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Roasting coffee transforms the chemical and physical properties of
green coffee beans into roasted
coffee
Coffee is a drink prepared from roasted coffee beans. Darkly colored, bitter, and slightly acidic, coffee has a stimulating effect on humans, primarily due to its caffeine content. It is the most popular hot drink in the world.
Seeds of ...
products. The roasting process is what produces the characteristic flavor of coffee by causing the green coffee beans to change in taste. Unroasted beans contain similar if not higher levels of
acids,
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
, sugars, and
caffeine
Caffeine is a central nervous system (CNS) stimulant of the methylxanthine class. It is mainly used recreationally as a cognitive enhancer, increasing alertness and attentional performance. Caffeine acts by blocking binding of adenosine to ...
as those that have been roasted, but lack the taste of roasted coffee beans due to the
Maillard and other chemical reactions that occur during roasting.
Coffee tends to be roasted close to where it will be consumed, as green coffee is more stable than roasted beans. The vast majority of coffee is roasted commercially on a large scale, but small-scale commercial roasting has grown significantly with the trend toward
"single-origin" coffees served at specialty shops. Some coffee drinkers even
roast coffee at home as a hobby in order to both experiment with the flavor profile of the beans and ensure the freshest possible roasted coffee.
The first recorded implements for roasting coffee beans were thin pans made from metal or porcelain, used in the 15th century in the
Ottoman Empire
The Ottoman Empire, * ; is an archaic version. The definite article forms and were synonymous * and el, Оθωμανική Αυτοκρατορία, Othōmanikē Avtokratoria, label=none * info page on book at Martin Luther University) ...
and
Greater Persia
Greater Iran ( fa, ایران بزرگ, translit=Irān-e Bozorg) refers to a region covering parts of Western Asia, Central Asia, South Asia, Xinjiang, and the Caucasus, where both Iranian culture and Iranian languages have had a s ...
. In the 19th century, various patents were awarded in the U.S. and Europe for commercial roasters, to allow for large batches of coffee. In the 1950s just as
instant coffee
Instant coffee is a beverage derived from brewed coffee beans that enables people to quickly prepare hot coffee by adding hot water or milk to coffee solids in powdered or crystallized form and stirring. Instant coffee solids (also called sol ...
was becoming a popular coffee drink, speciality coffee-houses began opening to cater to the connoisseur, offering a more traditionally brewed beverage. In the 1970s, more speciality coffee houses were founded, ones that offered a variety of roasts and beans from around the world. In the 1980s and 1990s, the gourmet coffee industry experienced great growth. This trend continued into the 21st century.
History
![Coffee roaster from Baghdad, 15th century](https://upload.wikimedia.org/wikipedia/commons/4/42/Coffee_roaster_from_Baghdad%2C_15th_century.jpg)
The first known implements for roasting coffee beans were thin, circular, often perforated pans made from metal or porcelain, used in the 15th century in the
Ottoman Empire
The Ottoman Empire, * ; is an archaic version. The definite article forms and were synonymous * and el, Оθωμανική Αυτοκρατορία, Othōmanikē Avtokratoria, label=none * info page on book at Martin Luther University) ...
and
Greater Persia
Greater Iran ( fa, ایران بزرگ, translit=Irān-e Bozorg) refers to a region covering parts of Western Asia, Central Asia, South Asia, Xinjiang, and the Caucasus, where both Iranian culture and Iranian languages have had a s ...
. This type of shallow, dished pan was equipped with a long handle so that it could be held over a
brazier
A brazier () is a container used to burn charcoal or other solid fuel for cooking, heating or cultural rituals. It often takes the form of a metal box or bowl with feet. Its elevation helps circulate air, feeding oxygen to the fire. Braziers ...
