Bitter has become a dreaded word in the specialty coffee industry. We all “know” that good coffee should be sweet and well-balanced, maybe a little acidic, and definitely not require sugar to make it palatable.
But what is it that causes coffee to be bitter – and is it always such a bad thing? Read on as we find out.
SEE ALSO: Coffee Science: Breaking Down Where Flavor Comes From
Coffee that hits the sweet spot. Credit: Matt Fury
Is Bitterness Bad?
Bitterness is not always a bad thing. In fact, if your coffee had no bitterness in it all, you might find it too acidic or sweet. The key is balance. A small amount of bitterness will help to ensure complexity and complement other flavors – without being overwhelming.
And that, really, is the problem. For most of us, throughout our lives, we’ve been more likely to be served a brew that’s far too bitter rather than one that’s too sour. This trend has led to a backlash against a trait that is actually essential to good coffee.
However, there’s no doubt that excessively bitter brews are a bad thing – so let’s take a look at what bitterness really is and how we can avoid tasting too much of it in our coffee.
Coffee being brewed on a Kalita Wave. Credit: Tyler Nix
What Is Bitterness?
Everyone can remember tasting something bitter. However, in many cases, that “bitter” food or drink might not have tasted bitter to someone else. This trait is a perceived taste, meaning it will change from person to person.
It’s also important to note that it’s not just taste that creates the experience of bitterness. Flavour is a combination of many things, including smell, emotion, music, and altitude. However, that’s a topic for another article; for now, we will be focussing on taste.
So let’s dig into some science and look at what causes bitterness in coffee. You may want to grab a cuppa and make sure you’re sitting comfortably!
A bitter brew? Hopefully not. Credit: Matt Fury
The Science of Bitterness
People used to think that the tongue was split into a kind of “taste map,” with different areas able to detect sweetness, saltiness, sourness and bitterness. But now, we know that flavour can be tasted in all parts of the tongue.
This is because our tongues’ sensory cells contain numerous proteins. And around 35 of these (according to the US America’s Institute for Quality and Efficiency in Health Care) react with the compounds in food substances to create the experience of bitterness.
This means that how bitter a coffee tastes comes down to these compounds, which are called phenolic compounds. Some of the most prevalent of these are the chlorogenic acids, which we’re going to talk about a lot. They account for up to 8% of the dry mass of green Arabica beans and have a large influence on the sensory elements of coffee.
There are many types of chlorogenic acid but there are two particular ones that you should know: 5-caffeoylquinic acid, which is the most common within green coffee, and di-CGA, which bears particular responsibility for coffee’s bitterness.
Although the majority of coffee’s bitterness comes from chlorogenic acids, Verônica Belchior, a Q-grader and PhD student researching the relationship between chemical compounds and flavours, explains that caffeine also plays a role. However, it is just a secondary one.
This coffee’s complex flavour compounds will interact with proteins in the tongue’s taste buds. Credit: Matt Fury
Bitter Green Beans
When we talk about bitterness in coffee, we often think of roasting (more on that to come!) – but some coffees are just more likely to create perceived bitterness than others.
To begin with, Robusta is far more bitter than Arabica. This is because it has more chlorogenic acids and caffeine. Chlorogenic acids in Robusta can make up to 10% of the dry mass – a whole 2% more than in Arabica. What’s more, Robusta has almost twice Arabica’s caffeine content.
SEE ALSO: Coffee 101: A VIDEO Guide to Arabica & Robusta
It’s not just the species and variety of coffee that affects its bitterness, however. In 2006, Adriana Farah and Carmen Marino Donangelo published a study on phenolic compounds in coffee in The Brazilian Journal of Plant Physiology. Their conclusion?
“Generic factors such as species and variety, the degree of maturation, and to some extent environmental conditions and agricultural practices, are important determinants of the composition of chlorogenic acids in green coffee beans, and will also affect the composition of the final beverage.”
They also drew attention to processing, particularly the monsoon method. This is an Indian tradition that exposes green coffee to humid monsoon winds. It has been found to reduce both chlorogenic acids and bitterness.
As for cherry ripeness, Verônica Belchior explains that “unripe beans, for example, have more chlorogenic acid content. That’s why the beverage is so astringent when there is a lot of these beans in a lot.”
Green coffee beans. Credit: Café Don Emilio
Does Roasting Increase Bitterness?
Okay, here comes the last bit of science, I promise!
Throughout roasting, something happens to the chlorogenic acids. They start to break down. And here’s the thing: while chlorogenic acids are responsible for the bitterness in coffee, they aren’t actually bitter. Not until the roasting process breaks them down into chlorogenic acid lactones and phenylindanes, that is.
Dr. Thomas Hofmann, a leading researcher on the topic, revealed in 2007 that these phenylindanes are what create the perception of bitterness. Moreover, the amount of phenylindanes in your coffee is linked to the roast profile
Light to medium roasts will have more of the acid lactones, which create what Hofmann describes as a “pleasant, coffee-like bitter taste quality.” Darker roasts, on the other hand, will have more phenylindanes, which creates a “lingering, harsh type of bitter sensation.”
So light to medium roasted coffees are likelier to taste less bitter and yet still contain those classic coffee aromas and flavours that we know and love. But of course, as we said before, bitterness is in the tongue of the taster (quite literally). Just because you dislike the taste of a dark roast doesn’t mean your friends will agree with you.
Freshly roasted beans at Saltspring Coffee Roasting Facility. Credit: Kris Krüg via Flickr, CC BY-SA 2.0
How to Avoid Overly Bitter Brews
So does this mean that, if you buy lightly roasted high-quality Arabica, you’ll be able to avoid a bitter coffee? Not necessarily. The brewer, whether that’s you or your barista, also affects the final taste in the cup.
To avoid bitterness, you’ll need to not over extract your coffee. This is because bitterness increases later in the brew. There are a lot of variables that affect extraction – brew method, grind size, water temperature, brew time… but there are some general guidelines that you can follow.
First of all, make sure you have the recommended grind size for whatever brewing method you’re using. As your grind size decreases, you increase the total surface area of your coffee and so may extract more flavour than you’d bargained for (plus, with drip/pour over brewing, it will increase your brew time and so also increase extraction).
The next thing is to check your water temperature. The hotter the water, the more efficiently flavour and aroma compounds will be extracted. If your coffee’s lip-curlingly bitter, you can try using water a couple of degrees cooler.
Then there’s the brew time: if your coffee has a bitter tang, you may be brewing those grinds for too long.
Remember, however, that extraction is a delicate balance of all these factors. And if you adjust one (such as grind size), then another (such as brew time) may also be affected.
A sweet filter coffee requires the right grind size, water temperature, and brew time. Credit: Matt Fury
Bitterness – it’s not bad, but when it drowns out all the other flavors in your coffee, it becomes a problem. Fortunately, as our understanding of the chemical compounds behind this flavor develops, we’re learning more and more about how to control its development and extraction.
So follow the steps I’ve listed for a perfectly balanced, complex coffee with just the right level of bitterness.
Written by Matt Fury, with thanks to Verônica Belchior and Scott Rao for their input.
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