One pesky test of the Q exam has to do with Organic Acids and Chemistry of Coffee. In a nutshell (or a coffee husk) we broke down five of the most prevalent acids found in coffee, differentiating between their tastes, and what happens in the roasting process to each of them.
The five acids we tested for were Citric, Acetic, Quinic, Malic, and Chlorogenic. We were given a control sample of coffee along with a cup of coffee having the added acid, as a means of comparing. Side-by-side it was easy to differentiate between the control and additive, an easy pass.
So here we go...
Citric Acid is known for having a sour and fruity taste to it and found in fruits like lemons and oranges. Like all living organisms, citric acid plays an important role as a key intermediate compound in the plant's metabolic life. In green coffee, citric acid along with malic and quinic acid constitute a significant portion of coffee's total acid content and in the development of perceived acidity. During roasting, citric acid reaches a maximum at light to medium roasts, then quickly diminishes as roasting levels progress. A typical medium roast will lose about 50% of its initial citric acid concentrationand diminishes further as roasting progresses . Generally, Kenya coffee tend to have lower levels of citric acid than those of Central America, suggesting a less advanced form of plant metabolism. Think of all the wonderful citrus notes in Latin American coffees.
Acetic acid, or more commonly known as the active ingredient in vinegar, is produced during two stages of coffee processing: post-harvest fermentation and roasting. This is found in high doses of wet processed coffees. With this particular acid, there comes a point in roasting where it starts to evaporate away, so the darker you roast the beans, the more it will be diminished. In low concentrations acetic acid imparts a pleasant clean, sweet-like charateristic to coffee infusions, but can quickly become ferment-like at higher concentrations.
Quinic acid has a bitter, astringent taste. Quinic acid is the result of Chlorogenic acid being broken down during the roasting process to make Quinic and Caffeic acid. Chlorogenic acid is bitter to start with so roasting the bean is a double whammy… the bitterness is enhanced. Brewed coffee that sits on the heater will develop a quinic taste. It is best to enjoy this taste in a gin and tonic rather in your cup of coffee.
Malic acid is found in apples and gives a tart taste and leaves a lingering flavor on your tongue. It is found in the coffee plant itself so roasting does not have much effect on it. I tasted and blogged about a lot of malic apple notes in our Panama Duncan Estate coffee.
Chlorogenic acid, CQA, leaves a bitter flavor on the backside of your tongue. Robusta coffee has twice as much Chlorogenic acid, which makes sense because I knew Robusta coffee typically has twice the amount of caffeine in it as Arabica, and CQA and caffeine go hand-in-hand. This is also another reason Robusta coffee is so robust...it has a double layer of defense. CQA and caffeine are too bitter and caffeinated that pests steer clear from the plants.
Phosphoric acid has no taste but it helps you to perceive sparkling acidity. Scott Labs botanists searched for different mutations of French Mission and Mocha, Yemeni Typica, and began cross breeding them. They wanted more naturally occurring phosphoric acid in their taste profile. Phosphoric acid has no taste but it adds an amazing dancing and sparkling acidity that remains in the front and back of the taste. There is some great info on SL28 Phosphorus in a 1971 article by Cannell called "Uptake and distribution of macro-nutrients in trees of Coffea arabica L. (SL28) in Kenya as affected by seasonal climatic differences and the presence of fruits." The article says something like 95% of total tree Phosphorus uptake during ripening goes to the fruit. A large amount of Phosphorus is diverted to the fruit from other parts of the tree.