"Acidity" is a term that I am sometimes afraid to use at public cuppings. It carries a very negative connotation in many people's minds. Many people who do not like to drink coffee claim that it is "too acidic," and worry that it will upset their stomach. When I talk about acidity for the general public, I often use code words that sound a lot nicer, like "life," "brightness," or "snap."
But a coffee with great acidity is not the same thing as a very acidic coffee. Coffee has lower acidity, on a pH scale, than most colas and diet colas and lower, in fact, than apple juice. It's simply not that acidic of a beverage, regardless of how it tastes.
But the perception remains, and there's a reason for it. The reason is this: bad coffee has bad acidity.
By "bad acidity" I mean, very simply, acidity that tastes bad. Of course, it gets very complicated very fast on a molecular level. There are many, many, many different acids present in a cup of coffee, each with its own distinct molecular composition and each with a distinct taste. But you don't have to know them all to begin to make sense of acidity.
Two major categories are organic acids and chlorogenic acids. Organic acids are our friends. Some common organic acids in the flavor of coffee include citric acid (think citrus fruits), malic acid (apples), lactic acids (often buttery flavors), and acetic acids (the major component acid of vinegar, actually). All by themselves these acids are way too intense. But mixed in with the innate sugars and other carbohydrates in roasted coffee, they provide a pleasing punch to the cup, or a refreshing kick.
Chlorogenic acids are not so great. They are present in any cup of coffee, though in what mix and in what proportion varies greatly. Generally, they are more intense in robusta than arabica, just like caffeine. Caffeic acid and quinic acid are two major chlorogenic acids. These taste pretty foul: bitter, harsh, and often metallic. Quinic acid is related to quinine, which is what gives tonic water its distinct taste. Think about the metallic punch in a gin-and-tonic and now relate that flavor to the taste of some terrible diner coffee you once had, the kind that's been sitting on a hotplate all morning... can you make the connection in your mind between those two tastes? That's quinic acid.
This latter category of acids is usually what makes people scowl when they hear the term "acidity" in relation to coffee. But, as I said, it doesn't have to be like this. Acidity can be lovely. Think of fresh squeezed orange juice, the really sweet kind; or the wonderful snappy taste of biting into a tree-ripe apple. Acidity at its best blends seamlessly with the sweetness of coffee to give you a pleasing sensation beyond the sum of its parts. When coffee judges score coffees on acidity, they are ranking the quality of the acidity, not the intensity. A very subtle, delicate acidity can score very high, just as an overpowering, sour acidity scores very low. Usually, the highest scores go to coffees that have acidity that is both salient and refined.
Organic acids are present in green coffee before it is roasted, but much of the acidity that ends up in the cup is created early on in the roasting process. But once sugars begin to caramelize, the acidity slowly goes down. As a rule of thumb, acidity goes down as coffee is roasted darker. This is a good thing if you have low-quality green coffee: you can basically cook off much of the offensive acidity. But with high-grade green, it's completely unnecessary: these coffees have a natural sweetness and are well-roasted enough to give caramel or chocolatey flavors, and only benefit from having a little citrus or apple-like snap in the cup. As the quality of green coffee arriving on the shores of the United States slowly creeps up the yumminess scale, you see more and more roasters taking their coffees a little lighter and more and more people buying these coffees. Dark roast is, of course, still exceedingly popular, but it's not the only way of dealing with "bad acidity" anymore... another solution is even more elegant: coffee with naturally good acidity!
There's lots more to say about acidity, but this is just a primer, not a paper for a chemistry journal. Here are a few more interesting things to know about acidity in coffee...
--The higher the elevation at which the coffee was grown, the more acidity it has. Speaking very generally.
--East Africa is probably the region most associated with incredible acidity, and especially Kenya.
--Astringent, or clinging soury acidity in coffee is often associated with coffee that was picked before the cherries were ripe. Cuppers often call these coffees "greeny" or "greenish."
--The anti-oxidant health benefit of drinking coffee is actually tied to the not-so-nice-tasting chlorogenic acids, and is more prevalent in ugly robusta than lovely arabica. But don't worry... there are still plenty of anti-oxidants in coffees with subtler chlorogenics. Any body-health benefit to robusta is offset by the price you pay in soul-health by drinking it.
If you want to train yourself to better evaluate acidity, here's a great activity: go to the supermarket and buy as many different kinds of apples as they have. Then take these home and cut them up and make a little chart. Try each apple and rank it in order of sweetness and tartness (a different rank for each category). Notice how the two can balance one another. This is very similar to how sweetness and acidity interact in a cup of coffee.
Another good training tool is mix lemon juice, sugar and water in different concentrations using a blind coding system and see if you can guess which is which (like in Q-grader training)
Of course, the best training tool is just to drink coffee and to slow down and notice carefully the sensations in your mouth and your mind as you taste it. Great coffee will always reward you for this activity with pleasure. As for less-than-great coffee... who needs it, right?
