is a classic value-added beverage crop. Mature, healthy, and intact green coffee beans appropriately washed, dried and cleaned, hold the full potential chemical arsenal for firing into good quality and tasting cupped coffee. Inside the living coffee beans are the flavor and aroma precursor compounds, which are the promissory notes for a good tasting brew.
The exact chemical combination is unique to each and every coffee origin, and governs its organoleptic signature, stimulating the taste buds on the tongue and sensory cells in the nose. There too, locked away in green coffee, are the chemicals responsible for the color and body in the brew. However, they all need priming and activation by roasting the physical force behind the chemical change.
The roaster, as the ‘gatekeeper’ of cupped coffee quality, has the enormous responsibility of converting the ‘slumbering’ physical and chemical qualities of green beans into real live taste and flavor. Green coffee beans delivered into his or her hands hold the hopes and aspirations of the grower, the farm processor and the sorter.
Coffee roasted by an individual and measured in minutes, pales to insignificance, when compared to the days, months and years invested by hundreds of workers in the field, washing facility, drying shed and sorting room. The hands of a skilled roaster, for just 10-15 minutes, will do justice to high quality green coffee, and retrieve a measure of respectability for mediocre blends. Otherwise, it will send years of effort and investment -- literally -- up in smoke.
An experienced operator makes the process look easy, although roasting coffee is highly complex. It is a clever art based on exact science, and is not for fools or the faint hearted. Any attempt to dissect roasting step by step (with its 200 compounds changing shape and form, structure and property), just turns a complicated process into a maze. The process is best considered as a series of themes, which is the essence of good coffee roasting.
Uniformity of Roast
A good roast is fine, but a ‘uniformly’ good roast is the best. Uniformity is -- first and foremost -- the responsibility of sorting and quality control to ensure that the beans delivered to the roaster are of even size, shape, and color, with quality well and truly sorted. There are two angles to uniformity. First, is uniformity of beans within the shipment received for roasting, which is well appreciated and catered for by sorting and quality control. In addition, there is non-uniformity within the origin at large, with its roots further upstream in the processing and drying stages and much more difficult to pinpoint.
Misshapen beans, broken beans, or simply a wide variation in bean size, delivers a ‘ragged roast,’ since different sized beans roast to different colors in the same batch. Good sorting will, however, usually overcome this problem, but the cause of a ‘dull’ roast rather than the desirable ‘bright’ roast is more difficult to identify. Over-drying or sub standard processing is generally the cause of dull roasts. Wide variation in bean color, especially the occurrence of ‘pales,’ is usually indicative of on-farm discrepancies, including immature picking, unsteady fermentation and drying, as well as poor quality control.
Although sorting will not necessarily deliver in every situation, when non-uniformity of the batch is caused by a small number of extreme pale-colored beans, it can definitely remedy the problem. If the variation is more general, gentle and less obvious, it will be more difficult to assess and approach by quality control.
In practice, every batch of beans from the same origin will be uniquely, if not marginally different. This is where “sample” roasting comes into play. With a small amount from every shipment, it will be easier to anticipate every eventuality for a commercial roast of approximately 100-kg or more. Errors that occur during trial runs are much less costly in the long run.
Less Weight Bigger Size
An obvious consequence of roasting coffee is that individual beans get much bigger, although it is also losing significant mass (weight) in the process. Loss in weight is mostly due to the water that is being driven off in the early stages of heating, while the increase in size is due to the expansion effect, which are the gases given off during pyrolysis.
When the temperature within the roaster approaches the boiling point of water (100°C), green coffee will have been dried to around 12% moisture (weight/weight). Moisture content of green coffee comprises of free water and moisture, which is chemically bound. During the first stage of roasting, free water is driven off, thus accounting for the single largest proportion of overall weight loss in beans.
In the later stages -- at higher temperatures -- some chemically bound water will also be lost, leaving roasted beans with a water content of up to 3%, and possibly reaching as low as 0.5%. As the process heats up, the water acting as steam hydrolyses (loosen and break up) long polysaccharide chains into individual sugar units for caramelizing with taste and color. The water content of roasted coffee should be maintained above an absolute minimum, or else the beans become bland brittle items that disintegrate into powder at the first touch of the grinding blade.
