What About Acids in Coffee?

In our last Pax Culinaria post, we talked about how acids affect the perception of water. But what about coffee? It's a special case, and this bonus Pax Coffea post explains it all.

After writing about how acid affects the perception of water, of course I had to follow up with a post about acidity in coffee. It’s an interesting topic- poorly understood by most, but very important nonetheless. One crazy thing- though acidity in coffee is prized by most coffee experts, it’s frowned on- at least in principle- by most consumers. What could cause this paradox?

First things first: like most foods and beverages, coffee is mildly acidic. The pH of a cup of coffee will vary according to several factors, but generally it will come in around pH 5: that’s more acidic than milk but less acidic than soda and wine. So there is definitely acid in coffee. But how much? And what do acids do, anyway?

As I mentioned in the previous article, acids generally stimulate the sensation we call “sour”. However, in coffee, we have something of a taboo against using the term sour, unless we are talking about a specific kind of defective or immature coffee. For this reason, coffee has adopted the term “acidity” when describing a coffee’s positive sense of sourness. Since elsewhere the term “acidity” is reserved for the chemical measurement of acids, and “sourness” is used for the taste of acids, coffee’s use of “acidity” to mean “sourness” is confusing. But it gets worse: since both sourness and acidity are thought of by consumers as being broadly negative in coffee, coffee marketers have learned to use euphemisms like “bright”, “crisp” or “citrusy” to describe the pleasant sensation of sourness in coffee. Confusing.

But that’s not the only confusing thing: sourness is a polarizing taste. We love sour foods- from citrus fruits to pickles to grapes. But too much sour turns us off. Flavor scientists refer to this as an “inverse U-shaped hedonic response” which is to say we like a little sourness more than no sourness, but too much sourness and we hate it. And this is exactly the case with coffee: the right amount of sourness makes coffee taste delicious and mouthwatering, balancing out coffee’s inherent bitterness and enhancing its natural sweetness. A coffee without acid tastes drab and lifeless, ashy and hollow. But a twinge of acidity- just enough- brings a coffee to life and makes it compelling and delicious. So, in coffee, “sour” in coffee is when there is too much or unpleasant sourness, “acidity” is when there is a good, balanced, pleasant sourness.

An illustration of an inverse U-shaped hedonic response, illustrating the effect of sourness in foods

For these reasons, “acidity” has been a term used by coffee experts for over a hundred years. Certain coffees- like coffees from Kenya and Costa Rica- are thought of as having an especially pleasant and intense acidity. Lots of processing and agricultural factors seem to influence acidity in coffee, which is one reason why coffee buyers obsess about these details. There is a whole section on the Coffee Taster’s Flavor Wheel dedicated to acids and sourness- right there next to the citrus fruits. But what about coffee drinkers? The inverse u-shaped response also explains why coffee buyers fetishize acidity in coffee but consumers avoid it- they are talking about different things. Coffee people are talking about a moderate, balanced acidity- they sometimes call it “sweet acidity”- that brings coffee to life. Consumers are talking about sourness- which they associate with fruit juices and vinegar. Different things.

The portion of the Coffee Taster’s Flavor Wheel focused on acidity

Ok, let’s talk chemistry. Sourness and acidity come from acids, and like many plants, coffee has several acids stored up in its tissues. For convenience, we can divide them into two categories, organic acids and chlorogenic acids. Organic acids seem to have a more powerful effect on flavor than chlorogenic ones, so we’ll focus on those. Of the organic acids, citric and malic acids are the most important ones in coffee. There is a persistent myth that citric acid tastes like citrus and malic acid tastes like green apples, but in truth they are hard to distinguish from one another: they both taste just sour. And, coffee tasters can’t distinguish between them in coffee anyway. Other acids exist, but mostly seem to play a supporting role (one of these is acetic acid, which is a special case I’ll discuss elsewhere). Both citric and malic acids degrade during roasting, which is why light roasted coffee tastes more sour than dark roasted coffee. Acidity is one of the important tastes that roasters control- a little extra roasting will temper the acidity in a coffee, but too much roasting will extinguish it, leaving the coffee flat and ashy tasting. For this reason, many roasters who like to roast dark buy coffees that are extremely high in acidity in the first place- then they can roast dark and still have a little bit of sparkle in the cup.

