According to the food industry definition, coffee roasting is a reactive drying process accompanied by simultaneous heat and mass transfer. The input material of the process is green coffee, whose main mass is composed of the 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 just like a cherry. When this fruit is "pitted," we call the seed inside the coffee bean. The endosperm is essentially the plant's "snack pack" – 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.
When this seed is roasted, these stored nutrients "cook." Under the influence of heat, they transform, and this is when the aromas and flavors we know as coffee taste are created. These nutrients in green coffee are mostly non-volatile, so they can barely or not at all be detected by smell. The volatile compounds necessary for the characteristic coffee aroma sensation only appear during roasting, under the effect of heat.
During roasting, the coffee bean undergoes significant physical and chemical changes: its mass decreases, its volume increases, while numerous new volatile and non-volatile compounds are formed. These volatile components play a crucial role in shaping the coffee's aroma profile, as they are directly responsible for the smell and taste sensation.
The essence of the whole process is one thing: the gradual increase of temperature. Everything that follows – the browning, the aromas, the crack, the bitterness – all depend on what temperature the coffee bean is at and how much time it has spent in each temperature range. The story of roasting is really the story of the coffee bean's temperature.
The Flavors
Before we start the temperature rise, we need to understand what exactly is in that green coffee bean waiting at room temperature in the roasting drum. These materials – we call them precursors – will determine what develops from them under the influence of heat. If these are not present in adequate quantity and quality, then no matter how we roast the bean, it won't make good coffee.
Carbohydrates (50%): The polysaccharides forming the bean's framework (cellulose, hemicellulose, arabinogalactans) are responsible for structural integrity – these hold the bean together until heat tears it apart. Free sugars (mainly sucrose: 6-9% in Arabica, 3-7% in Robusta) are the primary sources of acidity and sweetness – these will caramelize under heat and create new flavors.
Lipids (12-18%): Triglycerides and diterpenes (cafestol, kahweol). Lipids carry aromas and are responsible for the creaminess of "mouthfeel." During roasting, they remain chemically stable – heat doesn't break them down – but their physical position changes: as the bean heats up and cracks, these oils migrate from the interior to the surface. This is what we call migration.
Nitrogen-containing compounds (11-15%): Proteins, free amino acids, and alkaloids (caffeine, trigonelline). Free amino acids will become particularly important when the temperature reaches 130°C – because at this point, the processes forming the basis of the Maillard reaction start, for which these amino acids are indispensable.
Chlorogenic acids (CGA): Green coffee is one of the plant world's richest CGA sources (mainly 5-caffeoylquinic acid). These phenolic compounds are precursors of bitterness, astringency, and acidity. As the temperature rises, these acids will transform – first into lactones, later, at even higher temperatures, into phenylindanes, which drastically changes the character of bitterness.
Water (10-12%): Not just a solvent, but a moderator of chemical reactions and a medium of heat transfer. This water will be the first to react to heat – around 100°C it starts to turn into steam, and this steam formation will be one of the main driving forces of the bean's later physical transformation.
Now that we know what's in the bean, let's start the temperature rise.
Heat Transfer
Before the coffee bean's temperature begins to rise, we need to understand how heat actually gets inside the bean. The roasting drum is hot, but it's not enough for the bean to just be in it – the heat must physically penetrate through the hard cellulose structure. Three main mechanisms exist, and all three operate simultaneously, but in different proportions during different stages of roasting.
Conduction: Direct contact of the bean with the hot drum wall. It's like placing our palm on a hot iron – direct, immediate heat transfer. In traditional drum roasters, this dominates at the beginning of the process, when cold beans first contact the heated metal surface. Conduction is very fast but risky: if the drum wall is too hot, the bean's surface immediately carbonizes before the heat can reach inside the bean. We call this error scorching (conduction shock), and we'll discuss it in detail later.
