Reddening
Pigments are the chemicals responsible for the colour of a fruit’s skin and flesh. As fruits ripen, pigments undergo many changes, including the following (M. Knee, 2002, pp. 9–10):
- Loss of chlorophyll (green color), influenced by pH changes, oxidative conditions, and the action of chlorophyllase (an enzyme that breaks down chlorophyll)
- Synthesis and/or appearance of carotenoids (yellow and orange colors)
- Development of anthocyanins (red, blue, and purple colors)
The exocarp (aka epicarp), more commonly called the skin, is green for most of the coffee fruit’s development. Close to the end of the fruit’s maturation process, though, chlorophyll pigments disappear and beans become yellow in colour. The cells that make up the exocarp begin to accumulate anthocyanins, a large group of red-pigmented flavonoids. This pigment can range in colour from pink to burgundy.
Anecdotal evidence suggests that coffees ripen to different shades of red depending on their cultivar or the terroir in which they are grown. However, research by Prata, E. R. B. A., & Oliveira, L. S., (2007) found ‘no significant differences in anthocyanin contents among five different coffee varieties studied.’ In particular, the researchers was looking at the untapped potential to harvest pigments from coffee skins as a potential source of new income for farmers. They examined the Red Catuai, Icatu, Mundo Novo, Acaiá do Cerrado, and Rubi varieties. Even though anthocyanin is a large group of red-pigmented flavonoids, Prata and Oliveira (2007) identified two chemicals from this family — cyanidin 3-rutinoside and a small amount of cyanidin 3-glycoside — as the major precursors to the red colour in the five varieties they studied. Their results suggest that ripe red coffee colouration may not be particularly varied across the globe.
Yellow and Pink?
Of the flavonoids found in coffee, a yellow pigment called luteolin is responsible for making certain varieties ripen to yellow instead of red. This pigment is found in red-ripening coffee cherries too,