Dr. Hyde, G.M.
April 15, 1999
The Roasting of Green Coffee Beans
Coffee drinking is an everyday routine for many Americans. Although, before it can reach the consumers cup it must go through several complex processing steps. The most important and critical step is the roasting process. Roasting is a chemical procedure which converts a green coffee bean into a delicate brown or black bean. Coffee beans are roasted to bring out the flavor and aroma, with the degree of roast depending on the temperatures and duration of heating.
Green coffee beans usually arrive at the roasting facility in 60-kg (132 lb.) bags (Clifford and Willson, 1985). The bags are emptied onto a conveyor that runs the coffee beans through cleaning and de-stoning equipment. At this point, the beans may be decaffeinated. Once the coffee beans are cleaned and/or decaffeinated, they are stored in silos until it is time for roasting. See Figure 1.
Figure 1. A continuous roasting plant for processing coffee (Clinton, 1992).
Bean Characteristics During and After Roasting
Physical Changes of the Bean
Physically, as the beans are heated moisture is lost and the bean shrinks in weight, but increases in volume. When bean temperature reaches 200°C (392°F) the actual roasting begins (Clifford and Willson, 1985). During roasting, the primary loss of dry mass is due to carbon dioxide escaping the bean. About half of the carbon dioxide produced is also retained within the bean. Combustible gases are produced from the roasting beans as well, increasing the rate of roasting.
As a coffee bean roasts, expansion occurs, reducing the density of the bean. Different varieties and ages of beans also differ in density and moisture content, and consequently roast at different rates. A more moist and dense bean will roast slower and require a higher temperature in the roasting chamber. For true commercial roasting some roasters will measure the density and moisture content to develop a roasting procedure particular for a set of coffee beans (Davids, 1998). Because optimum flavor occurs at a certain roast color for each bean type, it is recommended that blending of varieties occurs after roasting rather than before. Bulk density influences the efficiency of coffee extraction during brewing, the relative strength of coffee beverage in a given amount of water, and requirements for packaging the ground coffee.
Besides the production of gases during roasting, a small amount of chaff is also produced. The chaff results from the heating and drying of small pieces of "silverskin" left behind in the crevice of the bean after hulling. Chaff is usually removed by way of air movement and a cyclone. A small amount of chaff is incorporated into the final ground coffee product. The chaff is added to soak up oil that surfaces on dark roast coffees, allowing the ground coffee to flow more easily (Clifford and Willson, 1985). The chaff is also added to regulate the bulk density to ensure uniformity in the final ground product and is called "normalizing" (Clifford and Willson, 1985).
Chemical Changes During Roasting
The entire roasting regime entails an intricate creation and destruction of the bean, which upsets the woody structure. During roasting one of the most noticeable changes of the coffee bean occurs from caramelization. Due to the intense heat the starches in the bean are broken down to simple sugars, where they begin to brown. This adds the recognizable color and also a slight sweetness to the coffee bean. As the bean continues developing a dark color the floral aromatics, fruit acids and caffeine weaken. Yet, as these characteristics diminish, the smoothness and other flavor compounds we associate with our coffee are developed. It is a misconception to think that the darker roasts contain higher caffeine levels. In fact, dark roasts such as the French or espresso roasts contain 15-20% less caffeine than a lighter roast (mindburp).
Major changes occur in the carbohydrates during roasting. Sucrose is lost rapidly during the roasting process so that even with a lightly roasted coffee only about 3-4% of its original content in the green bean remains. At a medium roast, some 1% may be found, whereas with very darkly roasted coffee it is completely lost. Other simple sugars present, in particular glucose, fructose and arabinose, are progressively destroyed. Polysaccharides are also greatly reduced. (Clarke, 83, 91)
One of the main flavor compounds developed from carmelization is coffoel. Coffoel, a coffee oil, is formed generally around 392°F in the heating process. Coffoel is a water-soluble compound that has a tendency to absorb other flavors easily. Hence the reason coffee is carefully stored away from other intense flavors.
Initially, roasting causes the removal of all free moisture and proceeds further to force out bound moisture. The force of the moisture leaving will expand the bean and produce crackling and snapping noises. At this point the color has not dramatically changed, and the oils have not volatilized. However, when the internal temperature reaches 400° F the oil will start to develop and the darkening of the bean will be seen (Davids, 1976). The initial crackle along with a light brown color is a good indication of the coffoel formation. The crackles are also an indication of pyrolysis, which is the temperature (around 465°F/240°C) at which gasses are produced in the roasting coffee beans that combust, causing them to emit their own heat. This results in an increase in temperature within the roasting chamber (Davids, 1998). In this stage the color will start to darken drastically and an oil formation will begin to develop (Davids, 1976). Roasting is subdued at the point the bean attains the sought after color.