Convection (Heat Flow): The flow of hot air between the beans. It's like when something is baking in the oven – the hot air envelops the bean from all sides. This is the most efficient way to get energy into the bean's core, because the air penetrates into the gaps between the beans and heats them evenly. Modern fluid bed roasters use this almost 100%. Convection improves acid retention because it provides more even heat distribution, but if the airflow is too strong, the thinner ends of the beans – where the germ is located – overheat and burn. This is called tipping.
Radiation: Heat radiation between hot metal surfaces and the beans. It's like warming up by a fire – we don't touch the flame, yet we feel the heat. Radiation is the finest, most uncontrollable form of heat transfer, but it plays a smaller role than the other two.
These mechanisms together begin to raise the coffee bean's temperature. Now we can follow the bean as it warms degree by degree and see what happens in each temperature range.
Below 100°C
Coffee beans enter the drum at room temperature, usually around 20-25°C. When they enter the already preheated roaster (which is usually heated to 180-220°C), their first reaction is to start absorbing heat like a sponge absorbs water. In this early phase, the bean's temperature is still very low, and heat transfer occurs primarily through conduction – the beans' surface contacts the hot drum wall.
The bean slowly warms. Nothing dramatic happens yet – the water is still in liquid form, the precursors are motionless, the structure is hard. But as we approach 100°C, the water molecules move faster and faster. This stage is preparation: the bean takes up heat, stores energy, and prepares for transformation.
During this stage, the roast master must be careful: if they apply heat too quickly, the surface overheats before the interior starts to warm. That's why in the first minutes they often reduce heat input – they let the bean "acclimatize," allowing the heat to spread evenly toward the interior.
100-130°C
As the bean's temperature crosses 100°C, a dramatic change occurs: the water begins to boil. The 10-12% water in the coffee can no longer remain in liquid form – it turns into steam. And here begins the first physical drama: steam needs about 1600 times more space than liquid water.
Think about it: in a coffee bean, which is a hard, solid, cellulose structure, suddenly something inside is swelling that wants 1600 times more space. But the bean is still strong. The cellulose cell walls – which are now still in a "glassy" state, meaning rigid and brittle – for now hold back this pressure. The steam can't break out yet, only slowly seeps through the pores.
Meanwhile, an important change occurs in the bean's material structure: it reaches the Glass Transition Temperature. This means that the polymer structure goes from a "glassy" state to a "rubbery" state. More simply put: the bean's hard shell begins to soften, becomes elastic. This makes it possible for the bean to expand later, at higher temperatures, instead of simply exploding.
In this stage, there are still hardly any chemical reactions. The most noticeable change is that the bean's color changes from green to yellowish – this is the thermal decomposition of chlorophyll. The bean is still "quiet," only water is leaving it, but it's already preparing for the big chemical explosion that will occur in the next temperature range.
The roast master is still cautious: if they drive the temperature too quickly, the surface dries out while there's still a lot of water inside. This later results in uneven roasting. The ideal is if dehydration proceeds slowly, evenly throughout the entire bean.
130-160°C
When the bean's temperature reaches 130°C, we cross a critical threshold. Until now, only physical changes have occurred – water left, the structure softened. Now, however, chemical reactions start, and this dramatically changes the bean's interior.
130°C is the point where sugars and amino acids suddenly "discover" each other. Until now they peacefully coexisted in the green coffee bean, but nothing happened between them. Now, however, there's enough energy for them to chemically bond. This is the beginning of the Maillard reaction – the most critical stage in all of roasting, which determines whether the coffee will have aroma or not.
Maillard's Test Tube
The reaction was named after Louis-Camille Maillard, a French doctor and chemist. The story begins in 1912, and the funniest thing 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 goo in the test tube didn't assemble into protein, but browned and began to emit completely new, characteristic smells. Maillard described the phenomenon, shrugged that "well, this is an interesting chemical process," published his study, then went on his way.
Maillard's discovery gathered dust in a drawer for decades. Scientists knew about it, but didn't attribute much significance to it. Then came World War II, and the Maillard reaction suddenly became important.
