Coffee roasting information - By Dave Corbey
Coffee roasters operate at high temperatures, and the green (un-roasted) coffee beans (Colombian Supremo shown in the picture) are roasted for a period of time depending on the degree of roast required. Commercial roasters are either horizontal rotating drums that tumble the green coffee beans as the heat energy is applied; the tumbling prevents burning and gives an even roast; or fluid bed roasters, where hot air is blown through the beans (which tumble in the air alone and roast twice as quickly as in drum roasters). The drum roasted method is slower, but preferred by speciality coffee roasters who believe it gives a better flavour in the final product.
When roasted green coffee beans expand, changing in colour and density. As the bean absorbs heat, the colour shifts to yellow, then to a light "cinnamon" brown and then a dark and oily colour. The roast will continue to darken until it is removed from the heat source. During roasting (or shortly after) oils can appear on the surface of the bean.
Coffee roasting coaxes flavour from a bland bean; un-roasted beans have all of coffee’s acids, protein, and caffeine, but none of its taste. It takes heat to spark the chemical reactions that turn carbohydrates and fats into aromatic oils, burn off moisture and carbon dioxide, and alternately break down and build up acids, unlocking the characteristic coffee flavour.