Coffee
Coffee was originally consumed as a food in ancient Abyssinia. The coffee plant is a relatively small tree or shrub, often controlled to a height of 2-3 meters, belonging to the family of the Rubiaceae. Coffea arabica accounts for 69%, Coffea robusta 30%, and Coffea liberta and other with 1% of the world production. Each of this species includes several varieties. After the spring rains, the plant produces white flowers. About six months later, fruit, approximately the size of a small cherry, replaces the flowers. The ripe fruit is red or purple.
The other portion of the fruit is removed by curing: yellowish or light green seeds, the coffee beans, remain. They are covered with a tough parchment and a silvery skin. Each cherry normally contains two coffee beans. When the coffee cherries are thoroughly dry they are transferred to hulling machines that remove the skin, pulp, parchment shell, and silver skin in a single operation.
Green coffee varies in composition according to the type of plant region from which it comes, altitude, soil, and method of handling the beans. Green coffee has no desirable taste or aroma; roasting develops these. Many complex physical and chemical changes occur during roasting including the obvious changes in colour from green to brown, and a large increase in bean volume. As the roast nears completion, strong exothermic reaction produce a rapid rise in temperature, usually accompanied by a sudden expansion, or puffing of the beans, with a volume increase of 50-100%. However, this behavior varies widely among coffee varieties due to the differences in composition and physical structure.
The flavour and aroma of roasted coffee are probably due in large part to breakdown and interaction of the amino acids derived from proteins.
| Constituents | Green % in dry basis | Roasted % in dry basis |
|---|---|---|
| Hemicelluloses | 23.0 | 24.0 |
| Cellulose | 12.7 | 13.2 |
| Lignin | 5.6 | 5.8 |
| Fat | 11.4 | 11.9 |
| Ash | 3.8 | 4.0 |
| Caffeine | 1.2 | 1.3 |
| Sucrose | 7.3 | 0.3 |
| Chlorogenic acid | 7.6 | 3.5 |
| Protein (based on nonalkaloid nitrogen) | 11.6 | 3.1 |
| Trigonelline | 1.1 | 0.7 |
| Reducing sugars | 0.7 | 0.5 |
| Unknown | 14.0 | 31.7 |
| Total | 100.0 | 100.0 |
Freshly roasted ground coffee rapidly loses its fresh character when exposed to air, and within a few weeks develops a noticeable stable flavour. The mechanism for the development of the staling is not known but is believed to be caused by an oxidation reaction that can be catalyzed by increasing levels of moisture.