Every coffee we drink is distinct in its flavours. Depending on the roasting and variety, coffee can be fruity, nutty, chocolate- or citrus-like. What is often overseen is that coffee is also particular in its taste and can show all kinds of acidity as well as bitterness. Speaking about bitterness, a coffee beverage can evoke various shades of bitterness from mild and pleasant to harsh and metallic.
What does research say about bitterness in coffee?
After decades of research, the question of bitterness is yet not so easy to answer. In former times and even today, it was assumed that caffeine makes coffee bitter. However, decaffeinated coffee tastes bitter too. In the end, science does not know completely which compounds make coffee bitter. That is why many researchers around the globe dedicated their work for elucidating the bitter taste of coffee and what can be done to have an alluring, balanced bitterness in the cup rather than a lingering, harsh one.
Many factors are taken into consideration to answer those questions for the perfect bitterness of coffee.
First of all, it is all dependent on the chemical composition of raw coffee. Chlorogenic acids have an impact on the bitter taste later on. Chlorogenic acid degradation products make up 60 – 70 % of the bitter taste in coffee. With up to 13% in raw coffee, these polyphenols play an essential role in the formation of all kinds of flavour-and-taste-active compounds in coffee. Besides, recent research shows that chlorogenic acids have all kinds of health beneficial effects such as reducing blood pressure, improving glucose regulation thus being Diabetes Type II preventive, as well as being anti-inflammatory.
A difference between good and bad bitterness
Back to the impact of chlorogenic acids on the taste of the coffee: Chlorogenic acids themselves are only tasting sour. How do they cause the bitter taste? During roasting, especially after the first crack and at bean temperatures around 200 °C, chlorogenic acids are converted into the corresponding lactones in a splitting of one molecule of water. That is the crucial chemical step turning the acidic chlorogenic acids into bitter tasting chlorogenic acid lactones. These lactones cause a mild, balanced, bitterness which is often described as “coffee-like” in the cup of coffee.
However, the reaction does not stop here. At darker roasts and higher bean temperatures above 210 – 220 °C, chlorogenic acid and the lactones undergo further degradation in a splitting of quinic acid (one moiety of the chlorogenic acid) and after some more steps, forming the metallic, lingering bitter phenylindanes. These compounds are somehow undesired as they make our cup of coffee harshly bitter tasting. Recently, research might also indicate that based on phenylindanes and lactones and a whole variety of minor tastes, active and bitter taste-enhancing compounds are formed in later stages of the roasting.
Extracting bitterness during coffee brewing
Besides the roasting, the preparation method is also highly influential on the bitter taste of the coffee.
Starting with the grinding of the beans, the grind size, respectively the particle sizes, have a significant impact on the bitter and sour taste of the coffee. Being said, a fine ground offers a larger extraction surface for the water to run through the grounds. As there is a larger surface, more of the bitter tastants can be extracted. If coffee is ground very coarsely, acidic compounds take over in the brew.
To put it short: the longer coffee is extracted, the more of the former mentioned bitter compounds, especially the harshly bitter-tasting phenylindanes are extracted into the brew.
Also, the temperature of the water influences the taste profile. Higher water temperatures result in a more bitter tasting cup than lower temperatures. It's recommended to keep the water temperature somewhere between 90 – 95 °C. Because if the temperature drops too low, it might lead to a flat flavour and taste profile (not applicable for cold brews, as the brewing time is way longer there).
When the coffee comes into our cup, it is also a matter of balance here. The grind size needs to be suited for the preparation method, and the method itself needs to be optimized in time and water temperature to make a well and balanced tasting cup of coffee.