Stand on the hillside at Buddha's Cup in Holualoa on a roasting morning and your senses get busy fast. The air carries something warm and sweet, then smoky, then almost chocolaty. The beans inside the roaster shift from pale green to gold to a deep, burnished brown you can almost taste with your eyes. And then comes the sound: a soft, rapid cracking, like the land itself is waking up. It is one of the most satisfying things we experience on this farm, and we have been here long enough to know that the romance of it never fades.
But behind that romance is science. Real, measurable, beautifully complex science that determines whether your morning cup is bright and floral or rich and bold, whether it lingers on the palate or fades fast, whether it carries the distinctive sweetness that makes Kona coffee one of the most sought-after coffees on earth.
Most coffee lovers know that roasting matters. Few know exactly why. And we find that the more you understand what is actually happening inside the bean, the more you appreciate what goes into every bag you buy, every cup you brew, every morning you take that first sip and feel genuinely glad you chose well.
So we want to explain it plainly. No lab-coat language. No chemistry degree required. Just the real story of what heat does to a green coffee bean and why the result tastes the way it does. We will cover the key reactions, walk through the stages of a roast, explain what different roast levels actually do to your coffee's chemistry, and show you why origin matters as much as method.
Before chemistry can do its work, the bean has to make a remarkable journey from cherry to roaster. If you want to understand that part of the story first, our guide to the journey of a Kona coffee bean on a family-owned farm will give you that foundation. Then come back here, and we will show you what happens next, when the heat turns on.
What Green Coffee Actually Contains Before Roasting
Before we can talk about chemical reactions, we need to talk about raw materials. A green coffee bean is not a blank canvas. It arrives at the roaster already carrying a remarkable library of chemical compounds, each one a potential ingredient in the flavor, aroma, and body of the finished cup. What roasting does is not create something from nothing. It transforms what is already there.
Understanding what the green bean contains is the foundation for understanding everything that follows. Think of it this way: a skilled chef can only work with the ingredients in front of them. A roaster can only develop the flavors that the green bean's chemistry allows. This is why we care so deeply about the quality of our beans before they ever reach the heat.
The Raw Ingredients: Sugars, Acids, Proteins, and Water
A green coffee bean is roughly 8 to 12 percent water by weight. Beyond that moisture, the bean is a dense package of organic compounds. There are reducing sugars, primarily sucrose along with smaller amounts of glucose and fructose. There are free amino acids and proteins. There are chlorogenic acids, which belong to a family of antioxidant compounds. There are lipids, primarily cafestol and kahweol trapped inside the bean's cellular structure. And there is caffeine, present and stable, waiting patiently through the entire roasting process.
Each of these compounds plays a specific role in what roasting produces. Sugars and amino acids are the primary reactants in the Maillard reaction, which we will cover in detail shortly. Sugars alone participate in caramelization. Chlorogenic acids transform into bitter and body-producing compounds. Lipids carry and release aromatic molecules into the cup. Caffeine, as we will explain, essentially rides through the roast unchanged.
One important myth to address now: many coffee drinkers believe dark roast coffee has less caffeine than light roast because the roasting burns it off. Caffeine is a chemically stable alkaloid. It does not degrade meaningfully during roasting at typical roast temperatures. The slight difference you might notice between roasts when measuring by weight is largely because roasting causes beans to lose moisture and expand, making dark roast beans physically larger and lighter per bean. Measure by weight rather than volume and the caffeine difference between roast levels is negligible. We will return to this point when we discuss roast levels in detail.
If you are curious about the full nutritional and compound profile of Kona-grown coffee specifically, our page on what nutrients are found in Kona-grown coffee goes deeper into what makes our beans distinctive from the inside out.
Why Origin Shapes the Chemistry
Not all green beans carry the same ratios of these compounds. Origin matters enormously, and it matters specifically because it determines the quality and variety of the chemical precursors that roasting has to work with.
Coffee grown at higher altitudes matures more slowly. The cherries spend more time developing on the tree because cooler temperatures slow the ripening process. That extended maturation period allows the plant to produce more complex sugars, more varied amino acids, and a richer overall chemical profile inside each seed. This is part of why altitude is so closely associated with cup quality in specialty coffee.