Figure 2. Chart showing audible cracks during roasting. (Janson, 1998).
THE ROASTING SCALE
Flavor of the coffee beverage varies with degree of roast, as does the color of the roasted bean. Table 1- Degree of Roast (Freeland-Graves and Peckham, 1996) provides a comparison of degree of roast versus flavor.
Table 1- Degree of Roast
Type Roast Color Flavor
Light city (New England) light cinnamon, light brown sour
City (American) medium brown sweet, slight sour
Full city high brown, shiny full-bodied, tangy
Viennese dark dark brown, shiny toasty, bittersweet
French heavy very dark brown, oily very burnt, bittersweet
Italian (Ice) very heavy extremely dark brown to very burnt, bitter
black, shiny, oily
Coffee roasting is the biggest factor in the flavor of the coffee bean. If a good bean is roasted poorly, stringent off flavors and aromas will develop. Both heating the bean to too high of a temperature or too low of a temperature can have drastic flavor consequences. Temperatures in roasters vary from 540°C, 430-480°C, to 370°C (1,004°F, 806- 896°F, 698°F) or lower, depending on air velocities (Clifford and Willson, 1985). The internal temperature of a roasting coffee bean should reach approximately 435° F. If the internal temperature of the bean only reaches 390°F the bean will have a light color, a grassy-sour taste, and lack aroma (Davids, 1998). If the internal temperature of the coffee bean reaches temperatures of 480°F, the flavorful oils will burn off and a charred or industrial flavor will develop (Davids, 1998). Other mistakes in roasting include heating the bean too slowly or too quickly. Under-roasted coffee beans produce a coffee that is bread, or nut-like in flavor (Davids, 1976). Beans that are over-roasted can lead to a burnt outer layer, while the internal structure remains green.
After the coffee beans have reached the desired degree of roast, the beans must be cooled immediately to prevent auto-combustion from altering the desired coffee grade. Beans cannot be allowed to cool on their own or over-roasting will occur (Davids, 1976). Currently there are three ways to achieve the cooling process.
1). Water-cooling by sprinkling the roasted beans with a slight shower.
2). Cooling by normal air.
3). Cooling in forced cold air.
The addition of water helps ensure uniform particle size after grinding and also increases the specific weight (Clifford and Willson, 1985).
Before the advent of mechanical roasters, roasting was done with the use of a frying pan or hand-driven cylinder over an open fire. This was a time consuming, smoky process which required some skill to avoid under or over-roasting the bean (Clarke, 1985). Today, commercial roasting uses large roasting devices such as in figure 3. This continuous batch roaster has the capabilities of four 100 pound roasts per hour.
Figure 3. Continuous Batch Roaster- it roasts and cools at the same time (Dodd, 1999)
Most of the large-scale roasters are made in the U.S. and Germany. The principle method most often used is a horizontal rotating drum in a batch or continuous flow process. A current of hot air is re-circulated through the tumbling green coffee beans. Re-circulation saves on fuel costs due to the retention of the combustible gases produced by the roasting beans. It takes on average 430,000 BTUs to roast 1200 pounds of beans (Clinton, 1992). A typical horizontal drum roaster is the Probat-Werke which roasts four 60kg bags at a time in 10-12 minutes (Clifford and Willson, 1985). The heat is provided by combustion of oil or gas; and a damper adjusts air intake. Typical problems with rotating drum roasters include: the scorching of some beans, and char and oil buildup on the inner drum walls (Clifford and Willson, 1985). The buildup leads to smoky tasting coffee, combustion of the oil buildup, and difficult cleaning. After the coffee beans are removed from the roasters, they are immediately cooled, and subjected to a small percentage of water.
A second type of roaster is the Gothot, made of a vertical drum. The beans are stirred with rotating paddles to distribute the hot air. Like the Probat-Werke, a hot air re-circulation system can also be added. The Gothot roaster also provides a short roasting time of six minutes. After the beans are roasted they are transferred into a "cool air" vessel.
