The Chemistry of Coffee: Understanding the Molecules Behind Each Cup
How can a coffee evoke jasmine, mango, dark chocolate, or caramel, when none of these ingredients have been added?
Why can two coffees from the same country, grown just a few kilometers apart, offer completely different sensory experiences?
And how can a simple variation in temperature, roasting, or extraction method radically transform the taste of a cup?
All these questions find their answers in a fascinating field: the chemistry of coffee.
Coffee is much more than just a drink. It is one of the most complex foods ever studied by scientists. To date, researchers have identified over 1,000 volatile compounds in roasted coffee, hundreds of which directly contribute to aroma perception. Few food products exhibit such molecular richness.
However, contrary to what one might believe, the taste of coffee does not depend solely on roasting or the origin of the beans.
Each cup is the result of a succession of biological, chemical, and physical phenomena that begin several months before harvest. The genetics of the coffee plant, the terroir, the maturation of the cherries, fermentation, drying, roasting, and extraction gradually modify the chemical composition of the bean to create the sensory experience we find in our cup.
At 94 Celcius, this scientific dimension plays an important role in our approach to coffee. Our goal is not only to roast excellent coffees but also to understand the mechanisms that allow them to express their full complexity.
In this guide, we will explore the major families of molecules that make up coffee, the chemical reactions that occur during roasting, the role of modern fermentation, and the influence of different extraction methods on aromas, body, and texture.
Whether you are a filter coffee enthusiast, an espresso lover, or simply curious to understand why a coffee can evoke peach, cocoa, or black tea, you will discover that behind every cup lies a true chemistry laboratory.
Why is coffee one of the most complex beverages in the world?
When we taste coffee, our brain does not perceive "notes of peach," "chocolate," or "jasmine." It actually detects an immense mixture of volatile molecules that stimulate our olfactory and gustatory receptors.
It is then our sensory memory that translates this combination of molecules into familiar images: a citrus fruit, a tropical fruit, a white flower, or a square of dark chocolate.
This complexity is what makes coffee so fascinating.
Today, researchers have identified more than 1,000 volatile compounds in roasted coffee, about 40 to 50 of which play a major role in the aroma perceived by humans. The concentration of some of these molecules is measured in parts per billion (ppb), or even parts per trillion (ppt), but they are enough to completely transform the perception of a cup.
For comparison, wine also contains several hundred aromatic molecules, but coffee is one of the most complex foods ever studied in food chemistry.
This molecular richness explains why two coffees grown on the same mountain can have completely different profiles.
A slight variation in temperature during maturation, a few extra hours of fermentation, a shorter roast, or a slightly different extraction can alter the concentration of dozens, or even hundreds, of molecules.
Each cup thus becomes a true chemical signature.
A cup of coffee is the result of several sciences
The taste of coffee is not only built during roasting.
It is the result of a succession of steps that begin several months before harvest.
Each step gradually modifies the chemical composition of the bean.
1. Genetics
It all starts with the botanical variety.
A Gesha, a Sidra, a Pink Bourbon, or a Caturra do not have the same genetic heritage.
This difference particularly influences:
-
the amount of sugars present in the cherry;
-
lipid concentration;
-
organic acids;
-
aroma precursors;
-
maturation speed.
In other words, two varieties grown under the same conditions will never produce exactly the same aromas.
2. Terroir
The terroir then influences the physiology of the coffee plant.
Altitude, temperature, rainfall, light, and soil composition directly modify the plant's metabolism.
Slower maturation generally allows for a greater accumulation of sugars, organic acids, and aroma precursor compounds.
This is notably why coffees grown between 1,700 and 2,300 meters often exhibit greater aromatic complexity.
3. Fermentation
After harvest, the coffee cherry enters a decisive stage.
For several hours, sometimes several days, yeasts, lactic acid bacteria, and other microorganisms naturally present — or intentionally added — transform the sugars contained in the mucilage.
This microbiological activity produces a multitude of new molecules:
-
organic acids;
-
alcohols;
-
esters;
-
aldehydes;
-
ketones;
-
volatile aromatic compounds.
Well-controlled fermentation increases the complexity of the coffee.
Conversely, poorly controlled fermentation can quickly generate sensory defects.
4. Roasting
Green bean is relatively discreet aromatically.
The magic truly appears during roasting.
Under the effect of heat, several hundred chemical reactions occur simultaneously.
Proteins react with sugars.
Lipids partially oxidize.
Chlorogenic acids transform.
Trigonelline degrades.
Carbohydrates caramelize.
In just a few minutes, coffee develops a huge part of the molecules that will make up its aromatic bouquet.