(a container of hot coals) until the coffee was roasted. The beans were stirred with a slender spoon. Only a small amount of beans could be heated at one time. The first cylinder roaster with a crank to keep the beans in motion appeared in
Cairo
Cairo ( ; ar, القاهرة, al-Qāhirah, ) is the Capital city, capital of Egypt and its largest city, home to 10 million people. It is also part of the List of urban agglomerations in Africa, largest urban agglomeration in Africa, List of ...
around 1650. It was made of metal, most commonly
tinned
Tinning is the process of thinly coating sheets of wrought iron or steel with tin, and the resulting product is known as tinplate. The term is also widely used for the different process of coating a metal with solder before soldering.
It is mos ...
copper or
cast iron
Cast iron is a class of iron– carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its color when fractured: white cast iron has carbide impur ...
, and was held over a brazier or open fire. French, Dutch and Italian variations of this design quickly appeared. These proved popular over the next century in Europe, England and the American colonies.
In the 19th century, various patents were awarded in the U.S. and Europe for commercial roasters, to allow for large batches of coffee. Nevertheless, home roasting continued to be popular. A man working at a commercial roasting plant beginning in the 1850s in
St. Louis, Missouri
St. Louis () is the second-largest city in Missouri, United States. It sits near the confluence of the Mississippi and the Missouri Rivers. In 2020, the city proper had a population of 301,578, while the bi-state metropolitan area, which e ...
, said that "selling roasted coffee was up-hill work, as everyone roasted coffee in the kitchen oven." Appliances catering to the home roaster were developed; in 1849 a spherical coffee roaster was invented in
Cincinnati, Ohio
Cincinnati ( ) is a city in the U.S. state of Ohio and the county seat of Hamilton County. Settled in 1788, the city is located at the northern side of the confluence of the Licking and Ohio rivers, the latter of which marks the state line wi ...
, for use on the top of a wood-fired kitchen stove, fitted into a burner opening. Green beans were available at the local general store, or even through mail order.
[ For roasting, many people used such simple methods as a layer of beans on a metal sheet in the oven, or beans stirred in a cast-iron skillet over a fire. Despite the wide popularity of home roasting, Burns felt that it would soon disappear because of the great strides made in commercial roasting in the 1860s and 1870s, including the benefits of the ]economies of scale
In microeconomics, economies of scale are the cost advantages that enterprises obtain due to their scale of operation, and are typically measured by the amount of output produced per unit of time. A decrease in cost per unit of output enables ...
. The commercial roaster inventions patented by Burns revolutionized the U.S. roasting industry, much like the innovations of inventors in Emmerich am Rhein
Emmerich am Rhein ( Low Rhenish and nl, Emmerik) is a city and municipality in the northwest of the German federal state of North Rhine-Westphalia. The city has a harbour and a quay at the Rhine. In terms of local government organization, it i ...
greatly advanced commercial coffee roasting in Germany. As well, the 1864 marketing breakthrough of the Arbuckle Brothers
John Arbuckle (1838 – March 27, 1912) was an American businessman who founded ''Arbuckle Brothers Company'', a coffee roasting and sugar refining company.
Early life
John Arbuckle III was born in Scotland in 1838 to Thomas Arbuckle, a Scotti ...
in Philadelphia
Philadelphia, often called Philly, is the List of municipalities in Pennsylvania#Municipalities, largest city in the Commonwealth (U.S. state), Commonwealth of Pennsylvania, the List of United States cities by population, sixth-largest city i ...
, introducing the convenient one-pound (0.45 kg) paper bag of roasted coffee, brought success and imitators. From that time commercially roasted coffee grew in popularity until it gradually overtook home roasting during the 1900s in America. In 1903 and 1906 the first electric roasters were patented in the U.S. and Germany, respectively; these commercial devices eliminated the problem of smoke or fuel vapor imparting a bad taste to the coffee. In France, the home roaster did not yield to the commercial roaster until after the 1920s, especially in rural areas. Coffee was roasted to a dark color in small batches at home and by shopkeepers, using a variety of appliances including ones with a rotating cylinder of glass, sheet iron or wire mesh, and ones driven by hand, clockwork or electric motor. Because of the smoke and blowing chaff, country dwellers generally roasted outdoors.
