![]() A perfect cup, every time (assuming, I should point out, you use the same coffee beans and roast). “If it is underextracted at this point, you use less coffee.” Find your happy place, and you can reach it again and again, without being flummoxed by trying to achieve a specific runtime. ![]() “If it is overextracted at this point, you need to use less water,” Hendon says. Progressively grind finer and finer until you get to a maximum extraction-after that, it’ll start to taste thinner again. Since looser, coarser grinds yield faster runtimes, that’ll yield a 10-second pull and a cup of coffee that’s acidic and thin. So Hendon suggests starting with a very coarse grind of roughly the usual amount of coffee a barista would go with, 20 grams. It’s all about the changing ratios of water, the solvent, and coffee, the thing from which that water is extracting flavor compounds. The runtime of the shot gets shorter, and the size of the espresso goes down a bit, but it also doesn’t overextract negative flavors. “The other option is to go to the maximum extraction yield and use a little less water,” Hendon says. It could be the worst-tasting cup of coffee you ever had and it could have the same extraction yield as the best.” But it’s something you can count. “It doesn’t tell you any qualitative information. “As the density of the coffee increases, or you dissolve more stuff, you see a proportional increase in refractive index,” Hendon says. So “extraction yield” uses the refractive index of the coffee, essentially the amount of light that can pass through it, to infer how much coffee’s in the coffee it’s roughly the equivalent of “brix” in wine. A cup of coffee is mostly water, mixed with dissolved solids (around 1 percent in pour-over and anywhere from 7 percent to 12 percent in espresso). “We wanted to be able to predict espresso extractions,” Hendon says. So Hendon started messing with more inscrutable variables in the lab, like the mineral content of the water or the age of the beans. Tweaking the kind of variables that shot-pullers generally use to try to pull a perfect cup-grind and tamping down, and sometimes water temperature and pressure-didn’t seem to fix the problem. ![]() If all this research seems like a tempest in a teacup (or a dustup in a demitasse?) well, coffee is a high-stakes game. To be clear, these are the kind of procedural, methodological matters that coffee adherents fight duels over. After six years of work, Hendon says he has a kind of formula for dealing with all those variables to ensure a heretofore unobtainable same-same espresso every time. But now one of them-a computational chemist with a sideline in championship-level espresso-pulling named Chris Hendon-is offering a solution to this specific reproducibility crisis. In fact, the one thing that coffee specialists agree on is that it’s all very complicated. No wonder the barista never gets your name right. It’s a process that 40 pages of partial differential equations can barely wrap their squiggly little symbolic arms around. Now imagine that even if you follow the boss’s recipe, which usually only specifies some of those things, you still don’t always get 40 milliliters of consistent espresso. You’d be a little emo too if the irreducible unit of your job involved juggling variables including not only the choice of coffee bean and how long it was roasted, but also water pressure, water temperature, total coffee mass, size of individual coffee grains (ranging from 100 to 200 μm-that’s grind setting), and the degree to which those grains are packed together, all working to optimally extract the more than 2,000 different molecules that contribute to taste and smell.
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