It is this step that transforms a plant bean into coffee.
5. Extraction
Finally, the last step is what the consumer performs.
Water acts as a true solvent.
It gradually dissolves different compounds present in the roasted bean.
However, not all molecules are extracted at the same rate.
Organic acids are generally extracted quickly.
Sugars follow.
Compounds responsible for bitterness often arrive later.
This is why a few seconds more or less during an extraction can profoundly change the balance of a cup.
An extraction does not create new molecules.
It simply selects which ones will be present in your cup.
The five major families of molecules that shape coffee's taste
Even though over a thousand compounds have been identified in coffee, the main players can be grouped into a few major families.
Carbohydrates
They represent a significant part of the green bean's dry matter.
During roasting, they become the main fuels for the Maillard reaction and caramelization.
They are responsible for a large part of coffee's gourmet notes.
Proteins and amino acids
They also participate in the Maillard reaction.
By reacting with sugars, they generate a multitude of new aromatic molecules.
It is thanks to them that coffee develops notes reminiscent of cocoa, toasted nuts, or warm bread.
Lipids
Arabica beans generally contain between 15 and 17% lipids, compared to about 10 to 12% in robusta.
These oils play a major role in the body of the coffee.
They also transport many aromatic compounds that are not very soluble in water.
Depending on the brewing method used, a greater or lesser part of these lipids will end up in the cup.
Organic acids
Coffee naturally contains a wide variety of acids.
Some are present before roasting.
Others appear or transform during fermentation or cooking.
They contribute to freshness, vibrancy, and sensory balance.
Contrary to popular belief, a very fruity coffee is not necessarily more chemically acidic.
The balance between sugars, bitterness, and aromatic compounds greatly influences the perception of this acidity.
Alkaloids
Coffee mainly contains two important alkaloids:
-
caffeine;
-
trigonelline.
While caffeine mainly influences bitterness and physiological effects, trigonelline plays a fundamental role during roasting.
By degrading under the effect of heat, it contributes to the formation of many molecules responsible for roasted, toasted aromas and vitamin B3 (niacin).
Chemistry… serving pleasure
Understanding coffee chemistry does not mean turning every tasting into a lab class.
On the contrary.
The more we understand what happens inside the bean, the more we can appreciate the immense work done by the producer, roaster, and barista.
Behind a simple note of strawberry or chocolate sometimes lie dozens of chemical reactions, hundreds of molecules, and months of work.
And yet, when everything is executed well, only one thing remains: a cup of coffee that seems astonishingly simple.
This is probably the true beauty of coffee chemistry.
Carbohydrates: the true architects of coffee aromas
If you had to remember one family of molecules responsible for the spectacular transformation that occurs during roasting, it would probably be carbohydrates.
Without them, coffee would never develop its notes of chocolate, biscuit, caramel, hazelnut, or toasted bread.
The green coffee bean is relatively discreet aromatically.
It already possesses certain precursors, but very few of the molecules we spontaneously associate with coffee.
The majority of these aromas appear only when carbohydrates encounter heat.
In other words, coffee does not naturally contain caramel aromas.
It rather possesses the necessary ingredients to create them.
What exactly are carbohydrates?
Carbohydrates encompass a large family of molecules composed mainly of carbon, hydrogen, and oxygen.
In coffee, they fulfill several essential biological functions.
They notably serve as:
-
energy reserve for the seed;
-
constituents of cell walls;
-
carbon source during chemical reactions of roasting.
The main carbohydrates found in a green bean are:
-
sucrose;
-
cellulose;
-
hemicelluloses;
-
arabinogalactans;
-
galactomannans;
-
various forms of polysaccharides.
Contrary to what many imagine, green coffee contains relatively few simple sugars like glucose or fructose.
The dominant sugar is sucrose.
Depending on the variety and origin, Arabica generally contains between 6 and 9% sucrose, while Robusta contains between 3 and 5%.
This difference partly explains why Arabicas often exhibit more natural sweetness.
Why is sucrose so important?
Sucrose acts as an immense reservoir of aromatic potential.
During roasting, it does not remain intact for very long.
From approximately 160 to 170 °C, it begins to decompose.
This degradation produces a multitude of new molecules.
Some will participate in the Maillard reaction.
Others will enter caramelization.
Still others will be used to create completely new aromatic compounds.
The sugar gradually disappears.
The aromas, however, appear.
It is a fascinating paradox.
The more coffee develops caramel notes, the less sugar it actually contains.
The Maillard reaction: the true aromatic engine of coffee
Most people know about the Maillard reaction without realizing it.