The American military sent tons of preserved foods to the front – egg powder, milk powder, potato powder.
But there was a problem: these powders during storage – over weeks, months, in warm warehouses, on ship decks – browned on their own and became disgusting-tasting, even though no one baked or cooked them. They just lay in the bag.
Military scientists started scratching their heads: "Why does egg powder brown in the bag? Why does it become stinky?" And they figured it out.
The Maillard reaction requires three conditions: sugars, amino acids (proteins), and heat. In egg powder, milk powder, and potato powder, all three were present. The sugars were provided by lactose in milk and starch in potatoes, while amino acids were present in all ingredients through proteins. As for heat, 130°C wasn't necessary, even 30-40°C proved sufficient, such as might occur in a warm warehouse or on a ship's deck.
Without control, the Maillard reaction leads to bad flavors and odors. Controlled – quickly, at high temperature, for the right duration – it gives the world's finest flavor, the aroma of coffee.
And the key is always controlling temperature and time.
Back to 130°C
So the bean is now around 130°C, and the Maillard reaction starts – quickly, controlled, as it should.
Under the influence of heat, sugars and amino acids chemically bond. Since this first bond is still very unstable, the structure of the resulting molecule quickly rearranges – this chemical "settling" initiates browning and the birth of flavors.
A bit scientifically: the carbonyl group of reducing sugars and the amino group of free amino acids condense, creating N-substituted glycosylamines. This is unstable and undergoes Amadori rearrangement.
The pH-dependent decomposition of Amadori products creates heterocyclic compounds that form the backbone of coffee aroma:
Pyrazines: Earthy, nutty, roasted aromas – this is what we sense as "coffee smell." Pyrroles: Cereal notes that add depth. Thiophenes: Roasted odors from sulfur-containing amino acids. Melanoidins: The reaction's end products are brown polymers that have antioxidant effects and increase coffee's body. They're also responsible for the characteristic brown color.
It's important to understand: these reactions occur now, at 130-160°C. This means that if the bean doesn't spend enough time at this temperature, there won't be enough Maillard reaction, there won't be pyrazines and melanoidins. The drink will be thin, acidic, characterless. That's why it's so important that the temperature increase is slow enough – we let the bean "sit" at this temperature so the reactions can occur.
140-170°C
Between 140-170°C, as the temperature reaches and exceeds 140°C, alongside the Maillard reaction, a new, closely related process also unfolds: Strecker degradation. In this temperature range, the Maillard reaction remains dominant, between 130-160°C it bonds sugars and amino acids, creating an increasingly complex, deeper aroma world. However, the reaction produces not only end products but also highly reactive intermediate compounds, so-called α-dicarbonyls. These molecules carry two carbonyl groups on adjacent, alpha-positioned carbon atoms, and are formed during the multi-step transformation of sugars and amino acids.
α-dicarbonyls are key players in aroma and odor formation because they react with remarkable ease with unused amino acids. This cooperation initiates Strecker degradation, during which amino acids are converted to aldehydes while carbon dioxide and ammonia are released. The resulting aldehydes are extremely intense and characteristically scented compounds, and fundamentally shape the aroma profile of many foods: they're responsible for almond's characteristic almond smell, vanilla's pure, recognizable vanilla aroma, and coffee's delicately honeyed, floral notes.
Three amino acids commonly found in foods play a particularly important role in the process: leucine, phenylalanine, and methionine. Leucine forms 3-methyl-butanal, which gives malty and chocolatey odor notes, phenylalanine converts to phenylacetaldehyde producing honeyed, floral aroma, while methionine becomes methional, responsible for the odor reminiscent of cooked potato and earthy tones.
Strecker
The reaction was named after Adolph Strecker, a German chemist. And here's the twist: Strecker described this process in 1862 – this was a full 50 years before Maillard discovered his own reaction!
