The goal of specialty roasting is to preserve the natural character and origin of the coffee bean. The roast master creates a unique profile for each batch to preserve and highlight the aromas resulting from the processing of the coffee cherry and the characteristics of the terroir.
Unlike mass production, the journey of the coffee bean here can often be traced back to the specific farm.
Roasting takes into account what happened to the coffee cherry after harvest. Whether it is washed, natural, or anaerobic fermentation, it determines the roasting method to balance fruitiness and sweetness.
Every coffee variety (e.g., Bourbon, Geisha) has its own flavor profile. The goal of the specialty roaster is not to 'burn off' the notes characteristic of the variety, but to let them shine.
As heat rises during roasting, the natural flavors of the coffee bean give way to roast aromas:
Processing methods did not come about by accident; their development was driven by necessity, environmental conditions, and the maximization of flavors.
Developed in places with water scarcity (e.g., parts of Ethiopia, Brazil). The coffee was simply spread out in the sun to dry along with the fruit flesh.
Developed where water was plentiful, but humidity was too high for slow drying. Here, water was used to quickly remove the fruit flesh to avoid rot.
Started in Costa Rica in the 2000s. The goal was to combine the cleanliness of washed coffees with the sweetness of natural coffees, while saving water.
Coffee beans (with or without fruit flesh) are placed in stainless steel tanks, which are then hermetically sealed. Lactic acid bacteria become dominant. Due to high pressure, sugars and aromatic compounds (esters) in the fruit mucilage (mucin) do not escape outwards but are 'pressed' into the coffee bean (the seed) through cell membranes.
This technique was adopted directly from winemaking. The coffee is not simply locked in a tank, but carbon dioxide (CO₂) is actively pumped in, displacing all oxygen. The coffee beans are placed inside as whole fruits (with skin).
While in most parts of the world the hard protective layer (endocarp) is removed only at the very end of drying during hulling, the Indonesian Giling Basah method breaks with this. In tropical Indonesia, due to extreme humidity, coffee would rot in the closed hull, so farmers mechanically crack the hull at 30-35% moisture content. This creates the deep color characteristic of Indonesian coffees, as well as the uniquely heavy, earthy-spicy aromas.
According to its food industry definition, coffee roasting is a reactive drying process accompanied by simultaneous heat and mass transfer. The input material of the process, green coffee, is a chemically stable endosperm.
Although we call it a "coffee bean," it actually has nothing to do with beans or legumes. Imagine a red fruit that looks exactly like a cherry. When this fruit is "pitted," the seed inside is what we call the coffee bean.
The endosperm is essentially the plant's "packed lunch." It is full of nutrients (sugars, acids, proteins) that are there so that if the seed is planted, it has the energy to grow out of the ground.
How does this become a characteristic coffee bean?
When this seed is roasted, these stored nutrients get "cooked." They transform under the influence of heat, and that is when the scents and aromas we know as coffee flavor are created. These nutrients are mostly non-volatile in green coffee, so they are barely or not at all perceptible by smell. The volatile compounds necessary for the characteristic aroma sensation of coffee only appear during roasting, under the influence of heat.
During the roasting process, the coffee bean undergoes significant physical and chemical changes: its mass decreases, its volume increases, while a large number of new volatile and non-volatile compounds are created. These volatile components play a crucial role in forming the coffee's aroma profile, as they are directly responsible for the sensation of smell and taste.
The formation of volatile aroma compounds is primarily the result of heat-driven reactions in which sugars and amino acids interact with each other or with their decomposition products. These reactions form the chemical basis of aroma formation during roasting; to understand this in detail, knowledge of the browning and decomposition processes discussed later is essential.
Flavor potential is determined by the presence of macro- and micronutrients. If these are absent, there is no point in roasting. We call these precursors.
For flavors to develop from precursors, proper heat regulation is required so that the entire roasting process yields the desired flavor result.
In the first half of roasting (from room temperature to approx. 170-180°C), the coffee bean behaves in an endothermic manner, absorbing heat. When the bean temperature reaches approx. 190-200°C (around first crack), the situation changes dramatically. The process becomes exothermic. From the Greek word exo (external). What does this mean? Chemical reactions occurring inside the bean (mainly the decomposition of organic matter, cellulose cracking, and pyrolysis) suddenly begin to generate heat. The coffee bean no longer just "asks" for energy, but "gives" it too. Because the processes occurring within the coffee bean generate heat energy themselves.
The coffee bean interacts with heat in several ways; knowledge of these is necessary because, as we will see at the end of the monograph, poor control of these contributes to bad taste sensations.
Roasting can be conceived as a chemical reactor where different reaction windows open as the temperature rises.
The first stage of the process is the removal of free and bound water. The structure of the beans transitions from a "glassy" state to a "rubbery" state (Glass Transition Temperature). This phase transition allows for the bean's volume expansion. Chemical reactions are minimal here; the breakdown of chlorophyll causes yellowing. As the temperature increases, we reach the Maillard reaction.
This is the most critical stage, a series of non-enzymatic browning reactions.