It's what gives a baguette its color.
What browns a steak.
What perfumes freshly baked cookies.
What transforms onions into a naturally sweet jam.
Coffee works on exactly the same principle.
When reducing sugars meet amino acids under the effect of heat, they initiate an immense cascade of chemical reactions.
A single reaction quickly leads to hundreds of others.
Scientists sometimes speak of a true reaction network, so numerous are the possibilities.
The result?
The gradual formation of many families of aromatic molecules:
-
pyrazines;
-
furans;
-
pyrroles;
-
aldehydes;
-
ketones;
-
thiols;
-
oxazoles;
-
lactones.
These are the molecules that build a large part of what we call "the taste of coffee."
Why do two coffees develop different aromas?
The Maillard reaction always follows the same chemical principles.
However, the initial ingredients change enormously.
Depending on:
-
the variety;
-
the altitude;
-
the maturity of the cherry;
-
the fermentation;
-
the drying;
-
the roasting,
the quantities of sugars, amino acids, and aromatic precursors vary.
Two coffees roasted in exactly the same way can therefore produce completely different profiles.
Chemistry follows the same rules.
The ingredients change.
The result also.
Caramelization and Maillard reaction: two often-confused phenomena
These two reactions are frequently presented as identical.
In reality, they are very different.
The Maillard reaction
It requires the presence of:
-
a sugar;
-
an amino acid;
-
heat.
It is responsible for the majority of coffee's complex aromas.
Caramelization
Caramelization is simpler.
It does not require proteins.
Sugar decomposes directly under the effect of heat.
It notably produces compounds reminiscent of:
-
caramel;
-
butter;
-
brown sugar;
-
cooked sugar.
In coffee, caramelization certainly exists, but its importance is often overestimated.
The majority of coffee's emblematic aromas come more from the Maillard reaction than from caramelization itself.
Melanoidins: the often-forgotten ones
As roasting progresses, Maillard reactions also generate very large molecules called melanoidins.
They are responsible for several important characteristics.
They contribute to:
-
the brown color of roasted coffee;
-
the mouthfeel;
-
part of the texture;
-
certain antioxidant properties;
-
aromatic persistence.
Melanoidins continue to be studied today.
Their structure is extremely complex and still imperfectly understood.
Why does very dark roasted coffee taste less sweet?
This is a question we often hear.
"If roasting develops caramel, why does dark coffee taste less sweet?"
Because perceived sweetness is not solely linked to sugar.
As roasting progresses:
-
sugars are consumed;
-
Maillard reactions slow down;
-
pyrolysis reactions become more prominent;
-
bitter compounds increase.
The brain then interprets bitterness more than sweetness.
A very dark coffee therefore contains fewer residual sugars and more molecules from thermal degradation.
It is this change in balance that explains why a light roast often seems sweeter, even when it contains very little sugar.
What this means for the roaster
For a roaster, managing carbohydrates is a real balancing act.
A roast stopped too early leaves certain reactions incomplete.
Conversely, an over-roasted coffee gradually destroys the most delicate aromatic molecules in favor of heavier, smoky, or charred compounds.
The whole difficulty lies in stopping the roasting at the moment when the coffee's aromatic potential reaches its maximum.
This point is different for each origin, each variety, each post-harvest process, and sometimes even for each harvest.
It is precisely for this reason that there is no universal roasting profile.
At 94 Celcius, we adapt each roasting curve to the chemical potential of the coffee we are working with.
Our goal is never to roast all coffees in the same way.
It is to allow each one to fully express the chemical reactions that make it unique.
Lipids: the invisible molecules that give coffee body
Ask a coffee lover what influences the taste of a cup.
They will probably talk about origin, roasting, or grinding.
However, another family of molecules plays a fundamental role in our perception of coffee: lipids.
Often called "coffee oils," lipids influence texture, body, aftertaste, and even how certain aromas are perceived.
However, they have no pronounced taste or marked scent.
Their role is much more subtle.
They serve as a support for some of the most interesting aromatic molecules in coffee.
In other words, they transport aromas more than they create them.
What is a lipid?
In biochemistry, the term lipid refers to a large family of molecules insoluble in water.
In coffee, we mainly find:
-
triglycerides;
-
diterpenes (cafestol and kahweol);
-
phospholipids;
-
sterols;
-
natural waxes;
-
free fatty acids in very small quantities.
In Arabica, lipids generally represent 15 to 17% of the green bean's weight.
In Robusta, this proportion is rather 10 to 12%.
This difference already partly explains why Arabica coffees often offer a richer and silkier mouthfeel.
