When the raw coffee beans are first added to the coffee roaster they are green in colour. As heat is applied they gradually change to yellow, then to light brown and these brown tones gradually darken until the exact degree of roast is reached. The roasting process is of course much more complicated than that. On a chemical and physical level, the roasting process is what produces the characteristic flavour of coffee by causing the green beans to expand and to change in taste, smell, and density. Unroasted beans contain acids, proteins and caffeine, but lack flavour. Roasting effects the Maillard reaction (named after the chemist Louis-Camille Maillard who 'discovered' it during 1910-1920) which is a chemical reaction between an amino acid and a reducing sugar using heat to produce odours and flavours. In layman's terms the coffee roasting process is broken down into three stages: sorting, roasting and cooling. In larger scale roasting houses the bags of green coffee beans are hand or machine-opened, dumped into a hopper and screened to remove debris. The green beans are then weighed and transferred by belt or conveyor to storage hoppers, and from there the beans are conveyed to the roaster. Roasters typically operate at temperatures between 370 and 540 °F (188 and 282 °C), and the beans are roasted for a period of time ranging from anything between 3 to 20 minutes. Commercial roasters are typically horizontal rotating drums that are heated from below and tumble the green coffee beans in a current of hot gases. The heat source can be supplied by gas, electricity or even wood. These roasters can operate in either batch or continuous modes and can be indirect- or direct-fired. Indirect-fired roasters are roasters in which the flame is never in contact with the beans, whereas direct-fired roasters contact the beans with the burner flame and the combustion gases. In large scale roasting operations, where price is the premium consideration, a Turbo Roaster is utilised. This machine can reduce roasting times from say 15 minutes in a traditional roaster down to as little as 3 minutes. Great for high volume, but unsurprisingly perhaps, there is a quality issue. Accelerated or turbo roasting precesses are not normally used for producing high quality coffees often used in high steeet coffee establishments. During the roasting process the bean undergoes a number of changes. The first change is endothermic, where the green beans are slowly dried to become a yellow colour and the beans begin to smell like toast or popcorn. The second step, often called the first crack, occurs at approximately 205 °C in which the bean doubles in size, becomes a light brown in colour, and produces a popping or cracking sound. In the next step the temperature rises from 205 °C to approximately 220 °C, and the colour changes from light brown to medium brown. The fourth step occurs between 225-230°C, when the roast colour is defined as medium-dark brown and goes through - and is called - the second crack. This second pop or crack is much quicker sounding and the beans take on an oily sheen. Further roasting produces a dark colour - the beans become smoky and the natural sugars begin to carbonise. These rich dark roasts are favoured for French or espresso blends in coffee machines. At the end of roasting, the roasted beans are cooled using a vacuum system. Roasted coffee beans can also be cooled using a fine water mist, which is known as "quenching", however this is considered inferior to air cooling as the water soaks the fresh beans with moisture and oxygen particles causing quick deterioration. The roasted and cooled beans are usually left to 'degas' for a period of time. The roasted beans are then packaged, usually in light-resistant foil bags fitted with small one-way 'aromalock' valves to allow further gasses to escape while protecting the beans from moisture and oxygen. For storage purposes roasted coffee beans are still considered to be fresh under 4 weeks old. However once the bean has been ground it should be used the same day.
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