Other roasters used or experimented with include "fluidized bed" roasting (the Wolverine Jet Zone Roaster) and the Smithern roaster. The Smithern roaster heats the green coffee beans under pressure of 50-150 psig with a closed circulation of nitrogen gas (Clifford and Willson, 1985). Normally beans are roasted under atmospheric pressure. The concept for the Wolverine roaster is to use the full benefit of high velocity air to increase the rate of roasting as compared to rotating drum roasters. Air velocity is so great that the beans are lifted off the bed they are resting on, giving the coffee a "fluid like" appearance. By floating the beans on air as they roast, charred beans, and buildup of oils and char are minimized, resulting in a higher quality end product (no smoky flavor taint) and cleaner roasting equipment.
According to Nagaraju (1997) fluidized bed roasting works well for large quantities of beans. However, for smaller batches, Nagaraju recommends using a spouted bed roaster (see Figure 4). The spouted bed roaster operates on the same concept as the Wolverine, floating and mixing the beans on a current of hot air. The difference is that the beans are contained within a vertical cylinder. The spouted bed roaster also works well for beans that are larger than 5 mm, whereas the fluidized bed roaster would tend to develop unstable fluidization (Nagaraju, 1997).
Figure 4. Schematic diagram of a spouted bed roaster. All dimentions are in mm.
Many restaurants, coffee shops, and home brewers prefer to roast their own coffee beans. There are many advantages to roasting coffee beans on-site and in small batches. It allows for custom brewing, it is cost effective, and the freshly roasted coffee bean will be at its peak flavor. A green coffee bean retains its full flavor for about two years, but once it has been roasted it starts to lose flavor in just three days, even if it has not yet been ground. Over the next three weeks, even more flavor is lost (Mean Bean). For these individuals there are small roasters available, many around $100 to $150. Designs of these small roasters vary, but all use the same principle as the large commercial roasters. However, instead of using gas to heat the beans, hot air from an electric element is used (Mean Bean).
Figure 5. A Fresh Roast coffee roaster made by The Mean Bean, Inc.
After Roasting and Cooling
After the coffee beans are roasted and cooled they can be packaged and marketed, or they can be further processed. Further processing includes blending for desired flavors, grinding, extraction, and packaging. Coffee is sold in various particle sizes. Regular grind is used in percolators, medium grind is for drip filter machines, and pulverized is for espresso machines. When extracting coffee for the purpose of instant coffee (soluble granules) a course grind is used. Finally, the roasted ground coffee is packaged into metal cans or vacuum packed into flexible bags.
Initially, there is little to no associated flavor from the green coffee bean. The roasting of green coffee beans is a crucial step in the production of good coffee. Type of bean and degree of roast affect the desired flavor. Without proper roasting the coffee bean cannot develop to its aroma and flavor potential. Additionally, the roasting process has evolved from pan frying the bean to high capacity and quality roasters. Commercial roasters, such as the rotating drum, perforated drum, and fluidized roaster each provide various benefits and restrictions. Given the spectrum of coffee roasters available, there is sure to be a degree of roast to please every coffee consumer.
Clark, R.J., and Macrae, R. (1985). Coffee Volume 1: Chemistry. New York:
Elsevier Applied Science Publishers.
Clifford, M.N and Willson, K.C. 1985. Coffee: Botany, Biochemistry, and Production of Beans and
Beverage. The Avi Publishing Co., Inc. Westport, Connecticut.
Clinton, William P. 1992. McGraw-Hill Encyclopedia of Science and Technology (Vo. 4, pp. 106-109), McGraw-Hill, Inc., London
Davids, Kenneth. 1976. Coffee: a guide to buying, brewing, and enjoying. 101 Productions.
San Francisco, California.
Davids, Kenneth. "Home Coffee Roasting." Rev. 10/98.http://www2.lucidcafe.com/lucidcafe/homeroast.html
Dodd, Sherma. "Coffee/PER" Rev. 3/99. http://www.coffeeper.com/CF/coffee2.htm
Freeland-Graves, J.H and Peckham, G.C 1996. Foundations of Food Preparation. 6th Ed.
Prentice Hall, Englewood Cliffs, New Jersey.
Janson. "Roasting" Rev. 1/98. http://www.estatecafe.com/roasting.html
Mindburp, "The Perfect Coffee is Only as Perfect as its Roast" Rev 4/99http://www.mindburp.com/roasting.html
Nagaraju, V.D, Murthy, C.T., Ramlakshmi, K., and Srinivasa Rao, P.N. 1997. Studies on roasting
of coffee beans in a spouted bed. Journal of Food Engineering. 31 (2): 263-270.
The Mean Bean, Inc. "Fresh Roast Home Page" Rev 4/99
Ukers, W.H. 1935. All About Coffee. The Tea and Coffee Trade Journal Company. New York