During the roasting process, the coffee changes physically in form and appearance as it absorbs heat and changes in flavor. The roaster can use these clues to monitor the progress of roast so they can respond by making adjustments in the amount of heat being applied (or noting what should be changed next time).
The main physical differences between roasted coffee and green coffee (besides the obvious color difference) is (1) the density of the beans and (2) the level of moisture.
Density is measured in terms of weight to volume. As an extreme example, metal is denser than plastic because when one weighs similar volumes of each, the metal will weigh more. Green coffee has a density of about 1.25 grams/milliliter (g/ml), while a medium-roasted coffee will have a density of 0.70 g/ml (Clarke, 1987, p. 85). This change is due to loss of moisture and (especially) sugar browning, which releases carbon dioxide and moisture (Sivetz, 1979, p. 250).
Change in density is of significance to the roaster because it influences how application of heat affects the bean at different stages. At the beginning of the roast, when the bean is at its maximum density, it is more resistant to absorption of heat and it takes a longer time for noticeable changes to take place. Conversely. when the coffee has lost a significant amount of density (later in the roast), changes in heat have a more direct effect.
Moisture content is important to the roaster since the moisture is the mechanism for conducting heat into the bean and causing the coffee to roast (Cellulose, which makes up much of the green bean, is an insulator, while moisture is more of a conductor of heat). Green beans have a moisture content of about 10-13%, while roasted coffee will usually have less than 2% moisture.
While moisture is more capable of conducting heat than cellulose, it is not as good a conductor as (for example) copper. It also has the property of retaining the heat it aborbs for a longer period of time (again, unlike copper, which cools very fast when the source of heat is removed). One can think of boiling water on a stove: it takes some time for the mass of water to heat up initially, but once it is hot it takes a certain period of time to cool down. The amount of time it takes to heat up and cool down depends on the amount of water and the level of heat being applied.
This is also the case with coffee. The more moisture that is present in the green coffee, the longer it will take to intially heat up, but more heat will be absorbed, causing the roasting process to accelerate at a later stage.
Application of heat: The rate at which heat is applied is the main issue for the roaster. A little knowledge of thermodynamcs (especially the second law) is helpful. The main items to remember are (1) that heat has a tendency to dissipate and (2) the direction that it dissipates is always from the hot item to the cooler one. In coffee roasting this is significant because it takes comparatively high temperatures to initiate and maintain roasting processes. Especially in the later stages of roast, if one does not maintain an environmental temperature in the roasting chamber higher than the bean temperature, the heat in the bean will dissipate ("exotherm") and the roast will stop. Once the roast has stopped, it cannot be started again.
Too much heat is as bad as too little, however. Charring and carbonisation of the bean surface can happen at any stage and adds a burned taste (Clarke, 1987, p. 79).
As with any cooking process, careful measurement of roast parameters are necessary. One measure to take is the amount of green beans to be roasted. This will vary with different roasting machinery; check the manufacturer's recommendations for the individual machine. One note: many (if not most) roasters come with a cup or other volumetric measurement. For more consistent results, weighing out the beans is recommended, since they vary in density. As the roaster develops their technique, measurement by weight will prove to be more useful in determining intended roast parameters for different coffee beans.
The roaster must be well heated up before introducing the beans into the roasting chamber. One has to initially depend on the machinery manufacturer for the recommended starting temperature, but this can be changed if satisfactory roast parameters are not attained. And finally one of the most important measurements is time; measuring using a stopwatch is recommended.
The following sections show how the roast should progress stage by stage. The roaster can monitor the changes in color and size of the bean and the amount of time it takes to roast. From this, what is happening physically in the bean and what flavor changes are occurring cna be inferred. It is recommended that one keep records of the process by filling in the roasting log.
Initial Charging of Roaster
Before putting the beans into the roaster, the color and condition of the beans should be noted. Getting an idea of the initial condition of the beans will help the roaster note how quickly they are changing in the initial phase of roasting. For more detailed examination of the green beans, see the green coffee page. When the roaster reaches the recommended temperature, start the stopwatch and place the beans in the roasting chamber.
Observable changes: The beans begin to expand in size and the color turns a deeper almost transluscent green. In natural processed coffees (which start out more yellow) or older (including "aged" and "monsooned") coffees the change may not be as noticeable. The aromatics smell slightly sour and vegetal at this point. This is called the "drying stage" by some
What is happening physically: The moisture in the bean has begun to absorb heat. The expansion is caused by the heated moisture pushing against the cell walls. The amount of swelling that takes place is due to orginal bean size, density, and percentage of moisture. The coffee has not lost much density, but is at the "rubbery" stage, where the moisture is dissipating through the cells and allowing expansion.
What is happening to the flavor: Not much. If one were to grind and brew coffee at this stage (assuming one did not destroy the grinder -the coffee is still too dense for most typical coffee grinders), it would taste mainly like green coffee, perhaps with a touch more acidity and cereal flavor. However, this is an important stage as the amount of heat absorbed will determine how quickly the coffee roasts at later stages.
Observable changes: The beans continue to expand but the color goes from green to golden-brown. The sugar browning has started and the aromatics move to sweet and caramel-like. From here on, the rate of browning will be an important indication of how the roast is progressing.
What is happening physically: Heat absorption continues and a "front" of heat is beginning to move from the outside to the center of the bean. The sugar browning reactions themselves also release heat and the roasting process accelerates. Moisture loss results in the coffee losing some of its density, but the bean is still expanding as the heat penetrates the bean.