In the 1950s just as instant coffee
Instant coffee is a beverage derived from brewed coffee beans that enables people to quickly prepare hot coffee by adding hot water or milk to coffee solids in powdered or crystallized form and stirring. Instant coffee solids (also called sol ...
was becoming a popular coffee drink, speciality coffee-houses began opening to cater to the connoisseur, offering a more traditionally brewed beverage. In the 1970s, more speciality coffee houses were founded, ones that offered a variety of roasts and beans from around the world. In the 1980s and 1990s, the gourmet coffee industry experienced great growth. Through the 1970s and 1980s, the Siemens Sirocco home roaster was made in West Germany and marketed globally. It was a small fluid-bed roaster made for the home enthusiast. The product was named after a commercial hot-air roasting process which itself was named after the hot Sahara winds called sirocco
Sirocco ( ), scirocco, or, rarely, siroc (see below) is a Mediterranean wind that comes from the Sahara and can reach hurricane speeds in North Africa and Southern Europe, especially during the summer season.
Names
''Sirocco'' derives from ...
. In 1976, chemical engineer Michael Sivetz patented a competing hot air design for manufacture in the U.S.; this became popular as an economical alternative. Sivetz called for the home roaster to focus on the quality of the bean. From 1986 through 1999 there was a surge in the number of patents filed for home roasting appliances. In the 1990s, more electric home roasting equipment became available, including drum roasters, and variations on the fluid-bed roaster. By 2001, gourmet coffee aficionados were using the internet to purchase green estate-grown beans for delivery by mail.
Process
The coffee-roasting process follows coffee processing
Coffee production is the industrial process of converting the raw fruit of the coffee plant into the finished coffee. The coffee cherry has the fruit or pulp removed leaving the seed or bean which is then dried. While all green coffee is proce ...
and precedes coffee brewing
Coffee preparation is the process of turning coffee beans into a beverage. While the particular steps vary with the type of coffee and with the raw materials, the process includes four basic steps: raw coffee beans must be roasted, the roasted ...
. It consists essentially of sorting, roasting, cooling, and packaging but can also include grinding. Bags of green coffee beans are hand- or machine-opened, dumped into a hopper
Hopper or hoppers may refer to:
Places
*Hopper, Illinois
* Hopper, West Virginia
* Hopper, a mountain and valley in the Hunza–Nagar District of Pakistan
* Hopper (crater), a crater on Mercury
People with the name
* Hopper (surname)
* Grace H ...
, and screened to remove debris. The green beans are then weighed and transferred manually, by belt, or pneumatic conveyor to storage hoppers. From the storage hoppers, the green beans are conveyed to the roaster.
Initially, the process is endothermic
In thermochemistry, an endothermic process () is any thermodynamic process with an increase in the enthalpy (or internal energy ) of the system.Oxtoby, D. W; Gillis, H.P., Butler, L. J. (2015).''Principle of Modern Chemistry'', Brooks Cole. ...
(absorbing heat), but at around it becomes exothermic (giving off heat). For the roaster, this means that the beans are heating themselves and an adjustment of the roaster's heat source might be required. At the end of the roasting cycle, the roasted beans are dumped from the roasting chamber and are typically cooled by air or water-quenching.
During the roasting process, coffee beans lose 15 to 18% of their mass due mainly to the loss of water but also to volatile compounds. Although the beans experience a weight loss, the size of the beans double during the roasting process due to the physical expansion caused by an increase in internal pressure from vaporized water. Green coffee can contain reasonable quantities of simple sugars. Sugars are quite reactive at roasting temperatures and, once the water has evaporated out of the bean, the sugars can begin to react to the heat in different ways. Some go through caramelization reactions, creating the caramel notes found in certain coffees.
There are several traditional variations to coffee roasting in different parts of the world. For example, in Vietnam
Vietnam or Viet Nam ( vi, Việt Nam, ), officially the Socialist Republic of Vietnam,., group="n" is a country in Southeast Asia, at the eastern edge of mainland Southeast Asia, with an area of and population of 96 million, making i ...
coffee is often coated with oil (traditionally clarified butter) and a small amount of sugar prior to roasting to produce a "butter roast". The roasting process results in an additional caramelized coating on the beans.
