Brewing Up an Acid Test for Coffee

In coffee circles, it’s long been taken as gospel that the presence of different acids in coffee are dictated by where the beans were grown, says Christina Birke Rune, a doctoral student at the University of Southern Denmark. “They say that you can specifically know where the coffee is from on the basis of which acidity you can taste in the coffee,” she says.

Identifying coffee origin by perceived acidity is part of coffee experts’ sensory evaluation process and even the Q Grader exam (a coffee quality evaluation certification) includes naming the acidity in different coffees. Yet when Rune’s colleague Ida Steen, a sensory scientist who specializes in coffee, took the Q grader exam, she couldn’t identify the coffees by their acidity. None of Steen’s fellow students could, either. That’s because it isn’t possible, say Rune and her co-researchers, who detailed their findings in an article in Science Talks titled Acids in brewed coffee: Chemical composition and sensory threshold.

Rune first did a literature search and found that there is no scientific proof that backs up these claims about acidity in brewed coffee. “Nobody really looked at brewed coffee,” she says. Some studies, she notes, examined acidity in brewed coffee without defining what brewing meant or offering specifics of the roasting process, which chemically alters coffee’s acid composition. “We all know that brewing has a great impact on the coffee,” she says.

So, Rune and her colleagues designed a study to test their hunch that the origin of the coffee was less important than the roasting for acidity. What they found proved their hypothesis that coffee drinkers can’t taste the different acids in their coffee and there’s no chemical evidence to support identifying the geographical origin of coffee based on its acid content.

The researchers roasted five arabica coffees, two each from Brazil and Kenya and one from Bolivia. The beans were roasted in a range from very light to light roasts and then brewed in a French press, chosen because it’s both a simple process and it’s the closest to the preparation method used for cupping (brewed coffee tasting) sessions.

They then did chemical analyses of the acids in each brew to find out the exact amounts of various acids in each. Coffee’s main acids are citric, chlorogenic, lactic, phosphoric, malic, acetic, formic, and glycolic. The degree of roasting changed the acid composition of the coffees, with some acids (citric and chlorogenic) decreasing, and some (glycolic and lactic) increasing as the roasts darkened.

Sensory Tests

Next, two sensory tasting sessions were held. The first was a threshold test using 40 untrained panelists who tasted a blend of Kenyan and Brazilian coffees that had acids added to them, along with a control. The tasters were tasked with judging which were more acidic. The second session used 13 coffee experts. They were asked to identify which of five specific acids—citric, acetic, phosphoric, lactic, and malic—they tasted in each preparation.

What the researchers found is that, with the exception of citric acid during the threshold test, none of the tasters could identify the acids, and there wasn’t a strong link between acids and coffee origin.

This research has prompted a host of questions, says Rune. She and her colleagues are currently working on how water composition might affect brewing. Aroma is another important topic, she says. Coffee from Kenya seems to have a more acidic taste than other types, but its chemical makeup isn’t any different; it does not, for example, contain more acids than a Brazilian coffee. This caused the researchers to wonder if aromas are influencing acidity perception.

“Coffees from Kenya and Africa, they’re more fruity,” says Rune. “And when you think fruit, you think acidic instead of chocolate, which is more sweet. So maybe that affects the perceived acidity,” she says.

The response from the coffee community has, in general, been one of relief, says Rune. Roasters, coffee championship competitors, and others, she says, “have reached out to us. They’re like, ‘This is super interesting, and I’m so glad because I thought I was dumb not being able to taste the difference between these acids and name them.’”ft