Here in Holualoa, we farm on the slopes of Hualalai volcano along the Kona belt, sitting between roughly 1,000 and 3,000 feet above sea level. The Big Island's unique combination of volcanic soil, afternoon cloud cover, warm mornings, and reliable rainfall creates conditions that coffee growers in most parts of the world cannot replicate. Those conditions produce green beans with a particular richness of precursor compounds, a natural sweetness in the sugars, and a complexity of acids that give our roasters something exceptional to work with before the heat is even turned on.
Soil composition contributes too. Volcanic soil on the Big Island is rich in minerals that influence nutrient uptake in the coffee plant. Those minerals affect the metabolic processes that produce flavor precursors inside the developing cherry. This is part of what terroir means in coffee, the same concept that wine lovers apply to grapes. The land leaves its fingerprint in the bean, and roasting amplifies and transforms that fingerprint rather than erasing it.
This is also why we believe so strongly in farm-direct sourcing. When you buy from a farm that controls the entire process from seed to roasted bag, you know that the green bean quality is the starting point and that every roasting decision builds on that foundation intentionally. To understand more about what makes Kona coffee what it is, our page on what is Kona coffee gives you the full picture. And if you have ever wondered why genuine Kona commands a premium price, our piece on why Kona coffee is so expensive explains the full story of value from farm to cup.
Chlorogenic Acids: The Antioxidant That Roasting Changes
Chlorogenic acids deserve their own introduction because they are one of the most significant compound families in coffee, both chemically and from a health perspective, and their story across the roasting process is one of the most interesting trade-offs in all of food science.
In a green coffee bean, chlorogenic acids (often abbreviated as CGAs) are the dominant antioxidant compounds. They are present at much higher levels in green coffee than in roasted coffee. If you have seen supplements or health products marketed around green coffee extract, this is largely why. The CGA content is high and largely intact before roasting begins.
As roasting progresses, CGAs begin to break down. By the time you reach a medium roast, a significant portion of the original CGA content has been degraded. By dark roast, they are nearly absent. But here is the nuance: the breakdown products of CGAs are not simply lost. Some of them contribute to the body, bitterness, and characteristic flavor of roasted coffee. The CGAs do not vanish into nothing. They transform into other compounds that become part of the cup's complexity.
This creates a genuine trade-off between antioxidant content and flavor development. Lighter roasts retain more CGAs and therefore more antioxidant activity. Darker roasts sacrifice most of that antioxidant content in exchange for the deep, bold flavors that pyrolysis and extended CGA degradation produce. Neither trade is wrong. It is simply chemistry, and knowing it helps you make an informed choice about what you want in your cup.
For anyone interested in the relationship between coffee, locally sourced ingredients, and wellness, our page on the connection between health benefits and using local ingredients explores how farm-direct, minimally processed foods carry the most intact nutritional profiles.
The Four Core Chemical Reactions That Define Roasting
Now we get to the heart of the matter. When heat is applied to a green coffee bean, four major categories of chemical reaction drive most of what happens to flavor, color, aroma, and body. These reactions do not happen one at a time in a neat sequence. They overlap, interact, and compete. But we can explain each one clearly on its own terms, and then the interaction becomes easier to picture.
These four reactions are: the Maillard reaction, caramelization, chlorogenic acid degradation, and pyrolysis. Together they account for the creation of more than 800 volatile aromatic compounds that researchers have identified in roasted coffee. That is an extraordinary number for something that starts as a simple seed.
The Maillard Reaction: Where Flavor Is Born
The Maillard reaction is the most important and complex of the four. It begins around 150 degrees Celsius and runs through much of the roasting process. It is named after Louis-Camille Maillard, a French chemist who described the reaction in 1912, though cooks had been harnessing it for thousands of years before anyone had a name for it.
Here is what happens in plain terms. When heat is applied to a system that contains both amino acids (the building blocks of proteins) and reducing sugars, those two types of molecules begin reacting with each other. They are not simply bonding together. They are rearranging, breaking apart, recombining into entirely new molecular structures that did not exist before the heat was applied.
The products of this reaction include hundreds of volatile aromatic compounds, the molecules that carry the aromas we associate with roasted coffee: nutty, caramel-like, floral, earthy, and fruity notes all trace back in part to Maillard chemistry. The reaction also produces a class of brown pigment molecules called melanoidins, which are responsible for the characteristic brown color of roasted coffee and which also contribute to body and mouthfeel in the cup.
