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

Roasting

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

French roasted coffee beans
French roasted coffee beans

At lighter roasts, the bean will exhibit more of its "origin flavor" - 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 flavor.

As the beans darken to a deep brown, the origin flavors of the bean are eclipsed by the flavors created by the roasting process itself. At darker roasts, the "roast flavor" 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 favor 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 pressurized cans and many roasters bag whole beans immediately after roasting in bags with pressure release valves. Today home roasting is becoming popular again. Computerized 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 flavor 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.

Process

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 °C (698 and 1004 °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 stabilize and dry (small amounts of water from quenching exist on thesurface of the beans). This stabilization 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.

Emissions and Control

Particulate matter (PM), volatile organic compounds (VOC), organic acids, and combustion products are the principal emissions from coffee processing. Several operations are sources of PM emissions, including the cleaning and destoning equipment, roaster, cooler, and instant coffee drying equipment. The roaster is the main source of gaseous pollutants, including alcohols, aldehydes, organic acids, and nitrogen and sulphur compounds. Because roasters are typically natural gas-fired, carbon monoxide (CO) and carbon dioxide (CO2) emissions are expected as a result of fuel combustion. Decaffeination and instant coffee extraction and drying operations may also be sources ofsmall amounts of VOC. Emissions from the grinding and packaging operations typically are not vented to the atmosphere.

Particulate matter emissions from the receiving, storage, cleaning, roasting, cooling, and stoning operations are typically ducted to cyclones before being emitted to the atmosphere. Gaseous emissions from roasting operations are typically ducted to a thermal oxidiser thermal catalytic oxidiser> following PM removal by a cyclone. Some facilities use the burners that heat the roaster as thermal oxidisers. However, separate thermal oxidizers are more efficient because the desired operating temperature is typically between 650°C and 816°C (1200°F and 1500°F), which is 93°C to 260°C (200°F to 500°F) more than the maximum temperature of most roasters. Some facilities use thermal catalytic oxidizers, which require lower operating temperatures to achieve control efficiencies that are equivalent to standard thermal oxidisers. Catalysts are also used to improve the control efficiency of systems in which the roaster exhaust is ducted to the burners that heat the roaster. Emissions from spray dryers are typically controlled by a cyclone followed by a wet scrubber>.

Gallery

Coffee
Coffee species: Coffea arabica | Coffea canephora (robusta)
Coffee varietals: Kenya AA | Kona | Jamaican Blue Mountain Coffee
Major chemicals in coffee: Caffeine | Cafestol
Processing of coffee: Coffee roasting | Home roasting coffee
Common coffee preparation methods: Espresso | Drip brew
Social aspects: Coffeehouse | Caffé
This article is licensed under the GNU Free Document License
It uses material from Wikipedia.

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