Strecker was the typical precise German scientist of the mid-19th century. He experimented with organic chemistry at the University of Tübingen. He wasn't roasting coffee. He was examining the reaction of a compound called alloxan (a derivative of uric acid) with various amino acids. He noticed that when he mixed these two, something strange happened: the amino acid broke down and characteristically scented compounds were formed.
Strecker realized that in this reaction, the amino acid loses a carbon atom (departs as carbon dioxide), so the molecule "becomes smaller," breaks down. The amino acid becomes an aldehyde.
But Strecker didn't know that this would later be the "partner" of the Maillard reaction. He only described that "if you attack an amino acid with a certain compound, an aldehyde is formed." Only decades later, in the 20th century, did they realize that this reaction also occurs during roasting – and in such a way that by-products of the Maillard reaction initiate it.
Maillard and Strecker
Between 130-160°C, the Maillard reaction is the determining process: sugars and amino acids react with each other, as a result browning begins and complex aromas are created, for example in the form of pyrazines, pyrroles, and melanoidins. As the reaction progresses, highly reactive intermediate compounds, so-called α-dicarbonyls, also form.
When the temperature reaches 140-170°C, while the Maillard reaction continues, these α-dicarbonyls take on a new role: they react with unused amino acids and initiate Strecker degradation. As a result, aldehydes are formed, which create an intensely, well-recognizable odor cloud with honeyed, floral, and malty aroma notes, further enriching the coffee's odor and flavor world.
These compounds are very volatile; as soon as they're created, they immediately begin to evaporate from the bean. That's why in light roasting – which doesn't go above 190°C – these aldehydes are still present, and this explains why floral-fruity notes dominate there. As the temperature continues to rise, these volatile compounds increasingly evaporate and give way to heavier, less volatile compounds.
160-200°C
As the temperature continues to rise and we cross 160°C, an important turn occurs. The Maillard reaction begins to slow because free amino acids are running out. Now pure sugar chemistry takes over. Here the sugar no longer reacts with protein, but with itself. This is what we call caramelization.
The sucrose in green coffee (6-9%) now begins to decompose drastically. Heat works harder and harder – the reaction is faster with each degree. Furans and maltol form – this is the classic caramelized, roasted sugar smell.
Notice the change: at 130-160°C, sweet, floral, honeyed aromas were still forming. Now, at 160-200°C, as sugars caramelize, sweet flavor decreases and bitterish, "roasted" character appears. The coffee's flavor deepens, darkens as the temperature rises.
The Acid Profile
Simultaneously with this sugar decomposition, the biggest change in coffee's character happens here: the acid profile doesn't simply "decrease," but completely changes.
Decomposition of "good" acids: Heat-sensitive acids that give fresh fruitiness, like citric acid and malic acid (we call these thermolabile acids, meaning heat-decomposable), begin to break down. These acids can't handle high temperature – above 170°C they rapidly degrade. Effect: The citrusy vibration of light roasting gradually disappears.
Birth of new acids: But meanwhile new acids are also created! During sugar decomposition, not only caramel-like flavors but also aliphatic carboxylic acids are formed – such as acetic acid and formic acid. Effect: These acids give coffee a heavier, more complex, sometimes fermented, wine-like or syrupy character.
So between 160-200°C this happens: light, fresh acids decompose, but heavier, more complex acids are created in their place. The coffee's character shifts from fruity to wine-like, syrupy, caramel-like. This isn't bad – it's simply the natural consequence of temperature. The roast master decides where to stop the process: whether they want to retain more fruity acids or seek the caramel-wine character.
Bitterness
As these reactions unfold, the character of the bitterness begins to develop. It is important to understand that bitterness does not stem from a single source, nor is it bound by rigid temperature limits. It is the result of a complex interplay between three main actors, which depends heavily on the chosen roast profile:
1. Caffeine (The Foundation): Thermally largely stable, it survives roasting almost unscathed. It provides a constant, "clean" base bitterness (accounting for about 10–20% of the perception) that remains a fundamental characteristic regardless of the roast level.