But their importance goes far beyond simple "body."
Lipids are aroma reservoirs
A widespread misconception is that coffee oils "give taste."
The reality is more interesting.
Many of the molecules responsible for coffee aromas are hydrophobic, meaning they dissolve poorly in water but very well in lipids.
During roasting, these aromatic molecules form in the heart of the bean.
Some of them then migrate to the lipids.
These then act as true reservoirs.
During extraction, a fraction of these oils is carried into the cup, where they gradually release the aromatic compounds they transport.
This phenomenon, in particular, explains why some coffees seem to develop new aromas as they cool.
Molecules continue to be gradually released from this lipid phase.
Why does an espresso seem richer than filter coffee?
The answer largely lies... in lipids.
An espresso machine operates at approximately 9 bars of pressure.
This pressure causes coffee oils to disperse as tiny droplets.
Scientists speak of an emulsion.
An emulsion is a very fine mixture of two liquids that normally do not mix: in this case, water and coffee oils.
This emulsion directly contributes to:
-
the syrupy texture;
-
the sensation of roundness;
-
the length on the palate;
-
the stability of the crema.
In other words, espresso contains more suspended lipids than most filter methods.
And that completely changes our perception.
Why does Chemex seem so clear?
Conversely, a Chemex uses a particularly thick paper filter.
This filter retains a large part of the oils and colloidal particles.
Result:
-
fewer lipids;
-
fewer suspended solids;
-
less body;
-
more clarity.
The aromas become more precise.
Floral, citrus, and delicate notes take center stage.
The coffee seems lighter, even if its concentration of aromatic compounds can be very high.
French Press: the opposite experience
The French Press works almost the opposite way.
The metal filter lets through:
-
oils;
-
colloids;
-
fine solid particles.
The cup then becomes much richer.
The body is denser.
The texture is smoother.
Certain chocolatey, spicy, or woody notes also seem more present.
This is not because the coffee contains more aromatic molecules.
It's because lipids change how our mouth and brain perceive these aromas.
Colloids: the forgotten elements of coffee
When we talk about texture, we immediately think of oils.
In reality, they are not alone.
Coffee also contains many colloids.
Colloids are tiny particles dispersed in water without being completely dissolved.
They come from:
-
polysaccharides;
-
degraded proteins;
-
melanoidins;
-
very fine ground coffee particles.
These colloids strongly influence:
-
viscosity;
-
mouthfeel;
-
aromatic persistence;
-
crema stability.
They work closely with lipids to build what we commonly call the "body" of a coffee.
Crema: much more than foam
Espresso crema is often perceived as a simple aesthetic element.
In reality, it constitutes an extremely complex physicochemical system.
It results from the interaction between:
-
carbon dioxide trapped in the bean after roasting;
-
lipids;
-
proteins;
-
melanoidins;
-
natural coffee surfactants.
Under pressure, the dissolved CO₂ is suddenly released from the portafilter.
The bubbles that form are stabilized by oils and certain surface compounds.
The crema then becomes a relatively stable foam capable of temporarily retaining several aromatic molecules.
During the first few seconds of tasting, some of these aromas are gradually released into the nose.
This is why the olfactory experience of an espresso is often particularly intense.
Diterpenes: molecules as studied as they are controversial
Coffee lipids also contain two well-known molecules:
-
cafestol;
-
kahweol.
These diterpenes have been the subject of much research.
They possess interesting biological properties, particularly concerning certain antioxidant and anti-inflammatory mechanisms studied in the laboratory.
However, they are also known to increase LDL cholesterol when consumed in large quantities, particularly in unfiltered brewing methods, such as Turkish Coffee, French Press, or boiled coffee.
Paper filters retain most of these molecules.
Thus, a V60 or a Chemex generally contains much less cafestol and kahweol than a French Press or an espresso.
This is not to say that one method is better than another, but simply to understand that the chemistry of your cup changes depending on the filtration used.
What this means for a roaster
When we develop a roasting profile at 94 Celcius, we never think only about aromas.
We also consider how these aromas will be transported to your cup.
A coffee primarily intended for filter will often emphasize clarity, finesse, and aromatic precision.
A coffee designed for espresso can be developed to offer more texture, sweetness, and persistence on the palate.
It's not just a matter of taste.
It's a matter of chemistry.
Lipids, colloids, dissolved gases, and aromatic compounds constantly interact to build the final sensory experience.
And it is precisely this invisible interaction that explains why two extraction methods using exactly the same coffee can offer two totally different experiences.
Coffee acids: why coffee can taste like lemon without being "acidic"
"I don't like acidic coffees."