Equipment
The most common roasting machines are of two basic types: drum and hot-air, although there are others including packed-bed, tangential and centrifugal roasters. Roasters can operate in either batch or continuous modes. Home roasters are also available.
Drum machines consist of horizontal rotating drums that tumble the green coffee beans in a heated environment. The heat source can be supplied by natural gas, liquefied petroleum gas (LPG), electricity, or even wood. The most common employ indirectly heated drums where the heat source is under the drum. Direct-fired roasters are roasters in which a flame contacts the beans inside the drum; very few of these machines are still in operation.
Fluid bed or hot-air roasters force heated air through a screen or perforated plate under the coffee beans with sufficient force to lift the beans. Heat is transferred to the beans as they tumble and circulate within this fluidized bed
A fluidized bed is a physical phenomenon that occurs when a solid particulate substance (usually present in a holding vessel) is under the right conditions so that it behaves like a fluid. The usual way to achieve a fluidize bed is to pump pressur ...
.
Roasts
Some coffee roasters use names for the various degrees of roast, such as "city roast" and "French roast", for the internal bean temperatures found during roasting. Recipes known as "roast profiles" indicate how to achieve flavor characteristics. Any number of factors may help a person determine the best profile to use, such as the coffee's origin, variety, processing method, moisture content, bean density, or desired flavor characteristics. A roast profile can be presented as a graph showing time on one axis and temperature on the other, which can be recorded manually or using computer software and data loggers linked to temperature probes inside various parts of the roaster.
One method of determining the degree of roast is to evaluate the bean's color. As the coffee absorbs heat, the color shifts to yellow and then to increasingly darker shades of brown. During the later stages of roasting, oils appear on the surface of the bean. The roast will continue to darken until it is removed from the heat source. Coffee also darkens as it ages, making color alone a poor roast determinant. It is common for roasters to use a combination of temperature, smell, color, and sound to monitor the roasting process.
There are two events called "cracks" that roasters listen for. At approximately , the coffee will emit a cracking sound. This point is referred to as "first crack", marking the beginnings of a very light roast. At first crack, a large amount of the coffee's moisture has been evaporated and the beans will begin to increase in size. When the coffee reaches approximately , it emits a "second crack". This sound represents the structure of the coffee becoming brittle and fracturing as the bean continues to swell and enlarge from internal pressure. If the roast is allowed to progress further, it begins to take on the characteristics of the roasting process and loses the characteristics of the coffee's origin.
Lipids present inside the coffee seed liquify from heat and pressure built up in the bean. These lipids can often be seen on the bean surface. An oily coating is more prevalent with darker roasts.
These images depict samples taken from the same batch of a typical Brazilian green coffee at various bean temperatures with their subjective roast names and descriptions.
Flavors
At lighter roasts, the coffee will exhibit more of its "origin character"—the flavors created by its variety, processing, altitude, soil content, and weather conditions in the location where it was grown. 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. Green coffee contains many different types of acids, some of which are pleasant to taste and some that are not. Of particular importance to the roaster are the chlorogenic acids (CGAs). One of the key goals of roasting is to break down these unpleasant acids.
Below, roast levels and their respective flavors are described. These are qualitative descriptions, and thus subjective.
Caffeine levels are not significantly affected by the level of roast. Caffeine remains stable up to and completely decomposes around . Given that roasting temperatures do not exceed for long and rarely if ever reach , the caffeine content of a coffee is not likely changed much by the roasting process.
Home roasting
Home roasting is the process of roasting small batches of green coffee beans for personal consumption. Even after the turn of 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. Later, home roasting faded in popularity with the rise of the commercial coffee roasting companies. In recent years home roasting of coffee has seen a revival. In some cases there is an economic advantage, but primarily it is a means to achieve finer control over the quality and characteristics of the finished product.
