Coffee fruits develop over the approximately 35 weeks after flowering (Cannell,1985). The sign of coffee cherries reaching maturity is their change from green to red (and, sometimes, pink, yellow or orange.) The colour change is the result of the loss of chlorophyll from the pericarp (skin), which is replaced by flavonoid pigments as the beans mature (Lopes et al, 1984 and Clifford and Kazi,1987).
After immature beans are roasted, they are usually referred to as quakers. Their presence in green coffee is due to a high proportion of immature fruits at harvest (P. Mazzafera, 1998). After processing and drying, they usually have a yellow-greenish silverskin that is tightly attached to the bean, and are concave (SCA). After roasting, quakers don’t undergo the usual colour change from green to brown. Instead, they acquire a peanut-yellow colour.
Quakers have an unpleasant peanut and grassy or strawy flavour. Almost every retail bag of coffee purporting to be ‘specialty grade’ contains at least one quaker. Most grading systems consider five quakers to be equivalent to one full black bean defect. The Specialty Coffee Association stipulates that a specialty grade coffee can contain no more than five quakers.
A video where BH Dean of Studies, Jem Challender conducts a blind cupping of quakers against ordinary beans using a commercial coffee. We recommend you try this as a training exercise, to help you become more aware of how tainted coffee beans can diminish cup quality.
Processing and sorting can tend to reduce the amount of quakers found in green coffee, but colour sorters, which are widely used on Brazillian plantations, are not effective in removing quakers from green coffee. This is because the colour change becomes exaggerated only after roasting. Using a colour sorter after roasting is one approach to reducing the number of quakers in a coffee.
Chemical Properties of Immature Beans
Compared with healthy,