2. Trigonelline (The Roast Aroma): This alkaloid begins to degrade under heat. This process forms nicotinic acid (Vitamin B3) and various pyridines. These compounds are sensorially interesting: they are not simply "bitter" but introduce smoky, roasted, and sometimes pungent, earthy nuances to the cup. They often act as flavor enhancers, making the perception of bitterness more complex and "aggressive."
3. Chlorogenic Acids (The Dynamic): This is where the most dramatic change occurs, strongly influenced by time and temperature:
Lactones (The Pleasant Bitterness): Starting at around 145–150°C, the dehydration of chlorogenic acids into chlorogenic acid lactones begins. These typically peak at medium roasts. They produce a mild bitterness characteristic of coffee, which we perceive as pleasant and structured—similar to the astringency of tonic water.
Phenylindanes (The Harsh Bitterness): As the roast progresses—whether due to very high final temperatures or extended roasting time (long "baking")—the lactones degrade further into phenylindanes. This is not an automatic process occurring above 200°C, but rather a consequence of the total energy input. Phenylindanes are responsible for that metallic, harsh, and lingering bitterness characteristic of dark roasts (e.g., Southern Italian-style espresso roasts).
190-205°C
Until now, throughout the heating, the coffee bean passively received heat. It behaved endothermically, meaning it absorbed heat like a sponge. The external heat source (the roasting drum, the hot air) heated it, and the bean "just" warmed up.
As the temperature reaches 190-200°C – roughly around the first crack – a fundamental change occurs. The process becomes exothermic, meaning from the Greek exo ("external"), it no longer involves heat absorption but heat release. This means that the chemical reactions occurring in the coffee bean suddenly begin to produce heat themselves.
In this stage, primarily the decomposition of organic materials, the cracking of cellulose structure, and pyrolysis processes dominate. The coffee bean is then not just "demanding" external heat energy, but actively "giving it back": the reactions occurring inside transform into autonomous heat production. This is a real turning point during roasting. The roast master must then hold back external heat input, because otherwise the process can easily get out of control, the bean can overheat, and the delicate aromas carefully built up earlier simply burn.
Simultaneously with this heat production, enormous gas pressure builds inside the bean. This has three sources, and now, at 190-200°C, all three are working at once:
Water vapor: The remaining water (which we haven't evaporated yet) is now definitely steam. Remember: steam needs about 1600 times more space than liquid water.
Reaction gases: The Maillard reaction, Strecker degradation, sugar decomposition – all produced by-products: carbon dioxide (CO₂) and carbon monoxide. These gases are now present in huge quantities in the bean.
Cellulose structure resistance: Coffee bean cell walls are thick, cellulose-based structures, similar to wood. This material is extremely hard and dense. Although it softened during the Glass Transition, it's still strong. Therefore, gases can't freely depart – they only slowly seep through the pores.
And then the moment comes. The cell walls have reached the limit of their load-bearing capacity.
And boom.
The cell walls suddenly crack. The bean practically explodes. This is the so-called "first crack." It gives a characteristic popping sound, as if we were making popcorn. The roast master hears: crack-crack-crack-crack – quick, sharp sounds.
What exactly happens? The gases suddenly break out, the pressure decreases, the bean expands. The cellulose structure cracks, the bean's volume increases – even by 50-100%. The surface becomes cracked. If we stopped roasting at this point, we'd get light roasted coffee – with lively acids, floral-fruity aromas, light body.
But there's another option too: we continue raising the heat.
205-224°C
If the roast master decides to continue raising the temperature after first crack, then we enter a new phase. Now it's no longer about aroma formation, but their transformation and partial decomposition.
As the temperature rises above 200°C, several things happen simultaneously:
1. The transformation of bitterness – second phase Remember the pleasant lactone bitterness that formed at 160-200°C? Now, above 200°C, these lactones further decompose and convert to phenylindanes. In taste sensation, this is a much rougher, more metallic, mouth-drying bitterness that lingers long on the tongue. This is characteristic of very dark roasted, for example Neapolitan-style coffees.