Packaging
Extending the shelf life of roasted coffee relies on maintaining an optimum environment to protect it from exposure to heat, oxygen, and light. Roasted coffee has an optimal typical shelf life of two weeks, and ground coffee about 15 minutes. Without some sort of preservation method, coffee becomes stale. The first large-scale preservation technique was vacuum packing in cans. However, because coffee emits CO2 after roasting, coffee to be vacuum-packed must be allowed to de-gas for several days before it is sealed. To allow more immediate packaging, pressurized canisters or foil-lined bags with pressure-relief valves can be used. Refrigeration and freezing retards the staling process. Roasted whole beans can be considered fresh for up to one month if kept cool. Once coffee is ground it is best used immediately.
Emissions and control
Particulate matter (PM), volatile organic compounds
Volatile organic compounds (VOCs) are organic compounds that have a high vapour pressure at room temperature. High vapor pressure correlates with a low boiling point, which relates to the number of the sample's molecules in the surrounding air, a t ...
(VOC), organic acids
An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO2OH, are r ...
, 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
A pollutant or novel entity is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. These can be both naturally forming (i.e. minerals or extracted compounds like o ...
, including alcohols, aldehydes
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
, organic acids
An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO2OH, are r ...
, and nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
and sulfur compounds. Because roasters are typically heated by combustion
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combus ...
of natural gas
Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbo ...
, carbon monoxide
Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
(CO) and carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
(CO2) emissions result from fuel combustion. Decaffeination
Decaffeination is the removal of caffeine from coffee beans, cocoa, tea leaves, and other caffeine-containing materials. Decaffeinated drinks contain typically 1–2% of the original caffeine content, and sometimes as much as 20%. Decaffeinate ...
and instant coffee
Instant coffee is a beverage derived from brewed coffee beans that enables people to quickly prepare hot coffee by adding hot water or milk to coffee solids in powdered or crystallized form and stirring. Instant coffee solids (also called sol ...
extraction and drying operations may also be sources of small amounts of VOC. Emissions from the grinding and packaging operations typically are not vented to the atmosphere.
Particulate matter emissions from the roasting and cooling 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 , which is 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. Catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
s 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
The term wet scrubber describes a variety of devices that remove pollutants from a furnace flue gas or from other gas streams. In a wet scrubber, the polluted gas stream is brought into contact with the scrubbing liquid, by spraying it with the ...
.
Gallery
File:Diedrich IR Series Coffee Roaster.jpg, Diedrich IR Series Coffee Roaster
File:Coffeeroasting woodfired.jpg, An old wood-fired coffee roaster
File:Coffee Roaster.JPG, A two-barrel coffee roaster for roasting coffee samples prior to purchasing or roasting green coffee beans
File:2013 Antique sample roasting machine Vits Munchen.jpg, An antique "Mignon" five-barrel test roasting machine
File:Probat Roasters.jpg, The roasting room at a midsize coffee company
See also
* Coffee
Coffee is a drink prepared from roasted coffee beans. Darkly colored, bitter, and slightly acidic, coffee has a stimulating effect on humans, primarily due to its caffeine content. It is the most popular hot drink in the world.
Seeds of ...
* Dry roasting
Dry roasting is a process by which heat is applied to dry foodstuffs without the use of oil or water as a carrier. Unlike other dry heat methods, dry roasting is used with foods such as nuts and seeds, in addition to some eaten insects such as ...
* Home roasting coffee
* Food grading
Food grading involves the inspection, assessment and sorting of various foods regarding quality, freshness, legal conformity and market value.Saravacos, George D.; Maroulis, Zacharias B. (2011''Food Process Engineering Operations'' CRC Press. p ...
* French press
A French press, also known as a cafetière, ''cafetière à piston'', ''caffettiera a stantuffo'', press pot, coffee press, or coffee plunger, is a coffee brewing device, although it can also be used for other tasks. In 1923 Ugo Paolini, an Ital ...
* Torrefacto
References
{{DEFAULTSORT:Coffee Roasting
Coffee preparation