The first change that occurs within the bean is the heating of the green bean's surface moisture. Some of this moisture is vaporized (for this reason, this stage is referred to as “the drying stage”), collapsing the first layers of cells while some moisture heats the interior of the bean. The roaster observes this by noting expansion in size and a change in bean color.

The beans themselves influence the rate at which this takes place. The conductivity of the system is a function of the physical state of the green coffee. In washed process coffees, the bean moves from a green or blue-green to more of a jade green. Natural process coffees (such most Indonesian or Brazil coffees), which begin as pale yellow, deepen in hue to a golden color. The amount of swelling that takes place is an indication of how quickly the bean is taking on heat. This is partially dependent on the amount of moisture in the bean. , but little of that energy is applied towards roasting reactions. The “drying stage” is a preparatory stage and the amount of time it takes will determine the heat difference between the outside and center of the individual beans. determining how quickly the absorbed heat will distribute throughout the bean. It is possible to overheat the beans at this stage, burning and charring the outside yet not allowing a gradual enough heat penetration. Conversely, if not enough heat is applied (or, as often happens, if the mass of coffee, the “charge,” to be roasted is too large) the beans will not heat evenly. At this stage, a great deal of the thermal energy in the roaster is required for evaporation.

With sufficient heating of the bean and the removal of a certain amount of surface moisture, the chemical changes associated with coffee roasting begin (referred to as “roasting proper” by Dr. Clarke 6 ) and the coffee moves from gold/yellow into a brown or reddish brown phase. This process begins when the bean temperature is around 160º C 7 . This color change indicates that the sugar browning reactions have begun to take place. It is these browning reactions (which include Maillard reactions, carmelization, and other reactions) that gradually move the thermodynamic process from one of an endothermic reaction (heat absorbing) to an exothermic reaction (heat releasing). The “activation energy” at which the sugars begin exothermic chemical reactions is 175º C 8 . The amount and rate of this reaction is determined by the concentration and type of sugars present in the bean (higher quality coffees have a greater amount of sugar than lower quality coffees; past crop coffees have less sucrose but more glucose) as well as moisture content and ambient heat (the increasing bean temperature). This is one of the most important phases of roasting; the rate of roast and resulting chemical reactions will be established as the result of the amount of heat taken on and the rate of sugar browning reactions.

Besides color, an important clue to the roaster is the aroma of the coffee as it develops. A sourish green aroma, possibly somewhat like peanuts in natural process coffees, is the first to develop. As the coffee moves from golden to reddish-brown, a sweet-roasty aroma develops. Finally, the more familiar coffee aromas develop at a temperature of 180-190º C 9 . The smell of burning may indicate carbonization.

A further consideration is the influence of air movement within the roaster. This creates a change in pressure and convective heat and removes humidity and gasses and will affect the rate of the process, with greater airflow intensifying the effect of the applied heat.

The exothermic reactions climax with first crack. At this point, a great deal of heat is released into the roaster and considerable mass transfer from the roasting beans (in the form of moisture, CO2, and other gasses) is taking place. Since the bean has expanded and loses mass during this process, it is considerably less dense and will not resist applied heat as easily. As a result, the batch is more subject to carbonization. The amount of exothermic energy released depends on the location, on the temperature, on the time a certain amount of organic compounds is being processed (reaction rate) and on the reaction enthalpy (heat loss as the result of the reaction).

At the point of first crack, the bean has already absorbed the potential heat energy that will produce the reaction. While the sugar browning reactions are occurring, the system is generating its own heat as well as absorbing heat. The exothermic energies lead to a further increase of the bean temperature without additional heat applied from the roast gas.

From a flavor perspective, the goal is to create a flavor profile made up of aromatics and a favorably perceived balance of sweet, acidic (tart), and sometimes bitter sensations, while minimizing the less appreciated astringent or bitingly bitter sensations. The purpose of roasting the coffee at a certain rate is not only to create the desirable flavors, but also to ensure a level of degradation of less desirable flavors.

