從科學角度,如何做好一杯咖啡 | Wall Street Journal

Science Factors in a Better Cup of Coffee

If you’re craving a good cup of coffee but can’t get to your local cafe, don’t worry.

Dr. Coffee has your back. The doctor, aka Christopher H. Hendon, teaches computational materials chemistry at the University of Oregon. But on the side, he uses his lab skills to help baristas improve their brews, and some of his lessons can be applied at home.

「I can’t tell you how to make the best coffee,」 he said, 「but I can tell you how to minimize the variation in a cup you produce.」

Several factors influence whether a brew will be delectable or disagreeable, including the amount of coffee used, the size and uniformity of the grind and the chemistry of the water.

But let’s start with the pot.

There are two varieties of devices used to make coffee from freshly ground beans: those that fully immerse the grounds in water (like a French press) and those that allow the water to flow through a bed of grounds (like a drip pot).

「The most foolproof method, and by far the easiest, is full immersion,」 Dr. Hendon said. 「All of the coffee contacts all of the water for the entire brewing process.」

To make a regular cup, he recommends a ratio of 60 grams of coffee to one liter of water—a formula that works for either brewing method.

「That’s way more coffee than you’re used to,」 he said. 「If you pass the same amount of water through too little coffee, you start to extract flavors that simply don’t taste good.」

All coffees have about the same pH—the scale used to measure how acidic or alkaline a water-based solution is—but the perceived level of acidity varies.

Coffees from Brazil, Indonesia and Hawaii naturally taste less acidic than the 「acid bombs」 from Kenya, Rwanda and Burundi. Coffees from Colombia and Ethiopia are somewhere in the middle.

Ultimately, how the beans taste in the cup depends on the water used for the brew.

「If you have hard water, you’re never going to be able to brew an acidic cup of coffee,」 Dr. Hendon said. 「It’s not possible.」

That’s because the high level of bicarbonate found in hard water neutralizes coffee’s acidic flavors. For that reason, at home, where it’s difficult to know how much bicarbonate is in the water, it’s safer to brew low-acid varieties.

「If the coffee is already low perceived acid, you won’t lose anything,」 Dr. Hendon said.

An armchair chemist can test the difference water can make by comparing coffees made with bottled water: Dasani is soft. Evian is hard.

For tap water, look for these clues: 「When you wash your hair, if there is a lot of foam, it’s soft,」 Dr. Hendon said. 「If your kettle has lime scale, it’s hard.」

To prepare coffee for brewing, Dr. Hendon recommends pulverizing frozen beans with a burr grinder.

「Every time you crack coffee, it makes dust and big pieces,」 he said. 「You want the dust to leave the grind chamber once the particles are sufficiently small. With a blade grinder, nothing leaves. If coffee is cold, the small particles will be about the same size. You』ve minimized the variance.」

For the grind setting, Dr. Hendon suggests brewing consecutive batches of coffee with progressively finer grounds to identify what particle size works best for your brewing method. 「Don’t change anything but the grind,」 he said. 「Once you arrive at a coarse or fine setting, the only variation is how long to brew and the temperature of the water.」

For optimal extraction, water temperature should hover between 195 to 205 degrees Fahrenheit, according to the National Coffee Association, but brew time will vary by device. A French press should take two to four minutes; a drip system should take about five minutes for a liter of water.

The percentage of coffee that dissolves in the water is key to a brew’s final flavor. Over-extracted coffee will taste unappetizingly bitter. Under-extracted coffee will taste unpalatably sour.

You might anticipate that a coffee devotee would scarf down a pot a day. But Dr. Hendon usually has a single cup.

He begins by heating his home’s soft tap water in a kettle fitted with a temperature probe set to 99 degrees Celsius, or around 210 degrees Fahrenheit—just below the boiling point.

While the water heats, he weighs out 15 to 15.5 grams of whole beans using a digital kitchen scale and grinds them with an electric burr grinder. He pours the grounds and about 250 milliliters of hot water into an AeroPress, waits for 1 minute and 15 seconds and presses the plunger, forcing the coffee into a ceramic mug.

When the concoction cools to around 150 degrees Fahrenheit, he takes his first sip.

About 20% of the time, he said, it’s exceptional. Another 20% of the time it’s very good.

But even Dr. Coffee isn’t perfect.

「About 60% of the time, I’m quite unhappy,」 he confessed. 「I make a lot of bad coffee.」

Wall Street Journal / 925 words / May 23&24 2020

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