What is happening to the flavor: The sugar browning reactions have started to develop the unique aromas of which the coffee is capable and the acidity is heightened due to sugar browning by-products. If one were to grind a brew the coffee at this stage, it would taste like sour cereal, with perhaps a touch of coffee aroma.
Observable changes: The beans continue to expand slightly and the color deepens gradually to a nut brown. The aromatics are beginning to develop some complexity, but are still mainly sweet and caramel. If the bean begins to look a little mottled or uneven, the heat may be turned too high. However, be careful in turning down the heat too much, as the roasting process will stop if the environmental temperature is allowed to cool below the temperature of the bean.
What is happening physically: Sugar browning is occurring throughout the bean and, in combination with the heated moisture, internal pressure is building. At this point, there are three sources of heat that are causing the roating process to take place, (1) the heat from the air in the roasting chamber, (2) the chemical release of heat due to sugar browning, and (3) the heat from the internal pressure buildup.
What is happening to the flavor: The more "coffee-like" aromatics are developing along with the aromas unique to certain coffees. The sugar-browning reactions continue to contribute to some types of acidity, but some of the fruit acids have begun to degrade. Coffee brewed at this stage of roast would still taste a bit sour and have some cereal flavor, but it would also have more recognizable coffee flavors and aromas with a bit of body. Not really ready for drinking, however.
Observable changes: There is an audible "pop" (or "crack") and the bean continues to darken. Aromatics are definitely coffee-like and unique origin aromas may be apparent. The heat may need to be lowered, but again the roasting chamber cannot be allowed to cool too much or the roasting process will stop.
What is happening physically: At this point, the internal pressure that has built up inside the bean as the result of moisture heating and sugar browning releases into the environment, raising the environmental temperature. At the same time, the bean loses a lot of density, opens up, and becomes more susceptible to changes in the environment. The pressure is released mainly through the seam in the bean and much density is lost. The bean is moving from the "rubbery" stage to a more brittle texture and not much expansion occurs beyond this stage.
What is happening to the flavor: The acidity is high and unique aromatics are high, but little body has developed. There is still noticeable cereal in the flavor.
Observable changes: The beans continue to darken and the browning evens out. Aromatics emerging definitely have the quality of roast coffee. Later in this stage, some smoke becomes evident. This stage requires careful heat management on the part of the roaster: too hot and the aromatics burn off, too little heat and the roasting process ceases.
What is happening physically: The bean has opened up as the result of first pop and responds more readily to heat in the environment. The bean continues to lose density as the sugar browning processes deepen the color.
What is happening to the flavor: This is the area in which a lot of flavor balancing can occur and is a favorite stage for single-origin coffee. Early in this stage, there is still plenty of acidity and aromatic development with roast-flavor quality replacing the cereal flavor. Later in the stage, the aromatics are at their most prominent and the body significantly increases, while intensity of acidity decreases. The beginning of this stage corresponds to the "Light Roast" stage illustrated on the roast flavor chart while the "Medium Roast" line on the same page corresponds to the end of this stage.
Observable changes: The beans pop again, but the sound is higher in pitch. The color starts to turn a slightly bluer shade of brown. Towards the end of this stage, oil begins to show on the surface of the bean. The bean may also "bloom" (show some oil) after cooling if the roast is ended at this stage. Bluish smoke is definitely in evidence.
What is happening physically: The popping is the result of the cell structure of the bean breaking down. This liberates the oils to flow through bean, ending up on the surface.
What is happening to the flavor: The acidity has almost vanished, the body increases, unique origin flavors are less apparent, and the "roasty/dark roast" flavors begin to dominate the flavor profile. Most espresso blends are roasted somewhere in this area and many prefer this stage of roast because of the emphasis of roast flavor, body, and sweetness. Most of the more delicate aromatics (fruit, floral) have been roasted off and those that remain are mainly those of roasted nuts, coffee-sulfurous, and earth. The early part of this stage corresponds to the "Medium Dark Roast" stage illustrated in the roast flavor chart.
Observable changes: The beans are shiny with oil and blue smoke is coming on strong (turn on the vent or open a window). At this stage one must be careful or the beans can burn or even ignite.
What is happening physically: The oil has pushed its way to the surface and the blue smoke is an indication of carbonization and late stage sugar browning.
What is happening to the flavor: With the acidity gone, the body hits it peak. Dark roast flavor increases as body is reduced. The aromatics are mainly in the dark-roast and earthy categories. The taste is mainly bitter and oily as the sugar carmelization is taken to its final stages and corresponds to the The beginning of this stage corresponds to the "Dark Roast" stage illustrated on the roast flavor chart.
Observable changes: The beans are totally blue-black and the oil becomes less apparent as it burns off. SAFETY NOTE: Under no condition should the roaster enter the realm of "third pop", which is a fire in the roaster that ruins the machinery as well as the coffee. Ask a few professional roasters, they all have stories about roaster fires.
What is happening physically: The oil begins to burn off and the charred cell walls are all that remain.
What is happening to the flavor: Dark, dark, dark. All coffee tastes pretty much the same at this level and most prefer it with plenty cream and sugar to add the body and sweetness that has been roasted away.
A major factor in this progression is the quality of the green bean and its physical attributes. Professional roasting operations usually have lab equipment that can more precisely measure some of these aspects, but the roaster can get some clues by close observation, as shown on the page on green coffee.