A good cup of coffee has a balance between acidic, bitter, and sweet compounds. When you properly extract a coffee – which is usually defined by achieving an extraction yield between 18 and 22 percent – these acids are well-balanced. They do not disappear, rather other compounds help to take away from some of the sour tastes you can experience with acids.
Under-extracted coffee tastes sour because of the acids in the coffee. The goal of coffee brewing is to extract enough from a coffee so that these acids are balanced, without bringing out the compounds associated with intense bitter flavours.
Coffee contains a number of acids. I am presently learning about what contributes to the flavours of coffee. I wanted to revise my knowledge by listing these acids:
- Citric acid: Citric acid converts into sugars as coffee develops on the plant. Unripe coffee cherries contribute more citric acid to a cup. Citric acid is responsible for the citrus flavours in a coffee, such as lemon or orange.
- Acetic acid: Acetic acid is the acid present in vinegar. This acid is created during the fermentation process, where coffee cherries are put in sealed tanks. In these tanks, yeast is allowed to eat the sugars from the cherry mucilage. Acetic acid imparts a vinegar flavour on coffee if it is in high quantities. This is why it is important that fermentation does not go on too long. This acid can accentuate the fruity characteristics in a cup of coffee, if present in the right quantities.
- Phosphoric acid: This acid imparts a sparkling sensation on your tongue. Soft drink companies add phosphoric acid into their drinks. The quantity of phosphoric acid increases in roasted coffee due to reactions in the roasting process.
- Malic acid: Malic acid degrades during the roasting process which means there is not as much left in the roasted beans. Malic acid tastes like apple.
- Tartaric acid: This acid is present in wine grapes. It imparts very little flavour on a coffee because almost all of this acid is degraded in the roasting process.
- Lactic acid: Like acetic acid, an increase in lactic acid is a by-product of the roasting process. This acid, like acetic acid, can make a coffee taste fruity.
It is estimated that there are around 1,000 compounds in roasted coffee and scientists are still studying these compounds. Of the acids in coffee, those listed above contribute significantly to the flavours of the final brew.
There are two other important acids that impact coffee: quinic and caffeic acid.
Both of these acids are the result of cholrogenic acids (CGAs) breaking down in a cup of coffee. This happens as coffee cools down to lower temperatures. Quinic and caffeic acid impart bitter flavours on a cup. This is why you cannot brew coffee on a batch brew machine and store it all day without a loss in quality. The CGAs will break down as the coffee cools and make the brew taste more bitter.