You encounter the Maillard reaction every day. It is the reaction that browns bread crust in the oven. It is what creates the sear on a steak. It is what makes toast smell like toast and not just dried bread. Heat forcing two types of molecules to swap partners and produce new ones: that is the essence of it. In coffee, the sheer variety of amino acids and sugars present means the range of Maillard products is exceptionally wide, which is part of why coffee aroma is so complex.
The timing of this reaction window is critical. Move through it too quickly and sweetness is underdeveloped, leaving the cup tasting thin or under-ripe. Linger in this stage too long without enough heat and aromatic compounds can stall. Push past it too aggressively and the delicate volatile compounds created in this stage begin to degrade before they can stabilize.
At Buddha's Cup, our roasters manage heat curves with care and intention, particularly through this phase. We are not simply applying heat and walking away. We are actively shaping the temperature trajectory to guide each batch of beans through this chemistry at a pace that honors the character of that specific harvest.
Caramelization: Sugar's Solo Performance
Caramelization is often confused with the Maillard reaction, and it is easy to understand why. Both happen during roasting, both produce brown color and sweet aromas, and both involve sugars. But they are distinct reactions driven by different chemistry.
Caramelization involves only sugars. No amino acids required. When sugars are heated above roughly 170 to 200 degrees Celsius, they begin to break down through a process of dehydration and polymerization, losing water molecules and rearranging their carbon structures into new compounds. The result is the family of flavors we recognize as caramel: sweet, buttery, slightly nutty, with an underlying richness that is different from the more complex floral and savory notes produced by Maillard chemistry.
When you taste a coffee with tasting notes of brown sugar, caramel, toffee, or butterscotch, you are tasting the products of caramelization. These are some of the most accessible and immediately pleasing flavors in a cup of coffee, the ones that make a first-time specialty coffee drinker stop and say, "I did not know coffee could taste like this."
In Kona coffee, the natural sweetness of beans grown in our volcanic soil and slow-ripened at elevation gives caramelization particularly rich inputs. The sugars present in our green beans are of high quality and abundant quantity, which means the caramelization products during roasting are correspondingly expressive. This is a direct connection between the terroir of our land and the flavor in your cup, mediated by chemistry that transforms origin character into tasting notes.
It is worth noting that both Maillard and caramelization reactions are happening simultaneously during the main browning phase of roasting, overlapping and interacting. Separating them conceptually is useful for understanding. In the actual roast, they coexist and their products combine in the finished coffee.
Chlorogenic Acid Degradation: The Antioxidant Trade-Off
We introduced chlorogenic acids in the section on green bean chemistry, and now we follow their story through the heat. This is one of the most practically important reactions for health-conscious coffee drinkers to understand, because it explains the relationship between roast level and antioxidant content in a nuanced and non-obvious way.
As temperature rises inside the roasting bean, CGAs begin to break down. This degradation starts even at light roast temperatures. The breakdown is not complete or instantaneous; it progresses as heat increases and roasting time extends. By the time a bean reaches medium roast, perhaps 50 to 70 percent of the original CGA content has been degraded. By dark roast, the remaining CGA content is minimal.
But here is the key nuance. CGA degradation is not purely a loss. The breakdown products of chlorogenic acids include compounds that contribute to bitterness, astringency, and body in the cup. Some of those products contribute positively to the flavor complexity we associate with well-roasted coffee. A coffee with zero CGA degradation, which would mean essentially no roasting, tastes grassy and sour and nothing like what most of us want to drink in the morning. Some transformation is necessary and desirable.
The practical takeaway is this. If you want maximum antioxidant retention, lighter roasts are your best choice. If you want the classic roasted coffee flavor with its depth of body and pleasant bitterness, you are accepting a degree of CGA loss in exchange for those flavor characteristics. And if you want something in between, a medium roast represents a genuine middle ground where meaningful CGA content survives and roast character has fully developed without overwhelming the cup.
This is not a reason to feel guilty about enjoying dark roast coffee. It is simply chemistry, and understanding it lets you make the choice that is right for you.
Pyrolysis: The Dark Roast Reaction
Pyrolysis is the reaction that takes over when temperatures climb above roughly 200 degrees Celsius and become the dominant force in the roast. The word comes from Greek roots meaning fire and separation. In chemistry, pyrolysis refers to thermal decomposition of organic compounds in the absence of sufficient oxygen. It is a kind of breaking-apart under intense heat.