This is probably one of the phrases we hear most often in a roastery.
But when asked what "acidic" means exactly, the answers vary greatly.
For some, it refers to coffee that is too aggressive.
For others, a sensation reminiscent of lemon.
Some speak of a hint of vinegar, while others simply think of a fruity coffee.
In reality, the word acidity is used to describe several completely different phenomena.
In chemistry, an acid is a molecule capable of releasing a proton (H⁺) in solution.
In tasting, acidity is a sensory perception.
And between the two, there are a multitude of interactions that explain why two coffees with almost identical pH can offer totally opposite sensations.
Understanding this difference completely changes how we taste coffee.
Coffee is naturally acidic... but less than several other beverages
A common misconception is that coffee is an extremely acidic beverage.
In reality, its pH is generally between 4.8 and 5.3, depending on the origin, roast, and extraction method.
For comparison:
| Beverage | Approximate pH |
|---|---|
| Lemon juice | 2.0 |
| Cola | 2.5 |
| White wine | 3.0 to 3.5 |
| Orange juice | 3.3 to 4.2 |
| Filter coffee | 4.8 to 5.2 |
| Espresso | 4.9 to 5.3 |
| Pure water | 7.0 |
Coffee is therefore acidic in the chemical sense, but much less so than many beverages consumed daily.
Yet, some coffees seem extremely bright while others seem very smooth.
Why?
Because our brain does not measure pH.
It interprets a set of sensory signals.
Not all acids taste the same
Coffee contains several dozen organic acids.
Fortunately, a few play a much more important role than others.
Citric acid
Naturally present in the coffee cherry.
It is associated with a fresh, bright acidity.
Depending on its concentration and balance with other compounds, it can evoke:
-
lemon;
-
lime;
-
orange;
-
mandarin.
Coffees from Ethiopia, Kenya, or certain regions of Colombia often have relatively high concentrations of citric acid.
Malic acid
It is also the main acid found in apples.
In coffee, it provides a rounder sensation.
It is often associated with:
-
green apple;
-
pear;
-
certain stone fruits.
It gives a milder acidity than citric acid.
Phosphoric acid
Phosphoric acid is fascinating.
At low concentrations, it provides an extremely clean sensation of freshness.
Several tasters associate it with:
-
certain grapes;
-
tropical fruits;
-
an almost sparkling sensation.
It is particularly sought after in several competition coffees.
Lactic acid
Produced notably during certain fermentations.
It does not resemble lemon.
It rather provides a creamy sensation.
Many tasters describe it as:
-
yogurt;
-
cream;
-
fermented milk;
-
certain pastries.
This is one reason why some controlled fermentations seem to make a coffee more "creamy" without adding more body.
Acetic acid
Acetic acid is the main component of vinegar.
At very low concentrations, it increases complexity.
At high concentrations, it becomes a defect.
The challenge is to stay below the threshold where it becomes dominant.
Chlorogenic acids
For a long time, they were presented as responsible for coffee's acidity.
Today, we know that their role is much more complex.
Chlorogenic acids represent one of the main families of polyphenols in green coffee.
They notably possess:
-
antioxidant activity;
-
a role in plant defense;
-
an influence on bitterness;
-
an indirect contribution to certain perceptions of astringency.
During roasting, some of these molecules degrade.
They notably give rise to:
-
quinic acid;
-
caffeic acid.
This transformation progressively alters the coffee's taste balance.
Why is a very fruity coffee not necessarily more acidic?
This is probably the biggest misunderstanding in specialty coffee.
Our brain never detects an isolated molecule.
It interprets a whole.
Let's take two coffees with exactly the same pH.
The first has:
-
more sugars;
-
very little bitterness;
-
many fruity esters.
The second contains:
-
less sugar;
-
more bitter compounds;
-
very few aromatic compounds.
The first will often seem fruitier, sometimes even more "acidic".
Yet, chemically, their acidity can be practically identical.
In other words, the perception of acidity depends enormously on the sensory context.
Why do some fermentations seem more acidic?
Modern fermentations indeed alter the chemical composition of coffee.
Certain yeasts and bacteria produce:
-
more organic acids;
-
alcohols;
-
esters;
-
aldehydes.
But they also produce a huge amount of new aromatic molecules.
These often reinforce our impression of fruit.
The brain naturally associates these fruity aromas with a sensation of freshness.
We then speak of a "bright" acidity, although it is often a multisensory phenomenon.
Why does roasting change acidity so much?
During cooking, several acids evolve differently.
Some decrease.
Others appear.
Still others transform.
