There are three main commercial varieties of coffee bean from amongst many, although just two of these are widely used: Arabica and Robusta. Arabica is the most widely grown and preferred bean, growing on higher land, usually between about one and two thousand meters. Robusta, a more hardy variety, is grown at lower altitudes and requires less rainfall. It is important for blending and is widely used for instant coffee. The third recognised commercial variety is Liberica, also hardy and low-altitude growing.
The shrub or small tree, 4.6 to 6 m (15 to 20 ft) high at maturity, bears shiny green, elliptic leaves and white, fragrant flowers that bloom for only a few days. During the six or seven months after flowering, the fruit develops, changing from light green to red and, ultimately, when fully ripe and ready for picking, to deep crimson. The mature fruit, known as a cherry, grows in clusters attached to the limb by very short stems, and usually contains two seeds, or beans, surrounded by a sweet pulp.
The coffee tree produces its first full crop when it is about 5 years old. Thereafter, it produces consistently for 15 or 20 years. Some trees yield 0.9 to 1.3 kg (2 to 3 lb) of marketable beans annually, but 0.45 kg (1 lb) is considered an average annual yield. This explains the high price of coffee compared to tea. Coffee picking is a highly skilled activity and it is only recently that the more prosperous growing countries have begun to use mechanised methods, in conjunction with optical detection systems to weed out the unripe beans.
Two methods are used for curing the coffee beans. The wet method produces more flavoursome coffee than the simpler and cheaper dry method.
In the wet method, machines remove the pulp of the ripe coffee cherries (softened in water), exposing the beans’ protective coat of parchment. The beans are then soaked and fermented in large tanks to loosen their covering which is washed away with water until the beans are clean. This washing process causes a light fermentation of the sugars in the coffee beans, leading to an increase in the cherished acidity of the coffee. When more acidic beans are roasted they can develop a delicious tang. However, now that people take less sugar, many drinkers prefer the flatter taste of dry-processed coffee. The beans are then dried by the sun or a machine to be put into a huller which removes the remaining covering. The polished beans (called green coffee) are then ready to be sorted by hand or machine to remove defective beans and extraneous material, and finally to be graded by size.
There is an unfortunate though very rare occurrence of over-fermented beans, which have been trapped in the machinery, escaping detection – one such bean will contaminate any coffee ground with it, making it foul and extremely bitter. These are sometimes known in the trade as ‘stinker beans’ and particularly affect Kenyan coffee.
Cheaper commercial coffees can aquire a characteristic sour fruity taste due to off-flavours from unripe beans and even stinker beans. Robusta coffee is often added to this which masks these rancid flavours with a thick dull body, leaving a stale rather bitter effect. This is the characteristic bitterness of cheaper instant coffees.
As with decaffeinated coffee, instant does not have to taste a lot worse. However, to pay for the extra processing, the giant corporations who control the instant market compensate by using inferior beans with plenty of robusta. Instant coffee that tastes as good as medium quality ground coffee would inevitably cost more.
Decaffeinated coffee is produced out of green coffee beans before they are roasted. All methods use the fact that caffeine is water soluble whereas the coffee oils, which produce coffee’s liquor, are not. Some methods of decaffeination use chemical solvents which have been associated with cancer in larger amounts.
In the dry processing method, the coffee cherries are spread thinly on mats or drying grounds. They are raked and turned frequently to dry evenly in the sun, which may take two weeks or more. Once dry, the cherries are put through hullers which remove the dry pulp, the parchment and the silver skins.