Understanding Roasting

23 October 2020 by Matt Lawrence

Roasting is the heating (cooking) of raw (green) coffee beans to develop their aroma, flavour and body. The basic process of heating the beans has been widely used for centuries, but today it is done on a larger scale and is a more highly sophisticated process. The skills needed at this stage is perhaps more crucial than others considered so far in determining the quality of the beans. Good beans can be ruined easily if they are not roasted properly. Highly sophisticated roasting machines are now available that can be accurately programmed to roast beans properly and produce good consistency from one batch to the next.

Roasting causes many physical and chemical reactions but some of the obvious ones are
- release of volatile substances responsible for flavour and aroma.
- evaporation of moisture and reduction in weight by approx 15%
- breakdown of the outer walls and increase in size by up to 60%
- change of colour from yellow or blue-green to varying shades of brown.
- release of large quantities of carbon dioxide.
- release of oils to the surface causing a shiny appearance

The Coffee Roaster

The coffee roaster, the person responsible for roasting coffee beans, must have a good understanding of the nature of different beans and how they will respond to heating. Someone who ‘cooks’ knows how long and at what temperature to cook foods In the same way, the operator of the roasting machine must know how long and and at what temperature to cook different types of beans. The length of time and temperature used are largely dependent on the size, density and moisture of the beans.

The Roasting Machine

The design of the roasting machine has evolved from small stone or metal vessels that were used hundreds of years ago to the large computerised models of today. In modern times there have been two basic desings, a drum roaster and a fluid bed roaster.

A Drum roaster heats the beans as they tumble in a heated roasting drum. They are heated mainly by conduction or contact with the hot sides of the drum.

a Fluid bed roaster, also known as a hot air roaster, keeps the beans suspended in hot air. They are heated primarily by convection or by hot air rather than by contact with the sides of the container they are in.

Some roasters are designed to be able to roast beans continuously, as successive lots pass through a heat lock. These are called continuous roasters.

The roasting process has three stages- drying, roasting and cooling.

Drying-

Green beans are placed in a heating chamber of the roasting machine and as they are heated, chemical and physical changes start to take place slowly. During this stage the amount of moisture contained in the beans reduces. The colour changes to yellowish and the aroma becomes a little like that of toasted bread.

Roasting The roasting temperature continues to rise and the temperature and pressure inside the beans build up, Held in by the very thick outer walls of the beans, the internal temperature and pressure lead to a great number of chemical and physical changes. The Beans expand as large amounts of carbon dioxide build up and many hundreds of new substances are formed inside the beans. The colour changes to an increasingly darker brown as the sugars caramelise.

Cooling-

The beans are moved from the heating chamber and into a cooling rack. Here they are quickly cooled with either jets or cold air or a fine mist of water that evaporates with the heat of the beans.

Blending-

Blending is mixing different beans from different areas. The beans are chosen for the complementary characteristics they can give to a coffee beverage. Hundreds of different bean varieties are grown in dozens of different areas throughout the world, all with different geographical influences. The coffee beans produced in a particular area usually produce the same year to year.

When making beverages using the espresso method good as well as bad characteristics of a blend become more pronounced then in beverages made using other methods. For this reason the choice of beans for a blend to make a perfect espresso is critical. The aim of blending is the selection of beans to achieve
- Good flavour and aroma
- full body
- balance of desirable characters
- consistency from one batch to another

When to Blend?

Blending may be done before or after the beans are roasted. If they are blended and then roasted together, the different beans must be much the same size and density and respond to the roasting process in the same way, to ensure that the beans roast evenly. If the beans are different, it is best to first roast them separately first and blend them afterwards. This takes more time and effort and is therefore more costly.