In parallel:
-
sugars gradually disappear;
-
bitter compounds increase;
-
pyrolysis reactions become more significant.
The result is a cup where the sensory balance constantly evolves.
A lighter roast generally retains more intact organic acids.
A darker roast often reduces this freshness in favor of roasted, chocolate, or smoky notes.
But, again, it's not just a matter of the quantity of acids.
It's about a new balance between all the molecules present.
What this means for a roaster
At 94 Celcius, we never seek to suppress acidity.
We seek to make it harmonious.
A beautiful acidity brings structure.
It provides length.
It highlights the sugars.
It reveals the aromas.
Without it, many coffees would seem flat.
Our job is therefore to find the roasting point where organic acids, sugars, lipids, aromatic compounds, and bitterness are perfectly balanced.
It is precisely this balance that transforms a simple acidic sensation into a vibrant, bright, and complex cup.
Chemistry teaches us one essential thing
Coffee never simply tastes "acidic".
It tastes balance.
Behind a note of mandarin, white peach, grape, or blackcurrant lies an extraordinarily complex combination of organic acids, sugars, volatile compounds, and hundreds of chemical reactions.
The more we understand this chemistry, the more obvious one thing becomes:
The best coffee is not the one with the least acids.
It is the one where each molecule finds its rightful place.
Aromatic molecules: why coffee can evoke mango without containing a single mango
Close your eyes.
Take a cup of excellent Ethiopian coffee.
You might perceive jasmine.
Lemon.
Peach.
Black tea.
Now take a fermented Colombian coffee.
You might recognize mango, passion fruit, strawberry, or even lychee.
Yet, none of these plants have been added.
Coffee is composed solely... of coffee.
How is this possible?
The answer lies in an extraordinary illusion created by our brain.
We never smell an isolated molecule
When we breathe in the scent of coffee, hundreds of volatile molecules simultaneously enter our nose.
These molecules reach the olfactory epithelium where approximately 400 types of functional olfactory receptors in humans are capable of recognizing different chemical families.
Each molecule activates several receptors.
Each receptor recognizes several molecules.
The brain then receives an immense combination of signals.
It does not try to identify each molecule individually.
It compares this combination to its memory.
In other words...
We don't smell chocolate.
We recognize a combination of molecules that resembles what our brain knows as chocolate.
The same phenomenon occurs for strawberry, mango, honey, or flowers.
More than 1,000 molecules... but only a few dozen truly dominate
Today, scientists have identified over a thousand volatile compounds in roasted coffee.
However, the majority are present in such low concentrations that they contribute little to the final aroma.
Conversely, a few dozen molecules have an extremely low perception threshold.
A few billionths of a gram can sometimes completely change a cup.
This is referred to as the Odor Activity Value (OAV).
A highly concentrated molecule can be practically odorless.
Conversely, a molecule present at only a few nanograms can dominate the entire aromatic profile.
Therefore, it is not the quantity that matters.
It is our brain's ability to detect it.
Pyrazines: the molecules of roasted coffees
Pyrazines are among the most emblematic compounds of coffee.
They appear mainly during the Maillard reaction.
Depending on their structure, they evoke:
-
cocoa;
-
roasted hazelnuts;
-
almonds;
-
toasted bread;
-
cereals;
-
peanut butter.
A slight variation in roasting already significantly alters their concentration.
This is one of the reasons why two very similar roasting profiles can seem completely different.
Furans: the scent of caramel
Furans primarily come from the thermal degradation of sugars.
They contribute to notes of:
-
caramel;
-
brown sugar;
-
maple syrup;
-
biscuit;
-
warm bread.
They add a lot of sweetness to coffee.
They are particularly abundant in medium roasts.
Aldehydes: freshness
Aldehydes often bring the brightest notes.
Depending on their structure, they can recall:
-
citrus;
-
apples;
-
flowers;
-
green fruits.
They play a major role in the sensation of freshness.
Ketones: sweetness and fruit
Several ketones are responsible for notes that are:
-
creamy;
-
buttery;
-
fruity;
-
milky.
They often contribute to coffees with a beautiful roundness.
Esters: the champions of fruity coffees
If we had to choose one family responsible for the explosion of modern fermentations...
It would probably be esters.
These molecules are mainly produced during certain fermentations.
Many have odors reminiscent of:
-
banana;
-
mango;
-
pineapple;
-
pear;
-
strawberry;
-
tropical fruits.
This is notably why some controlled fermentations develop extremely exotic profiles.
Esters are also very present in wine.
Terpenes: flowers
Terpenes are widely distributed in the plant world.
