are warmer, they become more malleable and break into different size pieces; when they get cold enough, they shatter, producing clean, even breaks.

“Every material can get to this point, where it will shatter rather than fracture. The question was, does coffee go through this, too,” says Christopher Hendon, a postdoctoral fellow in chemistry at MIT and coauthor of the paper.

To test this, the researchers compared the grind of beans frozen by progressively more extreme measures — a normal freezer, dry ice, liquid nitrogen — and the grind of beans at room temperature. They found that coffee beans do indeed have a shatter point, though they haven’t yet tried to pinpoint it. What they do know is that somewhere between room temperature and a few degrees below zero, beans undergo a chemical transition, which causes them to shatter in the grinder, producing smaller average grinds and a narrower particle size distribution — the perfect conditions for making coffee. But as grinders heat up with more frequent use, they work at cross-purposes with the final product.


The new study may explain why coffee quality declines as customer traffic picks up, but that doesn’t mean the problem will be easy to fix.

One solution would be to run coolant through the grinder. This is hard for two reasons. First, such a grinder does not currently exist. Second, if the burrs are cooled to too low a temperature, water will start to condense on them. “If the burrs get water on them, you start making little cups of coffee on your burr,” says Hendon.

The other approach would be to freeze the beans. This is actually what Howell has been recommending customers do at home for a long time, based on anecdotal evidence that coffee made from frozen beans just tastes better. Hendon cautions against doing this, however. He explains that without a vacuum sealer, air could freeze along with the beans, causing ice crystals to form and undermining the effort.

At a coffee shop, freezing beans is especially impractical. Baristas cope with the morning rush by having a hopper full of beans ready to grind at the flip of a switch. Making coffee from individually frozen packets of beans would take too long. In this sense, the current study may be tantalizing as much as anything else — after all, lots of folks need coffee to wake up, but it’s only going to be the early risers who get the best cups.

Kevin Hartnett is a writer in South Carolina. He can be reached at

Correction: A previous version of this article misstated where the paper was published. It appeared in Nature’s “Scientific Reports” and was highlighted in the journal Nature.