Coffee is roasted using a chemical process to balance, create or alter aroma, aftertaste, acids and other components of flavour to suit the varying taste preferences of customers. Roasting coffee makes it release its rich flavour and taste. Coffee beans are store green as harvested to avoid losing their taste and quality. However, green coffee beans cannot exhibit these characteristics which necessitate roasting.
Roasting results in the transformation of coffee’s physical and chemical properties due to a rapid increase in temperatures under which the beans are subjected. Coffee beans are cooled by spraying water on them upon reaching the required heat to produce the toast of choice. Coffee is made to remain at the required heat levels by continuously fanning it which cools the air above.
The coffee is then separated from any debris that may be present before being transferred and kept at room temperature. Water is then drained from the beans after they have normalized temperature. Dried coffee is then packed grounded or in grain, form to be ready for sale. Roasted coffee should be used kept in airtight containers to prevent oxidation which reduces flavour.
The Process of Roasting
The first stage in the processing of coffee is endothermic. During this stage, the green coffee beans are dried slowly to obtain a yellow colour. At this stage, the coffee grain begins to emit the smell of popcorn or toast. The beans are kept in a continuous motion to avoid burning. The internal temperature of the beans change which equally affects the colour outside. Coffee grain roasts can be of different profiles such as;
Light roasts have a brown colour and are preferred for coffee varieties which are mild. The bean’s surface contains no oil since temperatures are not high enough to make oils permeate to the outer surface of the coffee bean.
The second step in the roasting of coffee beans is an exothermic step known as the first crack. The stage is a shorter endothermic period compared to the first. During this stage, the temperature is raised to 205 °C or 400 °F. The bean’s size doubles and its colour is now light brown. The bean’s weight decreases by 5 percent.
The roast at this stage has a strong flavour with its surface having no oil. The roast is at times referred to as American roast owing to its preference in the country.
During the third stage, the coffee beans go through a process known as pyrolysis. The coffee beans’ chemical composition is altered and carbon dioxide is released. The beans have a shiny black colour at this stage, oiled surface and a mild bitter-sweet aftertaste. Coffee from dark roast can be slightly/charred dark.
To make espresso, one has to enhance coffee’s aroma and sweetness while at the same time minimize its acidity and bitterness during roasting. Heating coffee at temperatures above 200°C makes its sugar start to caramelize. Uncaramelized sugar is much sweeter compared to caramelized sugar. When coffee’s sugar caramelize, the grains achieve a dark colour. Medium roast coffee has the best blend of flavour since it coffee beans have been moderately subjected to heat between 205-215°C thus maintaining natural sweetness. The following are the colours and aroma of coffee through the roasting process:
- Pale – coffee beans will attain a pale colour that is almost white briefly after they are put in the roaster. The coffee will have an aroma similar to that of green beans.
- Yellowing – the coffee beans will start turning yellow with a hay smell. The Maillard reaction starts when the seeds attain temperatures ranging from 145, 150°C. The seeds attain a colour similar to that of an apple which has been exposed to the air.
- Orange/Tan – the coffee beans change colour from yellow to orange. The beans now have a sweet aroma similar to that of baking a bread.
- First crack –The coffee beans pops up or cracks. The coffee is now ready for grounding and brewing depending on one’s preference.
- Development time – This is the time after the first crack where coffee registers a swift change in colour and flavour. Careful monitoring is required to establish temperature and time under which coffee is roasted.
- The second crack – continuing to roast the crack takes coffee to the second stage. The coffee crack silently emitting oil on their surface.