The Science of Sustainability

Brewing the Perfect Cup of Coffee

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coffee and beans

There were many shaky hands at a regional coffee competition in Santa Cruz one weekend in early March, though it was hard to tell if the tremors were due to nerves or just the free-flowing coffee at the tasting bar.

Baristas from Colorado to Hawaii competed to earn a trip to the national championship in April. Brewer’s Cup participants, mostly from California, brewed a mystery coffee, tweaking their favorite brewing method to bring out the best flavors of that coffee. Winners from the first round used their own beans to brew in the finals.

As the brewing finalists’ recipes show, making the perfect cup of coffee is not as simple as adding hot water to ground coffee beans. The grind size, water temperature and brewing method can change the flavor. Coffee connoisseurs delight in controlling every detail. And even the corporate coffee giant Starbucks is experimenting with brewing. They’ve just opened a brewing “laboratory” in Amsterdam.

We describe an ideal cup of coffee in terms of its strength, aroma, flavor, acidity, and finish. Coffee enthusiasts quantify those descriptions as the concentration of coffee compounds in the final brew and the amount of flavor extracted from the beans.

Brewing coffee in SouthWest Regional Barista Championship

Peter Molignano, of Fortnight Coffee Company in Los Angeles, competes in the Brewer's Cup of the South West Regional Barista Championship, Santa Cruz, CA, March 9, 2012. Image credit: Melissae Fellet

There are 2000 different flavor and aroma compounds in roasted coffee beans. Only a fraction of those flavors dissolve in hot water, and even fewer are palatable. A cup of coffee brewed to full extraction is bitter and undrinkable. So brewers aim to pull the sweet and complex flavors from the grinds before the beans leach their bitter compounds.

Reaching that extraction sweet spot depends on several factors, including the ratio of water to beans by weight. (Notice that all the winning recipes from the competition specify water and bean weight.) The size of coffee grounds and the brew time matter too. If your coffee is weak, grinding the beans finer increases the surface area of the grounds exposed to the hot water and thus increases the amount of flavors extracted. But watch the clock once the hot water hits the grounds. Brew any pot too long and those bitter flavors seep out from the beans.

Water temperature is another way to control the flavor of coffee. Water warmed to 195-205 degrees Fahrenheit is best, says Lalo Perez-Varona, an independent coffee researcher from San Francisco.

Coffee associations around the world recommend numerical standards that label the perfect cup of coffee in terms of the percentage of extracted flavors, but ultimately individual taste preferences determine excellence.

For a tasty cup of joe, pick beans with flavors that you like, use your favorite brewing method, and troubleshoot using this chart, changing one factor at a time.

Factors influencing coffee flavor

Image credit: Melissae Fellet

Brewing method influences the flavor of the coffee too. Completely immersing the coffee in hot water, as in a French press, tend to brew coffee with more sweetness and body, says Perez-Varona. Paper filters trap some of the oils and solids, clarifying the flavors in the coffee.

Brewing with a siphon, also called a vacuum pot, gives you excellent temperature control due to the consistent heat source underneath the pot. This process is beautiful to watch:

I think a person's preferred brewing method is a chance to infuse their coffee with some personality and flair. Peter Molignano, of Fortnight Coffee Company in Los Angeles, brought his handmade wooden siphon holders to the competition in Santa Cruz. He warmed the upper part of the siphon with halogen lightbulbs mounted in a matching wooden box.

From precision analysis to the ritual of brewing, a cup of coffee is a union of science and art. Because my morning routine doesn’t leave time for experimenting with water temperature, brew time, grind size, and water and bean amounts, I’ll leave the science – and the art — to the experts at my favorite local coffee shop.

UPDATE 4/2/12: The underextraction-overextraction chart has been updated. The original picture had the last entries for the water to coffee mixtures switched.
UPDATE 3/22/12: The optimal water temperature for brewing coffee was originally reported as 192-203 degrees Fahrenheit. It has been changed to 195-205F.

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Melissae Fellet

About the Author ()

Melissae Fellet is a freelance science writer obsessed with electrons, atoms and molecules. Writing about chemistry, physics and technology, she hopes to reveal how the invisible building blocks of matter influence things like plastics, perfumed shampoos and the speedy computer chips we use everyday. She holds a BS in biochemistry and microbiology from the University of Florida and a PhD in chemistry from Washington University in St. Louis. She spends sunny days at her home in Santa Cruz either watching otters in the bay or tromping around the redwood forests.