Coffee contains over a thousand different substances but only 50 or so are responsible for its typical taste and aroma. Many of these only emerge during roasting, a process which increases the size of coffee beans by 50-100%, reduces their weight, changes their colour from green to brown and releases numerous aromatic substances. The ultimate taste of the coffee is dependent on the roasting process, which is a mixture of art and science.
The two most common roasting methods are drum and hot-air roasting. Speciality coffees are generally drum-roasted in small batches for around 12 minutes at temperatures of up to 230° C. Since such small quantities could never quench the thirst of coffee's mass market, most coffee beans are industrially roasted on a current of hot air at much higher temperatures for as little as 3 minutes.
Industrially roasted beans have a more acidic taste whereas the slower roast eliminates unpleasant acids. During roasting sugars and other carbohydrates in the bean are caramelised to create a substance known as coffee oil. Though technically not an oil, this fragile substance is what gives a coffee its flavour and aroma. The longer a bean is roasted, the more oil is drawn to the surface.