In coffee roasting, pyrolysis breaks down the complex aromatic molecules created during the Maillard and caramelization phases and converts them into simpler, more volatile compounds. These simpler compounds include the smoky, bitter, and charred notes that characterize dark roast coffee. The phenolic compounds produced during pyrolysis are responsible for the sharp, almost medicinal bitterness that some drinkers love and others find overwhelming in a French roast or Italian roast.
Pyrolysis also drives the physical changes you can observe. The oils trapped inside the bean's cellular structure begin to migrate to the surface, producing the shiny appearance of dark roast beans. The cellular structure continues to break down, making beans more porous and brittle. The bean's internal pressure builds and releases in what roasters call second crack, a series of rapid pops that is quieter and faster than the first crack that preceded it.
The trade-off with pyrolysis is real. The delicate, complex compounds produced during the Maillard reaction and caramelization earlier in the roast do not survive extended pyrolysis intact. They degrade into simpler molecules. This is why a very dark roast from two different origins often tastes quite similar: the pyrolysis reactions are so dominant that they mask much of the origin character. The nuanced chemistry that makes a Kona bean distinct from a Guatemalan or Ethiopian bean is largely replaced by the universal chemistry of intense heat.
For premium single-origin coffee like ours, this matters. We are not suggesting dark roast is inferior as a category. Some drinkers genuinely love the bold simplicity of a dark roast, and that preference is completely valid. But it is worth knowing that when you choose a lighter or medium roast from Buddha's Cup, you are preserving more of the chemical complexity that the Kona growing environment worked months to build inside each cherry.
Stage by Stage: What the Bean Experiences From Start to Finish
Now that we understand the four core reactions, we can walk through the roasting timeline and see how those reactions map onto the physical stages that a roaster observes. This is where abstract chemistry becomes something you can almost watch and hear and smell.
A complete roast from green bean to finished product typically takes between 8 and 15 minutes depending on the method, batch size, and target roast level. Within that window, the bean passes through distinct phases, each with its own chemistry, appearance, and aroma profile.
Drying Stage: Evaporation and Early Heat Absorption
The roast begins with the drying stage, which runs from the moment beans enter the roaster up to around 150 degrees Celsius. During this phase, the most significant thing happening is the evaporation of the bean's moisture content. That 8 to 12 percent water needs to leave the bean before the chemical reactions that drive flavor can proceed efficiently.
The bean is absorbing enormous amounts of thermal energy during this phase. Most of that energy goes into evaporating water rather than raising the bean's temperature, which is why the temperature curve tends to level off or rise slowly during drying. The bean acts almost like a wet sponge: it cannot get much hotter until it dries out.
Visually, the beans shift from their characteristic blue-green to a yellow color during this stage. The smell in the roasting room is not yet what most people would call coffee. It is more grassy, almost like baking bread dough or toasted grain. The cellular structure of the bean begins to soften as heat penetrates. The bean is not yet transforming chemically in a dramatic way, but it is preparing for everything that comes next.
Experienced roasters pay close attention to this phase because the rate of drying affects how evenly the bean enters the subsequent reaction stages. A bean that dries too quickly on the outside but retains moisture at its core will develop unevenly. The external surface will begin Maillard reactions while the interior is still wet and cool, producing a finished bean that is inconsistently developed through its cross-section. Getting drying right is a foundation for everything that follows.
Browning and First Crack: Reactions Accelerate
Once moisture has largely left the bean and temperatures climb into the 150 to 200 degree Celsius range, the chemistry changes dramatically and rapidly. The Maillard reaction kicks in with real intensity. Caramelization begins. The bean's color shifts from yellow to tan to light brown. The aroma in the roasting room begins to smell recognizably like coffee for the first time, complex and warm and inviting.
Inside the bean, something important is building. The Maillard reaction and other chemical processes produce carbon dioxide gas as a byproduct. That CO2, along with steam from residual moisture, creates increasing internal pressure inside the sealed cellular structure of the bean. The bean is also expanding physically as heat causes the cellular walls to soften and stretch.
At around 196 degrees Celsius, something happens that every roaster waits for and listens to closely: first crack. The internal pressure becomes sufficient to rupture the bean's cellular walls, and the beans begin to crack audibly, one after another, in a rapid series of pops that sounds something like popcorn in a pan. This is a physical event driven by chemistry, and it marks a critical milestone in every roast.
