Bitter truths: The science and perception of bitterness in coffee

Bitterness is one of the most contested flavor notes in coffee. For some, it signals strength and depth. For others, it’s a turnoff. Yet behind this one word lies a rich tapestry of chemistry, biology, and psychology. To appreciate it fully, we need to understand how bitterness arises, how we perceive it, and how it’s influenced by roast, brew, and even our genes.

The basics of taste perception

Bitterness is one of the five fundamental tastes, alongside sweet, salty, sour, and umami, and it has evolved as a biological warning system. In nature, bitterness often indicates the presence of toxic or alkaloid-rich compounds.

Humans detect bitterness via a family of approximately 25 bitter taste receptors, known as TAS2Rs. Here’s where it gets personal: not everyone has the same set. Some people have genetic variations that make them more sensitive to bitterness. These so-called 'supertasters' can perceive certain bitter compounds at concentrations 100 times lower than others.

So when one person calls a coffee 'pleasantly bitter' and another calls it 'undrinkable', they might both be right from their biological perspective.

What makes coffee bitter?

A single compound doesn't cause the bitterness of coffee, it's the result of dozens of chemical transformations that occur during roasting and brewing. Let's look at the main contributors.

The perception of bitterness depends not only on the presence of these molecules, but also on how they are extracted during brewing.

Caffeine alone contributes only ~10–15% to coffee’s overall bitterness (Ginz et al., 2000). Most of the perceived bitterness comes from the clorogenic acids' breakdown products.

Roast profile: how bitterness develops

Roasting transforms the raw precursors in green coffee into aromatic and flavourful compounds, but also into bitter ones.

Light roasts retain more chlorogenic acids, which taste sour rather than bitter.

Medium roasts convert chlorogenic acids (CGAs) into lactones, which are responsible for a more rounded bitterness.

Dark roasts produce phenylindanes, which taste sharper, harsher, and more persistent.

So, the darker the roast, the stronger the flavour of your coffee. But not all bitterness is the same.

Things that can affect how bitter your coffee tastes

Bitterness is a property that changes depending on how coffee is brewed. Here are the main things to consider:

1. Grind size and extraction time:
   
   

If you over-extract your coffee (brewing it for too long or using too fine a grind), you end up with more bitter compounds.
If you don't extract the coffee enough, the opposite happens, it's not sweet or full-bodied, it's kind of sour and thin.

2. Water: The hidden ingredient in bitterness
   
   You might have the best beans, the perfect grind, and a great brewer, but if your water's not right, your coffee will be too.

In Medellín, the water is pretty good for brewing coffee, it's considered soft to moderately hard, which means it doesn't have too many minerals that mess with the flavour. That's a win.

But there's a catch: the chlorine content is often high. Chlorine is added to disinfect tap water, but it can give your coffee a sharp, bitter, or even chemical aftertaste.

What can you do about it?

You can filter your tap water, and it's as easy as that. Even a basic charcoal filter pitcher (like a Brita or Aqualogic) can get rid of most of the chlorine and make your water clearer and more balanced. Don't use distilled or purified water; these have no minerals, so your coffee will taste flat and acidic. Make sure you taste your water, if it doesn't taste good on its own, it won't make good coffee. The water should taste clean and neutral, not metallic, not salty, and not like a swimming pool.

If you're brewing with Medellín tap water straight from the faucet, you're not far off a good baseline. Just filter it, and you'll be surprised how much smoother your coffee tastes.

3. Dose and brew ratio:
   
   If you use too much coffee and not enough water, you might end up with a bitter brew. For most pour-overs and filter brews, you should aim for 60–65g of coffee per litrer that's a 1:15 to 1:16 ratio.
   

Aroma, memory, and the illusion of bitterness

Here’s where it gets interesting: your nose influences your tongue more than you think.

Studies have shown that aromas can either amplify or suppress the perceived bitterness.  A 2022 sensory study (Rune et al., International Journal of Gastronomy and Food Science) found that adding citrusy or floral aroma compounds reduced bitterness perception, even though the brew's chemistry remained unchanged.

This is a classic example of cross-modal sensory interaction where our senses blend and reshape the experience.

And it’s not just scent. Even music, the color of the cup, and serving temperature can influence how bitter coffee tastes.

Why bitterness isn’t a defect

In the specialty world, bitterness often gets labeled as a flaw. But that’s a misread.

Bitterness, like acidity or body, is just one dimension of flavor. When balanced, it brings structure, complexity, and even elegance to a cup. A good shot of espresso, for instance, often combines acidity, bitterness, and sweetness in harmony.

What we call “too bitter” is often imbalanced bitterness untempered by sweetness, or over-extracted due to brewing errors.

How to train your palate

Bitterness, like acidity or fruitiness, is a skill your brain learns to interpret. It’s not just a matter of “liking” or “disliking” — it’s about developing awareness.

Here’s how to start training your taste buds:

Compare brews side by side

Brew the same beans two ways — one slightly under-extracted and one slightly over, and note how bitterness shows up differently. Try the same roast across different origins, or different roasts from the same farm.

Use a simple tasting journal

Even just noting "more bitter" or "less harsh" can help build your internal flavor map over time. Don’t worry about fancy terms at first; trust your sensory vocabulary.

Get structure and guidance

If you're feeling a bit lost or overwhelmed, our Beginner's Sensory Analysis Course is exactly what you need. It's designed to help you identify the main flavour notes in coffee, like bitterness, by tasting our farm's top-quality coffees and using simple tools that anyone can use.

Final sip: Rethinking bitterness

Rather than avoiding bitterness, try to understand it. It’s the unsung hero of flavor depth in many brews. Whether it’s a smooth Colombian washed or a bold dark roast from Brazil, bitterness carries complexity, structure, and character.

References

• Drewnowski, A. (2009). The science and complexity of bitter taste. Nutrition Reviews, 59(6), 163–169. https://doi.org/10.1111/j.1753-4887.2001.tb07007.x 
  
• Gao, C., Tello, E., & Peterson, D. G. (2023). Identification of compounds that enhance bitterness of coffee brew. Food Chemistry, 415, 135674. https://doi.org/10.1016/j.foodchem.2023.135674 
  
• Masi, C., Dinnella, C., Monteleone, E., & Prescott, J. (2015). The impact of individual variations in taste sensitivity on coffee perceptions and preferences. Physiology & Behavior, 138, 219–226. https://doi.org/10.1016/j.physbeh.2014.10.031 
  
• Rune, C. J. B., Münchow, M., Perez-Cueto, F. J. A., & Giacalone, D. (2022). Pairing coffee with basic tastes and real foods changes perceived sensory characteristics and consumer liking. International Journal of Gastronomy and Food Science, 30, 100591. https://doi.org/10.1016/j.ijgfs.2022.100591