Typically a green coffee cherry has little smell and contains compounds that are bitter to the taste, therefore it is essential that they are roasted prior to use; indeed one of the main reasons for roasting coffee is to enhance aroma.
The roasting process itself involves subjecting the fresh cherries to either a hot gas or laying them on a hot metal surface; this results in chemical and physical changes which in turn leads to an end-product that tastes and smell wonderful.
The Roasting Process
There are many different ways in which cherries can be roasted, and many different mechanisms have been scientifically developed with the aim of creating a better tasting product. Nonetheless the basic processes of roasting remain the same. The cherry is first heated up, resulting in a loss of water through evaporation. Once a temperature of 210 degrees celsius has been reached complex chemical reactions occur, resulting in increased pressure within the bean. This in turn forces carbon dioxide and water vapor to leave the it. The bean will then expand and pop. The bean is left in the roaster until it has reached the optimal roast, and is then cooled quickly by water and air cooling mechanisms.
As seen in the previous paragraph the roasting process is harsh and results in massive changes in the physical appearance of the green cherry: in addition to the obvious colour changes, a roasted coffee bean will lose about 40% of its density, increase in volume by up to 50%, and lose 75 to 85% of moisture content.
Types of Roasting
In conventional roasting, the beans are either placed in a vertical or horizontal rotating drum and subjected to hot gasses (up to 450 degrees Celsius); they are then roasted for eight to twelve minutes, depending on the roast required. The higher the proportion of gas to materials is, the lower the temperature, and the quicker in time, that a coffee bean can be roasted.
A commonly used method is that of 'Fluidized bed roasting'. In this method the beans are subjected to high speed, hot gas from below; this results in them being moved along the machine in a controlled manner, and in turn leads to a high quality product. As a coffee bean increases in volume during the roasting process it is important that the speed of hot gas can be modified in different parts of the roaster to maintain quality.
Another method is known as Fast Roasting. As the name suggests this method involves the rapid roasting of beans, typical times range from one to four minutes. This quicker roasting time is possible because the ratio of gas to material is very high in this method. Beans that have been roasted by the fast method have a different appearance than those from conventionally methods, they tend to be more dense and larger in size. This in turn leads to 'fast roasted coffee beans' having a stronger brew strength.
Concluding remarks
The roasting process involves subjecting beans to high temperature and pressure gasses. This process leads to physical and chemical changes within. The method used has a large impact upon the finished product, with fast roasting typically producing a product with higher brew strength than that of more conventional methods. Overall the process is very beneficial as it helps to enhance how coffee beans look, smell and taste.
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