First crack signals that the bean has undergone sufficient Maillard development to be drinkable. Light roasts are pulled shortly after first crack, while development is still young and the origin character of the bean is at its most intact. The acidity is bright, the aromatics are complex, and the sweetness is delicate. If you want to taste the terroir of a specific farm or region as clearly as possible, this is the roast window where that is possible.
Development and Second Crack: Flavor Lock-In or Pyrolysis
After first crack, the roaster enters the development stage. This is arguably the most consequential phase in the entire roast because it is where the roaster's decisions have the most visible impact on the finished cup. The beans are chemically active, the reactions are running, and the question is: how long do you let this continue before stopping the heat?
Medium roasts are developed in this window, after first crack has completed and before the second crack begins. During this phase, Maillard compounds continue to develop and stabilize. Caramelization deepens. The balance between bright acidity and smooth sweetness shifts gradually toward sweetness and body as development time extends. Many specialty roasters, ourselves included, find that medium roast is where Kona coffee's distinctive character, its natural sweetness, clean finish, and nuanced complexity, is best expressed.
As temperatures continue to climb above 200 degrees Celsius, a second crack occurs. This one is different from the first. It is quieter, faster, and more hissing than popping. It represents further cellular breakdown inside the bean, and it marks the beginning of pyrolysis becoming the dominant force in the roast. Oils begin migrating to the surface of the bean. Color deepens to dark brown.
Beyond second crack lies the territory of French roast and Italian roast. In this range, the roast character becomes the overwhelming flavor influence. The nuanced compounds from earlier phases degrade rapidly. What remains are the bold, smoky, bitter compounds of extended pyrolysis. For drinkers who love that profile, it is entirely legitimate. But for anyone who wants to taste the particular character of a Kona-grown bean from a specific farm on a specific hillside in Holualoa, stopping well before this point preserves far more of that story.
Our piece on the coffee roasting process goes into more detail about how we approach these decisions for each batch we roast.
How Roast Level Changes What's in Your Cup
Everything we have covered so far translates into practical outcomes in your cup: flavor, acidity, body, sweetness, bitterness, and even the antioxidant content of your morning coffee. Let us look at each major roast level through the lens of what the chemistry actually produces and what that means for the drinking experience.
This section addresses many of the most common questions we hear from customers who want to understand their coffee more deeply, not just choose a roast by habit or marketing language.
Light Roast: Origin Forward, Acid Rich, Antioxidant Heavy
Light roast coffee is pulled from the roaster shortly after first crack concludes. The Maillard reaction has had meaningful time to develop, but it has not been pushed into its later stages. Caramelization has begun but is not fully developed. Pyrolysis has barely started. And CGA content, while already reduced from the green bean baseline, is at its highest for any drinkable roast level.
What you taste in a light roast is primarily origin. The flavors present in the cup reflect the terroir of the farm: the soil, the altitude, the particular characteristics of that year's harvest. In a light Kona roast, you might find bright citrus notes, floral aromatics, a sweetness that is delicate rather than heavy, and an acidity that is clean and vivid. It is a coffee that asks you to pay attention because it has a lot to say.
The antioxidant profile of light roast is the strongest of any roast level. More residual CGA content means more antioxidant activity in the brewed cup. For health-conscious coffee drinkers, this is worth knowing. Light roast is not just a flavor preference; it is a nutritional choice if antioxidant intake is part of your reasoning for drinking coffee.
Now, the caffeine myth. Many people believe light roast has less caffeine than dark roast because the heat has not had as long to work. The reverse is sometimes claimed as well. The truth is that caffeine is chemically stable through roasting and does not degrade meaningfully at typical roasting temperatures. Any difference between roast levels in caffeine content is negligible when measured by weight. Where confusion arises is measurement by volume. Because dark roast beans have expanded more and lost more moisture, they are physically larger and lighter per bean. If you scoop your coffee by volume (tablespoons) rather than weight (grams), you may actually be getting slightly more caffeine from a light roast because the denser beans pack more mass per scoop. When measured by weight, the difference essentially disappears. This is a persistent myth worth putting to rest.
For a full comparison of how roast levels differ in flavor, body, and character, our detailed look at the difference between a medium roast and a dark roast is worth reading alongside this article.