Other components contributing to total weight loss in beans are volatile chemicals dissipating in the vapor phase and gases, mostly carbon dioxide but also carbon monoxide and nitrogen, formed and released during pyrolysis (oxidation /combustion) of insoluble components like polysaccharides, proteins and lipids. Depending on the degree of roast, the remnants of the ‘silverskin’ that is lost as chaff, is responsible for a small proportion of the overall weight loss of 14 - 23%.
The first expansion of the bean occurs when free water driven off as steam, expands the beans and causes a distinctive crackling noise. However, most occurs during pyrolysis at 140-160°C, when the temperature inside the bean approaches 200°C. Coffee beans break open violently -- like pop-corn -- and swells, causing the silverskin to fly off as chaff.
At this point, the gas -- mostly in the form of carbon dioxide -- has been generated by oxidation of carbohydrates. About half is retained inside the beans, causing high pressure and the beans to double in size. Entrapped carbon dioxide is very important in maintaining storage life of roasted coffee, because under normal circumstances un-ground, roasted coffee beans would be very susceptible to oxidation. The protective cushion of carbon dioxide is largely liberated during grinding, and is the chief cause of ground coffee losing its pleasing aroma and flavor.
Change in Color and Aroma
Color and aroma are the two changing characteristics most closely associated with roasting coffee. In simple terms, roasting turns green coffee beans brown, and the longer they are roasted the darker they become. Focus is then placed on the desired end result, which is commonly simplified as a light, medium, and dark roast. However, the dynamic nature of bean color during roasting, how it ties in with changes in aroma and the chemistry underpinning the whole roasting experience, is more important.
The first detectable color change happens early on, while free water is evaporated in the boiling process. Around the three-minute mark, the green color is lost as the beans brighten and turn yellow, though virtually no chemical changes will have taken place. This is confirmed by the grassy, cereal smell of hay and wheat to match the corn-colored beans.
Approximately four to five minutes later, with the temperature approaching 200°C, the chemical changes begin and oil forms in the beans, slightly darkening into a light tan color. Potentially volatile compounds already in the bean, or new ones produced by pyrolysis, are driven off in the vapor phase, and produce the characteristic pungent smell familiar in coffee shops that roast on the premises. If stopped here, the ‘reward’ is a ‘light’ roast of tan brown beans that is high on citrus acidity, low on body, but ideal for breakfast coffees, if mellowed with milk or cream.
A comprehensive chemical change will take effect about one to three minutes further down the line (9-11 minutes of roasting), with a correspondingly marked darkening in bean color. Acids start to break down, and sugars and crude fibers are partially caramelized to provide color and body in the cup. There are about 200 different flavor and aroma chemicals that are uniquely combined, as the basis of taste and aroma, and general feel of cupped coffee unique to the origin or blend. This is also known as the medium roast time, but there is nothing medium about this stage, except for the color. It is the level of delicate balance, yielding as much body as possible, without falling from the pinnacle of acidity and losing signature aroma. In the final seconds of this stage, the beans will have become as dark as possible without incurring an oily surface, a point of no return beyond what is a darker and riskier side of roasting.
Just one to two minutes later, the roast will enter a real ‘dark’ territory. The beans are no longer dry, as coffee oils - once inside - seep out freely on to the surface of the bean. Acids and trigonelline are disappearing fast, and at some point, body smothers acidity, and caramelised sugars overwhelm aroma.
Only the most resilient origins can withstand and benefit from this length and intensity of roasting, to provide body and silky spice with just enough acid sparkle. If moving further than 13-minutes, luck runs out for all, but those who like coffee beans black and burnt, with the coffee character having finally gone up in a puff of smoke.
Fast Moving Process
From the roaster’s and coffee connoisseur’s point of view, demarcation of roast degree into light, medium, and dark will seem a rigid and simplistic way of describing a continuous process of continually changing characteristics. Borderlines are in the eye of the beholder, with labels placed accordingly.
About the Author: Dr. Terry Mabbett is a technical writer with a PhD degree in Tropical Agriculture. He has worked in crop production and processing throughout the tropics - India, South East Asia, West Africa, and the Caribbean - and in his home country of the U.K. Dr. Mabbett has been writing professionally for over 20 years.