Roasting Coffee
There are many factors that can influence the taste of coffee, from where it is grown and the particular cultivar chosen as well as post-harvest methods. However, perhaps the greatest influence on the final flavour is created during the roasting process.
In green bean form the coffee bean has no taste, flavour or smell it is simply the seed of the coffee tree. It is only at temperatures above 200 degrees centigrade that the green coffee is transformed into a roasted product giving rise to colour, aroma and taste.
The green bean is primarily made up of carbohydrates (40-65%), nitrogen containing compounds, proteins & caffeine(11-15%), lipids (8-18%), organic acids (8%) and water (18-23%). Robusta coffee typically contains more caffeine and acids than arabica, the later gives robusta its characteristic harsher flavour.
Coffee flavour is made up of more than 1000 volatile and nonvolatile compounds and so is a highly complex product. The art of creating coffee blends is therefore a very skilled job which requires many hours of tasting to understand the complexity of individual origins and how different roast colours affect the final flavour.
In order to roast coffee correctly the roasting drum needs to be pre-heated to at least 200 degrees centigrade. Failure to do so will result in the coffee taking too long to roast which results in a flat and unappealing baked taste. The amount of coffee roasted in each batch will also alter the final flavour of the product. Over filling the drum to its potential maximum capacity will result in the coffee taking longer than is ideal to roast, conversely putting too little in results in the coffee roasting too quickly and not developing its best flavour. In our traditional style roasters, it typically takes around 20 minutes to roast the coffee.
The initial stage of roasting (under 200degree C) is the drying stage where moisture held within in the bean is driven out. The amount of water retained in the bean when it is processed in raw form is a key factor in the final taste in the cup as it will greatly affect the time taken to roast. Too much moisture will lengthen the roast time, too little will result in a much quicker roast. Both will result in overall lower and poorer quality taste & flavour. As the bean dries out and higher temperatures are attained the complex changes in the bean begin to happen. The bean begins to swell up, releasing carbon dioxide, and eventually we reach the “first crack” stage. It is from this point onwards that the coffee can potentially be cooled, ground and drunk. Prior to this point the bean will not have developed sufficiently to release its full potential and will result in an unpleasant grassy and overly acidic flavour.
From now on it is very much down to the individual roaster to determine the degree of roast colour they require to achieve the final taste and flavour of a particular origin or blend. The lighter the roast colour the more of the original flavour of the coffee will be tasted whereas the darker the roast the more origin flavour is lost and the “roast” flavour takes over. I like to use the analogy of cooking a steak. If you cook it rarely you taste the true meaty flavour whereas a well done steak tastes more of the cooking than the actual meat.
Lighter roasting tends to lead to sweet, floral, fruity, and nutty characteristics whereas a darker roast brings out cocoa, spicy ashy and pungent dark roast flavours. The darker you go the more chance there is of over roasting and creating a coffee that is too bitter. It is very much down to the skill and knowledge of the roaster to be able to judge the correct colour for a particular origin which will best highlight its unique taste and flavour. This can only be achieved by many hours of tasting and evaluating individual origins. There are many roasters in the UK, and elsewhere who complete a one- or two-day course in coffee roasting and believe they have the knowledge required to actually competently roast coffee. Judging by the standard of many of the poor-tasting products I find in the market this is definitely not the case.
I believe many roasters actually approach roasting from the wrong direction, ie they may understand the natural flavour of a particular origin but they fail to take into account the final brewing method that they are designing a blend/roast for. Every brewing method will bring out slightly differing qualities in the final product and as such this is a crucial element that needs to be considered when deciding on the final roasted colour.
As previously mentioned in the initial stages of roasting moisture is driven out of the bean. The result of this is that there is considerable weight loss during the process. Typically, a medium roast may lose up to 18% of its starting weight and a dark roast up to 23%. This weight loss in a considerable factor in the final selling price of roasted coffee as if we put 100kg of raw beans in to roast we may only get 77kg out after roasting!
As the roasting coffee nears the required colour the heat being applied to the beans is turned down to slow the rate of change thereby allowing a greater degree of control over the end product. We use the traditional method of using colour samples to check the actual colour of the roasting beans to ensure we achieve the roast we require. This is done by using a sampler which allows us to remove a small sample of roasting beans from the drum during roasting. We then check this against our reference sample until the correct colour is achieved. At this point it is critical that the beans are emptied from the drum and cooled sufficiently and quickly to prevent over roasting. This is where the coffee continues to cook in the cooling tray resulting in a darker final colour than expected.
Once the coffee is completely cold it is put through the destoner. This is a vacuum lifter that is set at the specific rate of air flow required to lift the roasted beans but to not lift anything heavier. The downside to coffee being a natural product is that it often involves a degree of manual handling, and it is not uncommon to find stones in amongst the raw beans. The purpose of the destoner is to prevent these being mixed in with roasted coffee beans which is never going to be beneficial to grinding the coffee for brewing! It has to be said though that a lot of coffee is grown in soil which is volcanic, and this results in very light weight soft stones which sometimes manage to pass through the destoner. However most quality coffee grinders are built to cut out if a stone enters the auger that feeds the beans into the grinding burrs thus saving the grinder from burning out or any damage occurring to the actual burrs.
Another by product of coffee roasting is what we call husk. This is a very thin skin that surrounds the raw coffee bean. During the roasting process, the bean swells up and eventually cracks open. It is at this point this skin breaks off and is carried out of the roasting drum with the smoke produced during roasting. The husker is specifically designed to create a swirling motion which results in the husk falling to the floor of the husker whilst the smoke rises and goes through the afterburner where temperatures are sufficient to burn off the smoke and smell.
This husk has been chemically analyzed and found to be high in nitrogen and is especially useful to the gardener in enrich soil. The only downside is that it is incredible light and therefore liable to be blown away unless immediately dug into the soil!