The reaction was named after Louis-Camille Maillard. He was a French physician and chemist, and the story began in 1912. The funniest part is that Maillard wasn't interested in coffee or gastronomy at all. He was interested in human cells. He was researching how proteins are built in our cells. While experimenting, he heated sugars and amino acids (the building blocks of proteins) together in test tubes, hoping to discover the secret of life.
Instead, what happened? The mixture in the test tube didn't assemble into protein but turned brown and began emitting entirely new, characteristic scents. Maillard described the phenomenon, shrugged saying "well, that's an interesting chemical process," published his study, and went about his business.
His discovery gathered dust in a drawer for decades. Scientists knew about it but didn't attach much importance to it. Then came World War II, and the Maillard reaction suddenly became important (or at least a moral issue). The US Army sent tons of preserved food (powdered eggs, powdered milk, powdered potatoes) to the front.
The problem was that these powders turned brown on their own during storage and became disgusting in taste, even though they hadn't been cooked. The soldiers hated them. The army scientists started scratching their heads: "Why does the egg powder turn brown in the bag?" That's when they pulled out Maillard's old study. They realized that the same reaction was taking place, just slowly, at low temperatures, and this was ruining the food.
These reactions trigger Strecker degradation at a certain point; they appear almost simultaneously.
By-products of the Maillard reaction (α-dicarbonyls) react with amino acids, inducing decarboxylation (CO2 release).
The reaction was named after German chemist Adolph Strecker. And here comes the twist: Strecker described this process in 1862 – exactly 50 years before Maillard discovered his own reaction!
The name sounds a bit scary ("degrading" = lowering in quality, deteriorating), but in a chemical sense, it simply means: falling apart into a simpler form. Strecker realized that in this reaction, the amino acid loses a carbon atom (leaving as carbon dioxide), so the molecule "becomes smaller," it degrades.
In roasting, Strecker degradation is a kind of "secondary" process that rides on the back of the Maillard reaction.
How does sweet become bitter, and fruity become winey? As the temperature crosses 160°C, the Maillard reaction begins to slow down (free amino acids run out), and pure sugar chemistry takes over. Here, sugar no longer reacts with protein, but with itself and heat.
This stage involves two intertwined processes:
The sucrose in green coffee (6-9%) begins to decompose drastically.
This is where the biggest change in coffee character happens. The acid profile doesn't simply "decrease," it gets exchanged.
But alongside chemistry, physics is also at work. The decomposition of sugars involves pressure. The hard structure of the coffee bean holds this increasing internal pressure for a while, but a point comes when the material gives in.
Coffee bean cell walls are made of thick cellulose (like wood). This material is very hard and dense, not allowing the generated steam and gases to escape. Therefore, internal pressure increases until the cellulose wall reaches its load-bearing limit (approx. 20-25 bar). This is when the explosion (crack) happens. This is the first crack; during the second, the charring of organic matter (carbonization) begins. A new wave of carbon dioxide formation cracks the bean. At approx. 224°C. Lipid migration: Due to cell destruction, oils are pressed to the surface through capillaries ("sweating" coffee bean). Flavor state: "Dark Roast". Acids are gone, sugars are burnt. Bitter, smoky, heavy profile.
While the bean physically cracks and pops, the character of bitterness also undergoes a dramatic transformation. Bitterness is not static but changes with temperature, in two main phases:
The coffee's final flavor profile is not the work of a single moment but the combined result of all the chemical processes discussed so far. The character is determined not only by how high a temperature we reached but also by how much time it took to get there. We call this the "Time-Temperature Integral": the end result is the joint imprint of heat and time.
This time window lasts from the moment of the first crack until the beans are dumped. In this phase, the roast master fine-tunes the end result not with force (heat) but with timing.
While the most conspicuous result of roasting is the browning of the beans, the human eye is often a deceptive instrument. Ambient lighting conditions or fatigue can easily mislead a roaster; therefore, modern industry utilizes the objective Agtron scale instead of subjective "eyeballing."
Measuring instruments do not actually see "color" in the human sense; instead, they monitor the Near-Infrared (NIR) spectrum. Every substance possesses a unique "optical fingerprint." The measurement is based on the differing light reflectance of materials:
In instrumental measurement, the rule is simple: a decrease in infrared reflectance indicates a higher presence of carbon—meaning the coffee is more highly roasted.
In the foregoing, we revealed how science – from Herschel's prism to Carl Staub's instrument – enabled us to objectively measure the chemical "maturity" of the coffee bean using infrared light. The Agtron scale shows exactly where we have arrived on the sugar-carbon axis.
However, roasting is not just about reaching the destination, but also about the journey there. Even behind a seemingly perfect Agtron average value (e.g., 55), defects can hide which the machine might not see, but the tongue immediately detects.
This defect typically occurs at the very beginning of roasting, in the Charge phase.
Although similar to Scorching, Tipping usually develops in the later stage of roasting, or due to overly aggressive air circulation (convection).
This is one of the most insidious defects for roasters because visually it is hard to notice (the bean might look nicely brown), but the flavor is lost. It usually occurs around or after the first crack.
This is the only defect for which the roast master is not responsible, but the producer (or lack of sorting) is.