They are found in particular in:
-
citrus fruits;
-
flowers;
-
aromatic herbs.
In coffee, several terpenes contribute to the notes of:
-
jasmine;
-
orange blossom;
-
lavender;
-
bergamot;
-
lemongrass.
Exceptional Gesha coffees often display a beautiful expression of this family.
Thiols: tiny but extraordinary molecules
Thiols are fascinating.
They contain sulfur.
Taken in isolation, some smell extremely bad.
Yet...
At very low concentrations, they become responsible for some of the most sought-after aromas in coffee.
They can evoke:
-
passion fruit;
-
grapefruit;
-
blackcurrant;
-
certain tropical fruits.
Their perception threshold is incredibly low.
A few parts per billion are sometimes enough to alter a cup.
Why can the same molecule smell differently?
Another fascinating phenomenon.
A molecule does not always have a single scent.
Depending on:
-
its concentration;
-
the other molecules present;
-
the temperature;
-
saliva;
-
our genetics;
-
our experience,
it can be interpreted differently.
The brain works by comparison.
It constructs a global image.
It's exactly like an orchestra.
A trumpet doesn't play the same music depending on the instruments accompanying it.
Aromatic molecules work in the same way.
Retronasal olfaction: the true secret of tasting
When we drink coffee, most of the aromas do not pass directly through the nose.
They travel up from the mouth.
This phenomenon is called retronasal olfaction.
As the coffee warms in the mouth, volatile molecules evaporate.
They then reach the olfactory epithelium through the back of the nasal cavity.
This is why coffee often seems much more aromatic after a few seconds than on first contact.
And it's also why nasal congestion greatly reduces our perception of flavors.
Taste rarely disappears.
Smell, however, is greatly diminished.
Why does coffee change as it cools?
This question greatly intrigues enthusiasts.
As it cools:
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certain molecules become more perceptible;
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evaporation slows down;
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bitterness decreases;
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acidity often seems more precise.
Our brain then accesses aromas that were previously masked.
This is why professional tasters always taste coffee at several temperatures.
A cup rarely tells its whole story when it's scalding hot.
What this means for a roaster
At 94 Celcius, our goal is not to artificially create aromas.
Rather, we seek to preserve the molecules that nature, the producer, and the roasting process have allowed to develop.
Every second of roasting influences their concentration.
Every additional degree can favor certain chemical families... while destroying others that are much more fragile.
Our job is therefore to find the balance.
The one where the greatest number of aromatic molecules express themselves harmoniously.
Because ultimately...
Coffee never simply tastes like coffee.
It tells an extraordinarily complex chemical story, composed of hundreds of molecules that play together the most beautiful aromatic symphony.
And it is precisely this complexity that makes coffee one of the most fascinating beverages ever studied by science.
Roasting: a few minutes that completely transform coffee chemistry
While green coffee is rich in potential, it is roasting that allows it to reveal much of its personality.
Before entering the roaster, a coffee bean already contains carbohydrates, proteins, lipids, alkaloids, organic acids, and a multitude of aromatic precursors. However, if you smelled it at that moment, it would evoke fresh cereals, green peas, or raw peanuts more than the coffee we know.
In just a few minutes, this relatively discreet raw material becomes one of the most aromatically complex foods in our diet.
Why?
Because several hundred chemical reactions occur simultaneously.
For the roaster, it's not just about cooking a bean.
It's about guiding a succession of chemical reactions that will evolve differently depending on temperature, time, rate of temperature increase, bean moisture, airflow, and even crop density.
Each decision alters the final composition of the coffee.
The first phase: drying
Green coffee beans generally contain between 9 and 12% water.
The first step in roasting is therefore to gradually evaporate this water.
During this period, very few new aromas are produced.
However, this phase is essential.
Too rapid drying creates significant temperature gradients between the surface and the core of the bean. Conversely, too slow drying can limit the development of subsequent reactions.
The roaster is already preparing the coffee's chemistry... long before the aromas appear.
The Maillard reaction: the heart of roasting
As the temperature rises, sugars begin to react with amino acids.
We then encounter the famous Maillard reaction.
This is probably the most important stage of the entire roasting process.
Hundreds of new molecules gradually appear:
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pyrazines;
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furans;
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pyrroles;
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aldehydes;
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thiols;
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ketones;
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oxazoles;
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melanoidins.
The color of the bean changes rapidly.
The first characteristic coffee aromas appear.
Sweetness increases.
Aromatic complexity explodes.
For a roaster, this period represents a delicate balance.
Too short, it leaves the coffee underdeveloped.
