Bitterness is often characterized by unpleasantness; if coffee is available on its own or in large quantities, so be it. But when combined with other flavor elements, such as sweetness and acidity, bitterness can add body and complexity to coffee.
Bitterness also balances out perceived acidity, which makes it an essential ingredient for a well-balanced cup of coffee. Several factors contribute to the bitter taste of coffee, some of which include: Quinic acid, trigonelline, alkaloids, caffeine, carbon dioxide, etc.
Coffee roasted for a long time usually provides more bitterness to the cup than coffee that is burned for a short time. This is partly because quinic acid continues to form as the coffee is roasted.
While many bitter compounds take longer to extract than sweet and acidic compounds, in return, bitterness is very tasteful; our senses are very “sensitive” to irritation, its compounds. Can quickly dominate the cup if given the opportunity.
Also, remember that Robusta coffee is more bitter than Arabica coffee, regardless of all other factors.