Medium Roast: The Balanced Sweet Spot
Medium roast is where Maillard and caramelization chemistry reaches its fullest, most balanced expression. Both reactions have had sufficient time to develop their products. The volatile aromatics from Maillard are present and stable. The caramel and brown sugar notes from caramelization are rich and well-developed. The organic acids from origin have been partially transformed, reducing sharp acidity while retaining enough brightness to keep the cup lively. And pyrolysis has not yet taken over as the dominant influence.
The result is a cup where both origin character and roast character are present and complementary rather than one overwhelming the other. For Kona coffee specifically, this is where the bean's distinctive sweetness is most accessible. The natural sugars of our volcanic-soil-grown cherries translate into a caramelized richness that is smooth and immediately enjoyable, while the complexity of origin notes adds dimension and keeps the cup interesting from first sip to last.
Medium roast is the choice that most specialty roasters and most of our customers return to, not because it is the safest choice, but because it genuinely showcases what well-grown, carefully processed Kona coffee can be. It is the range where the bean has enough to say and the roaster has given it the right conditions to say it clearly.
Our Mokulele Reserve roast is a beautiful example of how careful roasting in the medium range honors a premium Kona bean's full chemical potential. If you have not tried it, we encourage you to explore what deliberate roasting in this sweet spot tastes like.
Dark Roast: Roast Character Takes the Lead
Dark roast is the product of extended pyrolysis and significantly reduced levels of the lighter, more volatile compounds from the Maillard reaction and caramelization. What remains after the heat has pushed into and past second crack is a cup dominated by bold, smoky, bitter, and deeply roasted flavors. The body is heavier. The acidity is low. The finish is long and assertive.
CGA content in dark roast is minimal. Much of the antioxidant chemistry that survived a light or medium roast has been degraded further by the extended heat. The compounds that replaced those CGAs, the pyrolysis products that contribute to bitterness and body, are present in abundance.
Origin character is largely suppressed in dark roast. This is an observable fact of the chemistry. The nuanced compounds that reflect the specific terroir of a farm, the altitude, the soil, the microclimate, are not stable enough to survive the temperatures involved in dark roasting. They have been converted into simpler pyrolysis products that taste broadly similar regardless of where the bean came from.
This is why commodity coffee is often roasted dark. A dark roast can make a mediocre green bean taste acceptably bold and familiar. It masks defects and lack of origin character behind roast character. For premium single-origin coffee like ours, we offer dark roast options because some drinkers genuinely love that profile and there is nothing wrong with that preference. But we want you to know that choosing a medium roast of genuine Kona means tasting more of what this land, this altitude, and this farming tradition actually produces. It is a deliberate trade-off either way, and knowing the chemistry helps you make it consciously.
How Roasting Method Affects the Chemistry
The same chemical reactions occur in every roasting method. Maillard, caramelization, CGA degradation, and pyrolysis happen whether you are using a drum roaster or an air roaster or any other configuration. What differs between methods is the rate of heat transfer, the distribution of heat around the bean, and the degree of control the roaster has over each phase of the reaction sequence. These differences are real and meaningful in the cup.
Drum Roasting: Convection, Conduction, and Thermal Mass
Drum roasters are the most traditional and widely used roasting technology in specialty coffee. The core design involves a rotating drum, usually metal, that tumbles beans continuously while heat is applied. The beans receive heat through two pathways: conduction, meaning direct contact with the drum surface, and convection, meaning the hot air circulating through the drum.
The thermal mass of the drum itself is an important factor. A large metal drum takes time to heat up, but once it is at temperature, it holds that heat with stability. This thermal mass creates a kind of momentum in the roast. Temperature changes are gradual rather than abrupt. The beans experience a slower, more even progression through each chemical reaction stage.
This slower progression is well-suited to developing body and roast character. The extended time in the Maillard window allows more complex flavor compounds to develop and stabilize before temperatures rise further. Many of the classic characteristics associated with traditional Kona farm roasting, full body, sweet finish, smooth acidity, reflect the kind of thermal stability that drum roasting provides.
Managing a drum roast well requires attention and experience. The roaster must anticipate temperature changes in advance because the thermal mass means adjustments take time to register. Skilled drum roasting is as much intuition and experience as it is data, which is part of why farm-direct roasting by people who have intimate knowledge of their own beans consistently produces exceptional results.
Air Roasting: Faster, Cleaner, More Uniform Heat Transfer
Air roasters, sometimes called fluid bed roasters, work on a fundamentally different principle. Instead of a rotating drum