The art of roasting

Coffee’s characteristic flavour and aroma develop during the roasting process. Coffee roasting was discovered by Arabs in Yemen in the 14th century, after a fire ignited some coffee trees.
Flavour is locked within the green coffee bean until it is roasted, heating green coffee beans sets a series of complex chemical reactions in motion that release the flavour compounds hidden within each bean.

Art and science

Roasting coffee beans to perfection is an art as much as a science since they may react differently to the roasting process, depending on the soil and weather conditions in which they were grown and the processing method used. Roasting coffee transforms the chemical and physical properties of green coffee beans into roasted coffee products.

The roasting process is what produces the characteristic flavour of coffee by causing the green coffee beans to expand and to change in colour, taste, smell, and density. Un-roasted 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 (named after the chemist Louis-Camille Maillard who discovered it in 1912) and other chemical reactions that develop and enhance the flavour.
Coffee contains hundreds of compounds, including sugars, oils, proteins, and minerals.

As a result of roasting, many of these compounds develop and change composition. During the roasting process, the green coffee beans gradually lose moisture. When the beans reach approximately 400 degrees Fahrenheit, a series of chemical changes known as “pyrolysis” occurs.

Roasting coffee releases moisture from the bean, causing the bean to expand creating the crack we see in the bean. The moisture loss makes the beans weigh less. Most commercial roasts lose around 10-14% of their weight; darker roasts like the Starbucks roast loses around 18% of its original weight, and up to a staggering 25% for French roast.

Roasters typically operate at temperatures between 370 and 540ºF (188 and 282ºC), and the beans are roasted for a period of time ranging from 3 to 30 minutes.

Roasting coffee is an art, and like all true art, it takes years of practice before one becomes a master roaster.

A bright rather than a dull roast is desired, as dullness can indicate over-drying or poor processing; also, a good quality sample of beans will appear even, since a variety of colours, particularly “pales”, among the coffee beans can indicate immature picking, poor sorting of cherries, and varying degrees of fermentation and drying on the plantation