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 change in taste.
Unroasted beans contain similar if not higher levels of acids, protein, sugars, and caffeine 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. The vast majority of coffee is roasted commercially on a large scale, but some coffee drinkers roast coffee at home in order to have more control over the freshness and flavor profile of the beans.
The coffee roasting process transforms these raw beans into the distinctively aromatic, flavorful, crunchy beans that we recognize as coffee.
It consists essentially of sorting, roasting, cooling, and packaging 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 roast profile is defined by: temperature and roasting time. Central components are temperature rise Rate-of-Rise (ROR) and reduction of roast temperature over time. This roast profile varies depending on the type of beans used and leads to different taste characteristics. The roast time (from small batch roasters) is between 10-20min. and takes place at temperatures between 180 and 250 ° C.
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.
The most popular, but probably the least accurate, method of determining the degree of roast is to judge the bean’s color by eye (the exception to this is using a spectrophotometer to measure the ground coffee reflectance under infrared light and comparing it to standards such as the Agtron scale). 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. Most roasters use a combination of temperature, smell, color, and sound to monitor the roasting process.