The principle observable change is in the browning of sugar. In the “roasting proper” period before first crack, the sugars are heating up and some browning reactions are taking place. During first crack, the greatest amount of browning rates is taking place. At this point, many chemical pathways can be taken, especially in the presence of free amino acids (some of which are themselves the product of heating), which can result in a multitude of potential flavors. These flavors are the result of the temperature already attained by the reactants and the amount of thermal energy in the environment that is continuing to drive the process. The actual flavors themselves are partially the result of what is available in the bean for reaction, which specific amino acids and the form of the sugar (sucrose, glucose, or fructose). As a result, the roaster will find that some roaster profiles work better for some samples than another in emphasizing the unique attributes of a particular coffee. However, the flavor possibilities of browning reactions are almost infinite.

In addition to the sugar browning reactions, other reactions that will result in flavor are occurring. Perception of acidity (tartness) in the cup is due to a combination of several acids. During roasting, chlorogenic acid (which tastes a bit bitter and/or astringent) degrades into quinic acid and acetic acid is created as a by-product of sugar degradation. Certain carboxylic acids are inherently present in the green bean. Some do not degrade with roasting, such as lactic acid, while others such as citric and malic gradually degrade as the coffee darkens.

Trigonolline is a nitrogen-based substance that is bitter in taste and degrades as the roast progresses. Carl Staub, inventor of the Agtron and roasting trainer, maintains that “For lighter roasts there will be more trigonelline, hence bitterness, but also less sugar carmelization. Caramelized sugar is less sweet in the cup than noncaramelized sugar, so when properly roasted these two constituents form an interesting compliment to each other. 13 ” This is the basis of his “ideal reaction rate” –to balance the degradation of trigonelline with the carmelization of sugar (caramelized sugar tastes less sweet). Degradation of trigonelline begins at a temperature of 193º C and the majority has degraded by the time the batch reaches a temperature of 230º C, a moderately dark roast. Degradation products include the aromatic classifications of pyrroles and pyridines, characteristic of dark roast flavor profiles, and nicotinic acid, generally regarded as favorable.

As mentioned, the degradation of chlorogenic acid produces quinic acid, usually perceived as pleasantly sharp. In addition, caffeic acid, quinones other than quinic acid, and phenols are produced through chlorogenic acid degradation. Many roast measuring systems, including the Agtron, use the degree of degradation of chlorogenic acids as a means to measure roast degree (by measuring the amount of quinones present). Depending on roast degree, a certain amount of residual chlorogenic acids, generally perceived as more bitter or astringent, may still be present. The aforementioned phenols have the effect of drying the salivary glands and creating an astringent sensation.

In addition to flavor (tastes and aromas together), an important aspect of coffee flavor is body, how the liquid feels in the mouth. This sensation is due to viscosity of the liquid and is influenced by different phases (oils, particles, etc.) dispersed throughout the beverage. Perception of body is mainly due to the cellulose structure of the bean changing form and becoming more soluble as the result of applied heat.

During second crack (which typically begins as the bean reaches a temperature of 230º C), the cellulose and other complex carbohydrates that compose the cell walls of the bean begin to fracture as the result of heat 17 . The carmelization of sugars is fully accomplished by the time the coffee reaches 230-240º C and most of the carboxylic acidity has been volatized. The primary consideration in terms of roasting at this level is maintaining the inherent structure of the bean while allowing the roast to progress.

The flavor profile has entirely shifted from the balance of sweetness, acidity, and bitterness to characteristic dark roast flavors dominated by bitterness. Since much of the soluble cellulose has volatized, the sensation of body begins to reduce as roast progresses. Many aromatics volatize completely and a few characteristic “dark roast” aromas dominate the aroma profile.

After second crack, there is danger of creating problematic unacceptable flavors due to overheating. If the heating is uneven, the cell matrix will be damaged, carbonization will occur unevenly, and the flavor quality of the coffee will be diminished. High levels of pyridine will adversely affect the flavor 18 . Earlier in the roast, trigonelline degrades into a mixture of pyridine derivatives 19 , some based upon combination with methyl and other alcohols (3-Methylkpyridine, 4-Ethylpyridine, etc). Thermally induced methanol loss leaves pure pyridine, described as ashy and smoky in flavor.