I received an email seeking clarification on my wild claims and speculation from last week. A quick recap: working against the one-note flavours of dark roast, modern coffee works to highlight the floral, citrus, berry, fruit, caramel and chocolate flavour notes tied to origin or terroir. The problem: roasting is quite difficult. Meaning, more often than not, modern coffee fails to create well-structured and balanced cups. Instead, we have roasts that emphasize sour and/or bitter tastes.

This week’s question: “I understand tasting sour and bitter flavours when dialing-in but not sure I totally understand how it relates to the roasting part”. I love this type of feedback! I frequently burn through the details, so if you are ever confused, do not hesitate to reach out. Let’s start with the basics of sour and bitter with a fun chart that outlines how these tastes commonly appear as flavours:

While it is absent from the chart, you can imagine sweetness right in the middle. When dialing-in espresso or batch brew, you will often find yourself bouncing around in between these types of tastes, like so:

Sadly, not all coffees have that sweet spot. It requires the expertise of a seasoned producer, favourable growing conditions, precise processing and an experienced roaster to properly develop the coffee. The usual logic mirrors the extraction process above with roast colour correlating to taste:

You will see this kind of rationale in many forms but most predominantly on tasting wheels. From underdeveloped to overdeveloped, from light to dark, you see the transition from sour to bitter. But something does not line up! Now it is true that as you lessen development in a roast, you will move from floral to citrus to green to vegetable flavours. However, this does not align or parallel the same progression away from bitter and towards sour that is present in espresso or filter extraction.

If you look at the flavours underneath the tastes in that first chart, citrus pulp is sour but zest is bitter. Moreover, all vegetables line up on the bitter side rather than the sour side. If we apply this to roasting, the chart should look more like this:

There are points on either end of the development spectrum in which bitter flavours will appear in the cup. When I speak of the danger of one-dimensional and universalizing flavours that mask origin, we can see that they emerge in both light AND dark roasts.

In a recent paper titled “Dual Mechanism for Bitter Avoidance in Drosophila” (available here), researchers discovered that bitter components in food can actually suppress perceived sweetness. The study used flies to examine how bitter notes will signal potential toxins and in order for the brain to receive these warnings, bitterness overwrites sensations of sweetness. They link this to other species, including humans, who also have an evolutionary history in which bitter tastes alert the brain to potentially harmful toxins. Interesting to consider given the abundance of possible bitter notes in a cup of coffee regardless of roast degree.