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Processing of coffee is the transformation of the chemical and physical properties of green coffee beans into roasted coffee products, including whole and ground beans and soluble coffee products. Additional operations associated with processing green coffee beans include decaffeination andinstant (soluble) coffee production.
Processes
Roasting
Espresso-roast coffee beans 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. 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 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 and Kenya 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.
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 a week after roasting even under ideal conditions.
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.
Decaffeination
Decaffeination is the process of extracting caffeine from green coffee beans prior to roasting. The most common decaffeination process used in the United States is supercritical carbon dioxide (CO2) extraction. In this process, moistened green coffee beans are contacted with large quantities of supercritical CO2 (CO2 maintained at a pressure of about 4,000 pounds per square inch and temperatures between 90° and 100°C [194° and 212°F]), which removes about 97 percent of the caffeine from the beans. The caffeine is then recovered from the CO2, typically using an activated carbon adsorption system.
Another commonly used method is solvent extraction, typically using oil (extracted from roasted coffee) or ethyl acetate as a solvent. In this process, solvent is added to moistened green coffee beans to extract most of the caffeine from the beans. After the beans are removed from the solvent, they are steam-stripped to remove any residual solvent. The caffeine is then recovered from the solvent, and the solvent is re-used. Water extraction is also used for decaffeination. Decaffeinated coffeebeans have a residual caffeine content of about 0.1 percent on a dry basis. Not all facilities have decaffeination operations, and decaffeinated green coffee beans are purchased by many facilities that produce decaffeinated coffee.
Main article: Instant coffee
In the manufacture of instant coffee, extraction follows the roasting and grinding operations. The soluble solids and volatile compounds that provide aroma and flavor are extracted from the coffee beans using water. Water heated to about 175°C (347°F) under pressurized conditions (to maintain the water as liquid) is used to extract all of the necessary solubles from the coffee beans. Manufacturers use both batch and continuous extractors. Following extraction, evaporation or freeze-concentration is used to increase the solubles concentration of the extract. The concentrated extracts are then dried ineither spray dryers or freeze dryers.
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 sulfur 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, andstoning operations are typically ducted to cyclones before being emitted to the atmosphere. Gaseous emissions from roasting operations are typically ducted to a thermal oxidiser or thermal catalytic oxidiser following PM removal by a cyclone. Some facilities use the burners that heat the roaster as thermal oxidisers. However, separate thermal oxidisers 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.
See also
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