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Coffee Roasting Process

bean1.jpgRoasting coffee is the transformation of the chemical and physical properties of green coffee beans into roasted coffee products.

The roasting process is integral to producing a savoury cup of coffee. When roasted, the green coffee bean expands to nearly double its original size, changing in colour and density. As the bean absorbs heat, the colour shifts to yellow and then to a light "cinnamon" brown then to a dark and oily colour. During roasting oils appear on the surface of the bean. The roast will continue to darken until it is removed from the heat source.

At lighter roasts, the bean will exhibit more of its "origin flavour" - the flavors created in the bean by the soil and weather conditions in the location where it was grown. Coffee beans from famous regions like Java, Kenya, Hawaiian Kona, and Jamaican Blue Mountain are usually roasted lightly so their signature characteristics dominate the flavour.

As the beans darken to a deep brown, the origin flavors of the bean are eclipsed by the flavours created by the roasting process itself. At darker roasts, the "roast flavour" is so dominant that it can be difficult to distinguish the origin of the beans used in the roast. These roasts are sold by the degree of roast, ranging from "Light Cinnamon Roast" through "Vienna Roast" to "French Roast" and beyond. Many consider that a "full city" roast is a great roast because it is "not too light" and "not too dark".

In the 19th century coffee was usually bought in the form of green beans and roasted in a frying pan. This form of roasting requires much skill to do well, and fell out of favour when vacuum sealing of pre-roasted coffee became possible. Unfortunately, because coffee emits CO2 for days after it is roasted, one must allow the coffee to get slightly stale before it can be vacuum sealed. For this reason two technologies have recently been employed: Illy has begun to use pressurised cans and many roasters bag whole beans immediately after roasting in bags with pressure release valves. Today home roasting is becoming popular again. Computerised drum roasters are available which simplify home roasting, and some home roasters simply roast in an oven or in air popcorn poppers. Once roasted, coffee loses its flavour quickly. Although some prefer to wait 24 hours after roasting to brew the first cup, all agree that it begins to get off-flavors and bitterness about 1-2 weeks after roasting even under ideal conditions like being stored in an airtight container or de-gassing valve bag.

Home roasting

Up until the 20th century, it was more common for at-home coffee drinkers to roast their coffee in their residence than it was to buy pre-roasted coffee. During the 20th century, roasting coffee in the home faded in popularity with the rise of the commercial coffee roasting companies. In recent years home roasting of coffee has seen a revival, and while there may be an economic advantage, primarily it is a means for the coffee aficionado or coffee snob to get access to better quality, fresher roasted beans. Home roasting is the process of buying green coffee beans and roasting them in your own home. Roasting coffee in the home is something that has been practiced for centuries, and has included methods such as heating over fire coals, roasting in cast iron pans, and rotating iron drums over a fire or coal bed.


The coffee roasting process consists essentially of cleaning, roasting, cooling, grinding, and packaging operations. 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 pneumatic conveyor to storage hoppers. From the storage hoppers, the green beans are conveyed to the roaster. Roasters typically operate at temperatures between 370 and 540 °F, and the beans are roasted for a period of time ranging from a few minutes to about 30 minutes. Roasters are typically horizontal rotating drums that tumble the green coffee beans in a current of hot combustion gases; the roasters operate in either batch or continuous modes and can be indirect- or direct-fired.

Indirect-fired roasters are roasters in which the burner flame does not contact the coffee beans, although the combustion gases from the burner do contact the beans. Direct-fired roasters contact the beans with the burner flame and the combustion gases. At the end of the roasting cycle, water sprays are used to "quench" the beans. Following roasting, the beans are cooled and run through a "destoner". Destoners are air classifiers that remove stones, metal fragments, and other waste not removed during initial screening from the beans. The destoners pneumatically convey the beans to a hopper, where the beans are stabilised and dried (small amounts of water from quenching exist on the surface of the beans). This stabilisation process is called equilibration. Following equilibration, the roasted beans are ground, usually by multi-stage grinders. Some roasted beans are packaged and shipped as whole beans. Finally, the ground coffee is vacuum sealed and shipped. Everyday alchemy, coffee roasting coaxes golden flavor from a bland bean. Unroasted beans boast 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 flavor.
Grades of coffee roasting; from left:
unroasted, light, cinnamon, medium, high, city, full city, French, and Italian.


A note on flavor: Describing the tastes of different roasts is as subjective as putting a wine into words. In both cases there is no substitute for your own personal taste, for sample;

  Aliases Roaster Watch Surface Flavour
Light Cinnamon roast, half city, New England After about seven minutes the beans "pop" and double in size, and light roasting is achieved. American mass-market roasters typically stop here. Dry Light-bodied and somewhat sour, grassy, and snappy
Medium Full city, American, regular, breakfast, brown At nine to eleven minutes the beans reach this roast, which U.S. speciality sellers tend to prefer. Dry A bit sweeter than light roast; full body balanced by acid snap, aroma, and complexity
Dark High, Viennese, French, Continental After 12 to 13 minutes the beans begin hissing and popping again, and oils rise to the surface. Roasters from the U.S. Northwest generally remove the beans at this point. Slightly shiny Somewhat spicy; complexity is traded for rich chocolaty body, aroma is exchanged for sweetness
Darkest Italian, espresso After 14 minutes or so the beans grow quiet and begin to smoke. Having caramelised, the bean sugars begin to carbonise. Very oily Smokey; tastes primarily of roasting, not of the inherent flavor of the bean

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