Time for coffee and tea
Commercial Coffee Bean Roasting
Some of the main reasons that coffee is roasted include removing toxins, enhancing taste and concentrating smell. When it starts out a green bean, is very hard, has little smell and contains compounds that are bitter to the taste. Therefore the coffee bean must be roasted in order to change the physiology and chemical make up of the green coffee bean. The roasting process involves exposing the fresh coffee beans to a hot gas, alternatively the beans can be lay on a hot metal surface; the resulting chemical and physical changes lead to coffee that tastes and smell wonderful.
Many different mechanisms have been scientifically developed to roast coffee in order to create better tasting coffee. Nearly all of these methods of coffee roasting use the same basic processes. Initially the green coffee bean is heated up; this results in evaporation of water in an endogenous way. Once the coffee bean reaches about 160oC exogenous chemical reactions take place, these reactions reach their ultimate potential at around 220oC. Further increases in pressure within the bean lead to carbon dioxide and water vapor being forced out of the bean, and a lowering of coffee bean density. This in turn leads to an expansion of the coffee bean in size, and a splitting and popping of the bean. Once the bean has been roasted long enough (from eight to twelve minutes depending on the roast required), it is rapidly cooled by use of water and cold air.
The process of roasting coffee results in many changes in a coffee beans appearance: The bean changes from green to brown or almost black in colour; The density will decrease by around 40%; there will be an increase in volume by almost half the size of the bean again; and there will be a massive loss (up to 85%) in water content.
Usually coffee beans are placed in either a horizontal rotating drum, or a vertical rotating drum, that has paddles to move the coffee around. They are then exposed to hot gasses, which reach temperatures of almost 450oC when they enter the drums, but which cool when mixed with the coffee beans. The coffee beans are then left for between 8 and 12 minutes until they have reached the required roast. In general the greater the ratio of Gas: Coffee then the quicker a bean can be roasted, and the lower the temperature that roasting can be achieved.
One of the most common ways of commercial coffee roasting is that of Fluidized Bed Roasting. In fluidized bed roasting coffee beans are given bursts of high velocity gasses from beneath; this enables the beans to move across the fluidized bed in a controlled way, and results in a high quality end product. During the roasting process coffee beans increase in size and therefore it is essential to modify the speed that the gas comes out in different parts of the machine to ensure uniform roasting.
Further methods of commercial coffee roasting include Fast Roasting. In the fast roasting process the coffee beans are rapidly roasted, and it can take as little as one minute to completely roast the beans (though often it may take up to four minutes). Fast Roasting makes use of a high Gas: Coffee ratio. The coffee that is resultant from the Fast Roasting process typically has a higher brew strength than normally roasted coffee; it is also notable that the beans are also greater in both density and volume.
As can be seen from above the roasting of coffee beans requires that they are subjected to gasses at high temperature on a rotating drum. During the coffee roasting process many physiological and chemical changes occur in the coffee bean. As there are many different methods that are used to roast coffee, the procedures used have a large impact upon the way a coffee looks and tastes. Indeed, the art of commercial coffee roasting enables a hard, aroma less green bean to be changed into the deep smelling, good looking, and great tasting coffee that we all love.