2. Further acid decomposition: The remaining citric acid and malic acid completely decompose. The coffee loses its acidic character and becomes increasingly bitter-bodied.
3. Sugar burning: Caramelization continues, but now goes too far – sugars don't just caramelize but begin to carbonize. This gives burnt, smoky flavors.
4. Beginning of carbonization: The carbonization of organic materials (carbohydrates, proteins, fats) accelerates. The bean gradually becomes black. Pyrolysis – heat-induced chemical decomposition without oxygen – proceeds at full steam.
5. Lipid migration: Due to cell structure degradation, lipids (oils) can now flow freely. Oils in the coffee bean are pressed to the surface through capillaries. We can observe this as the "sweating" coffee bean phenomenon – the bean's surface becomes shiny, oily.
Around 224°C
As the temperature reaches about 224°C (again, this can vary), another explosion occurs. Another wave of carbon dioxide formation further increases internal tension, causing additional cracks within the coffee bean. This is the second crack.
This comes with a finer, crunchy sound – not as sharp as first crack.
In this state, the coffee already belongs to the dark roast category. Acids have mostly disappeared, sugars have burned, the flavor profile becomes bitter, smoky, and strongly bodied. Lipid migration is complete – the bean sweats with oil.
If we went further, the bean would completely carbonize and we'd get undrinkable smoky-burnt flavored coffee.
Where's the End?
This decision determines the coffee's final character. And by learning one concept, we can answer the question.
Development Time
After first crack begins the development phase (Development Time, DT), which lasts from first crack to the end of roasting. This period is crucial because here the coffee's final character forms: the fine balance of acidity, sweetness, body, and aromas.
The length of DT determines whether the coffee will be underdeveloped, optimally balanced, or overdeveloped – regardless of what high temperature we reached.
Short DT (underdevelopment): When we stop roasting a few seconds after first crack, the process is interrupted too early. Although the bean reached the necessary temperature, the chemical reactions don't have time to fully develop. Caramelization is incomplete, melanoidins don't form properly, and acids remain sharp and disorganized. The finished drink is thin-bodied, watery feeling, often acidic, raw, with greenish-vegetal notes. The bean is then "underdeveloped": the reactions occurring within it haven't concluded.
Optimal DT: If the bean spends 15-25% of total roasting time in the Development Time phase – which is usually 2-4 minutes – then all important processes can proceed at the right pace. Acids round out while fruity aromas remain lively. Sugars caramelize, creating sweet, caramel-like flavors, and melanoidins fully develop, giving the drink rich body and creamy sensation. The end result is harmonious, balanced, clean coffee profile – this is the point where roasting becomes true mastery.
Long DT (overdevelopment): If after first crack we keep the bean at high temperature for too long, even 5-7 minutes, the reactions exceed the optimal range. Acids and fruity aromas gradually decompose, flavors flatten, and increasingly toast-like, ash-like notes appear. The coffee loses its complexity and liveliness, replaced by bitter, overly roasted character. The bean is then like "someone who talked too much": all exciting details disappear, only empty, heavy bitterness remains.
So it's clearly visible that during roasting, timing is as determining a factor as temperature itself. The coffee's flavor profile is not the result of a single moment, doesn't depend solely on what temperature we finish roasting at. Rather, it's the imprint of the entire journey the bean travels during gradual temperature rise. It's determining at what pace we heated, how even the heat input was, and how much time it spent in different temperature ranges.
Error Possibilities
The path of coffee's heating is full of pitfalls. The master must constantly balance between heat transfer modes, watch timing, and respond to the bean's signals. Let's see what happens when something goes wrong – and how these errors relate to each stage of temperature rise.