All of this talk of chemistry might make you want to whip out your pH meter and start measuring your coffee’s acidity to predict how it will taste. Well, funny thing: it turns out that just measuring the pH of a coffee will tell you little about its flavor. This is true in wine and other foods as well, which is why there is another measure of acidity used for foods, called titratable acidity. Instead of measuring hydrogen ions as pH does, titratable acidity (TA) measures how much of a substance needed to neutralize a certain amount of a base. TA is a much stronger predictor of sourness in foods and beverages than pH, but it’s still not perfect. That means other factors are at work in coffee to make it taste more or less sour- perhaps the coffee’s bitterness is amplifying the perception of sourness, or the sweetness is muting it. Aromas can play a role too- fruity aromas might increase the perception of sourness in coffee. All this means that a coffee’s measured acidity (its pH or TA) might not be the same as its perceived acidity (sourness). From here on out, I’ll try to be specific when I talk about the two.

So, as a coffee drinker, how do you optimize the perceived acidity you like in a coffee? First, learn to recognize it: if you drink a coffee and you feel a pleasant tang, or mouthwatering sensation, that’s perceived acidity. High perceived acidity might taste sour, like in a very light roasted espresso. If you taste fruit, there is a good chance you’re tasting acidity too. Lower-acid coffees tend to taste nutty, or like cocoa powder. Both can be delicious. Brewing can modulate acidity too- using a higher coffee- to- water ratio has been shown to increase sour tastes in coffee. Cold brewing has been shown to reduce acidity in coffee too.

If you want to explore acidity in coffee, here are some tips: since perceived acidity lessens during roasting, light roasted coffees tend to have much higher perceived acidity than dark roasts. Coffees from Kenya, Ethiopia, and some places in Central America will tend to have high acidity. Look for terms like “bright”, “crisp”, and “citrusy” on the label.

An example of a coffee likely to have a high acidity: “bright”, “lemon”, and “light roast” are the giveaways.

If you want to taste lower acid coffees, try darker roasts- the roasting minimizes much of the perceived acidity in the coffee. Coffees from Indonesia, especially Sumatra, are well known for having gentler acidity, as do many coffees from Brazil. Look for terms like “nutty”, “chocolatey”, and “spice”- they often accompany lower-acidity coffees.

So, though acidity is an essential and desirable flavor characteristic in coffee, in excess it can turn people off. And because taste is personal, that place is likely to be in different places for different people. I happen to love intense acidity in coffee- I find it cleansing and exciting- but I know and respect lots of coffee drinkers who favor milder, less-acidic tastes. Now, go drink some coffee!

p.s. In my view, the perfect and best representation of acidity in coffee is in iced coffee, my favorite summertime drink. A crisp and bright acidity in cold coffee does exactly the same thing that acid does to water- it makes it more refreshing, thirst-quenching, and satisfying on a warm day. That’s why I you will find me drinking washed Ethiopian and Kenyan coffees, served cold over ice, between May and September every year.

p.p.s. You’ll notice I did not mention anything about the relationship between coffee acidity and gastric discomfort, which has been a hot topic (no pun intended) in coffee. This is very intentional. I am not an expert on coffee and health issues, so I generally try to scrupulously avoid discussing the health implications of coffee. I will say that, in my reading, the evidence linking coffee and gastric distress just isn’t there. But that’s not uncommon for dietary studies- it’s very very difficult to study the effects of foods on the health of human subjects. I believe people who report gastric distress after drinking coffee, but neither the effect nor the cause seems to have been demonstrated very well. I always give the same advice to people who ask me: listen to your body, and try a variety of coffees in the marketplace- some are designed to offer better compatibility to those with sensitive stomachs. If low-acid coffee works for you, by all means drink it!

p.p.p.s. For coffee science geeks, please do check out the papers linked in this post. While writing, I was reminded of the amazing work coffee scientists have been doing- most within the past 5 years- on coffee acidity.