Too long, it can reduce aromatic freshness and shift the profile towards heavier notes.
First Crack: A Major Physical Change
Around 195 to 205 °C, depending on the coffee, the bean reaches sufficient internal pressure to crack.
This is the first crack.
Contrary to popular belief, this sound is not solely caused by water vapor.
It results from a combination of phenomena:
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increase in internal pressure;
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release of carbon dioxide;
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gas expansion;
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progressive weakening of cell walls.
The bean then significantly increases in volume while its density decreases.
This transformation profoundly changes its structure.
It will then facilitate extraction during coffee preparation.
Development: A Few Seconds That Change Everything
After the first crack begins what roasters often call the development phase.
Even if it sometimes only lasts one or two minutes, this period strongly influences the final result.
Aromatic compounds continue to evolve.
Some reach their maximum concentration.
Others are already starting to degrade.
The longer the development, the more certain chocolatey, roasted, and spicy notes gain prominence.
Conversely, several molecules responsible for floral or fruity notes are relatively fragile.
The goal is therefore never to develop "as much as possible," but to find the point where each coffee expresses the best balance between complexity, sweetness, and identity.
At 94 Celcius, there is no universal recipe.
Each origin, each variety, and each post-harvest process has its own optimal development window.
Pyrolysis: When Heat Destroys as Much as It Builds
Beyond a certain threshold, the reactions change in nature.
The heat becomes intense enough to cause the direct thermal decomposition of many molecules.
This is called pyrolysis.
This stage notably produces compounds responsible for notes:
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smoky;
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roasted;
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woody;
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spicy;
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sometimes charred.
At low intensity, these molecules enrich the complexity of the coffee.
At high intensity, they eventually mask the terroir and standardize the aromatic profiles.
This is why a very dark roast often tends to make several coffees surprisingly similar.
Chemistry progressively dominates over origin.
Roasting Adds Nothing
An idea often comes up among enthusiasts:
"Roasting adds aromas."
In reality, it adds absolutely nothing.
It transforms.
It reorganizes.
It destroys certain molecules.
It builds new ones.
The bean already contains almost all the necessary potential.
The roaster's role is to reveal this potential without erasing it.
This is precisely the philosophy that guides each of our roasts at 94 Celcius.
Our goal is never to impose a signature.
It is to allow the coffee to tell its own story.
Conclusion: Chemistry Explains Coffee, But It Doesn't Explain Why We Love It
After exploring carbohydrates, lipids, proteins, organic acids, alkaloids, aromatic molecules, and the reactions that transform a green bean into a cup of coffee, one fact becomes clear: coffee is much more than a simple beverage.
It is an extraordinarily complex biological system.
Each bean represents the result of several years of growth, thousands of hours of human labor, and millions of chemical reactions that chain together from the coffee tree's flowering to your first sip.
A different variety.
A few extra meters of altitude.
A slightly longer fermentation.
A roast stopped thirty seconds earlier.
A slightly finer grind.
Water that is slightly more mineralized.
Each of these details modifies the chemistry of coffee, sometimes imperceptibly, sometimes spectacularly.
And yet, despite all this complexity, our brain never thinks in molecules.
It thinks in emotions.
It recognizes a cup that evokes jam made by a grandparent, the smell of hot chocolate in winter, flowers encountered during a trip, or fruit savored under a summer sun.
Chemistry does not create these memories.
It simply gives them a language.
This is probably what makes coffee so fascinating.
Unlike many foods, there isn't just one way to produce excellent coffee. There are an infinite number of possible balances, each revealing a different facet of the bean, the terroir, and the producer's work.
At 94 Celcius, we believe that understanding chemistry does not detract from the magic of coffee.
Quite the opposite.
Each scientific discovery helps us to better respect the work done before us.
Understanding the Maillard reaction reminds us of the essential role of the roaster.
Understanding fermentations allows us to better appreciate the producers' know-how.
Understanding aromatic molecules helps us to better interpret what we perceive in a cup.
But no equation will ever replace the experience of tasting.
No chromatographic analysis can explain why one coffee affects us more than another.
No graph will replace the emotion felt when discovering a coffee capable of surprising us.
Science teaches us how coffee becomes exceptional.
Tasting reminds us why it deserves all this attention.
This is precisely where chemistry and art meet.
And it is in this place, between scientific rigor and the pleasure of sharing a cup, that we try to work every day at 94 Celcius.
Because ultimately, behind the thousands of molecules, chemical reactions, and roasting profiles, there remains a simple truth:
The best coffee is the one that makes you want to prepare the next cup.
Modified on July 13, 2026