Conduction Shock
This error happens right at the beginning, in the first minute of heating. If the roasting drum's metal wall is too hot relative to the beans' heat absorption capacity – for example, heated to 220°C when we add cold beans – then the bean's surface immediately receives heat shock.
Since the coffee bean's cellulose structure conducts heat poorly, the heat can't flow quickly enough toward the core. Energy accumulates on the surface. This initiates premature pyrolysis: surface sugars and fibers immediately carbonize before the bean's interior even reaches 100°C. We skip the fine phases of Maillard reaction and caramelization.
The result is a visual trap: the bean's exterior is already carbonized (Agtron level 20), but its interior is still completely raw – perhaps only at 150°C. The drink will be bitter, burnt, metallic tasting, with vegetal off-flavors.
Lesson: Controlling conduction is critical in the first minutes. We must start more slowly so heat has time to penetrate evenly.
Tipping
This error appears later, around 170-190°C, and is most often triggered by too strong hot air flow, that is, convection.
Here a kind of geometric overheating occurs: since the coffee bean isn't a perfect sphere, the ends – where the germ is located – are much thinner and more vulnerable than the bean's middle. If heat delivery is too aggressive (for example, too rapid airflow), these points heat faster than the rest of the bean.
In this case, water evaporates from them in moments, which is problematic because the natural cooling effect of evaporation ceases. Evaporation cools the surface – but if there's no water, there's no evaporation, no cooling. Without protection, organic material immediately blackens at the bean ends – while the bean's middle is still developing normally.
Flattening
This is one of the trickiest errors in the process, and can occur at any time during heating, but most commonly appears in the 160-180°C stage.
This occurs when the rate of temperature rise (Rate of Rise, RoR) drastically slows or stops. For example, the bean is at 170°C, but in the next 5 minutes only rises to 175°C – that is, almost stagnates.
What's the problem with this? At this point roasting loses momentum. The chemical reactions necessary for aroma formation simply stop. Remember: these reactions need energy – rising temperature. If temperature stagnates, reactions slow down or stop.
Instead of sugars beautifully caramelizing and converting to sweet flavor compounds, they just slowly dry out. The Maillard reaction doesn't complete. The bean's surface remains matte and dull because the internal pressure that would press oils to the surface is missing – gases slowly seeped out, but there wasn't enough energy for the big explosion.
Quakers
Finally, those immature beans for which not the roast master but lack of sorting on the farm is responsible.
In these beans, there simply isn't the chemical "fuel" necessary for roasting – the precursors. They lack the sugars and amino acids needed for roasting. Remember that at 130°C the Maillard reaction starts? Well, this requires sugars and amino acids. If they're not there, nothing happens.
These beans don't participate in the processes during roasting: they only lose water (dehydrate), but are unable to brown. Even at the darkest roast – at 224°C – they remain yellowish-hazelnut colored.
Summary
Below 100°C: Preparation. The bean absorbs heat, acclimatizes.
100-130°C: Water departs. Dehydration, Glass Transition. Structure softens.
130-160°C: Maillard reaction. Chemistry explodes. Browning, aromas, pyrazines, melanoidins.
140-170°C: Strecker degradation. The odor cloud. Aldehydes, floral-fruity notes.
160-200°C: Sugar decomposition, caramelization. Acid profile changes. Pleasant bitterness forms.
190-200°C: Exothermic turn. The bean produces heat. Gas pressure builds.
200-205°C: First crack. Cell walls crack. Expansion.
200-224°C: Toward darkness. Carbonization, lipid migration, transformation of bitterness.
224°C: Second crack. Additional cracks.
The roast master's true artistry lies in consciously directing the entire heating process. They don't merely apply heat to the bean, but control its pace: how quickly they raise the temperature, how much time they allow for reactions occurring in each temperature range, and exactly when they intervene to close the process.
These decisions determine whether acids remain lively or become rounded, whether sugars transform into fruity sweetness or deep caramel notes, and whether the roasting result is light and clean in character or darker, more intense, more bitterish.