Roasting coffee transforms the chemical and physical properties of green coffee beans into roasted coffee products. The roasting process is what produces the characteristic flavor of coffee by causing the green coffee beans to expand and to change in color, taste, smell, and density. Unroasted beans contain similar acids, protein, and caffeine as those that have been roasted, but lack the taste. It takes heat to speed up the Maillard and other chemical reactions that develop and enhance the flavour.

As green coffee is more stable than roasted, the roasting process tends to take place close to where it will be consumed. This reduces the time that roasted coffee spends in distribution, helping to maximize its shelf life. The vast majority of coffee is roasted commercially on a large scale, but some coffee drinkers roast coffee themselves in order to have more control over the freshness and flavour profile of the beans.

 

Process

The coffee roasting process consists essentially of sorting, roasting, cooling, and packaging operations but can also include grinding in larger scale roasting houses. In larger 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 (188 and 282 °C), and the beans are roasted for a period of time ranging from 12 to 30 minutes. 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 natural gas, liquefied petroleum gas (LPG), electricity or even wood. These roasters can operate in either batch or continuous modes and can be indirect- or direct-fired.

An alternate to the drum coffee roaster was developed by Michael Sivetz for which he was given US patent 3,964,175 on June 22, 1976. It involves roasting the coffee beans while they are levitated on a cushion of heated air. The process is called a fluidized bed and is commonly used in other food processing applications.

Many people who roast coffee prefer to follow a "recipe", or roast profile, when bringing out the flavour characteristics they wish to highlight. Any number of factors may help a person determine the best profile to use, such as the coffee's origin, varietal, processing method or desired flavour 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.

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, the roasted beans are cooled using a vacuum system. Roasted coffee beans are also cooled using fine water mist, this method is known as "quenching" and is considered inferior to air cooling as the water soaks the fresh beans with moisture and oxygen particles sending it stale almost instantly. Following roasting, the beans are cooled and stabilized. This stabilization process is called de-gassing. Following de-gassing, the roasted beans are packaged, usually in light resistant foil bags fitted with small one-way valves to allow gasses to escape while protecting the beans from moisture and oxygen. Roasted whole beans can be considered fresh for up to one month. Once coffee is ground it is best used immediately.

Packaging

Extending the useful life of roasted coffee relies on maintaining an optimum environment for the beans. The first large scale preservation technique was vacuum packing. 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.

As the bean absorbs heat, the color shifts to yellow and then to varying shades of brown. During the later stages of roasting, oils appear on the surface of the bean, making it shiny. 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, Kenya, Hawaiian Kona, and Jamaican Blue Mountain are usually roasted lightly so their signature characteristics dominate the flavor.^[1] 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.

A note on flavor: Describing the tastes of different roasts is as subjective as putting a wine into words. In both cases there’s no substitute for your own personal taste. As a guide, if you can see the oil on the beans as in the image above, you are more likely to taste the roasting flavours, than the individual characteristics of the beans.