Coffee Roasting

Coffee roasting is a traditional process that, despite its great importance, is still designed and managed on an empirical basis, through trial and error.

A less empirical and more scientific vision regarding coffee roasting comes from understanding the raw material and the compounds it contains.

The following text aims to investigate in a simple way the main chemical compounds present in roasted coffee and the main physical modifications that occur during roasting, with particular focus on the chemical reactions that lead to the formation of different compounds, in general terms and without making distinctions between coffee species for now, in order to lead to future reading of more specific and targeted insights.

Coffee Roasting: the Raw Material

When discussing coffee roasting, the first fundamental role is played by the selection of green coffee beans.

They already contain some of the chemical substances that will bring the perceptible characteristics to the coffee beverage’s taste, and partly the precursors that will lead to the formation of chemical substances that will evolve during coffee roasting.

The expert roaster knows that the raw material must be guided towards the development of its components that undergo changes and that these cannot be created from nothing. Consequently, excellent green coffee can produce either excellent or terrible roasted coffee, but poor raw beans cannot be turned into good ones.

What is Coffee Roasting

Coffee roasting is an intense thermal process through which heat is transferred to coffee beans, initially green, through direct or indirect heating, leading to both physical and chemical changes in the beans.

Roasting is essential to bring coffee beans to have the chemical and physical characteristics necessary to be ground and to produce, through extraction, the coffee beverage.

The tastes, flavors, aromas, and all potentially perceptible characteristics in the cup from the raw material depend on the coffee roasting process.

The Joule and Temperature

It’s impossible to talk about heating without becoming familiar with the unit of measurement for heat.

Since heat is one of the various forms that energy can take, it is measured in the International System in Joules (J).

Heat is a form of energy directly connected to the thermal agitation of particles that make up a body. In short, the more the particles that make up a body move, the hotter this body becomes.

Since it is difficult to indicate exactly how much thermal energy is contained in a body, it was necessary to create another physical quantity: temperature.

Temperature defines a body’s ability to exchange thermal energy (heat) with the outside (or other bodies); in other words, temperature can be considered as an indication of the average state of movement of particles present in a body.

To measure temperature in the simplest way, a thermometer is used. When a thermometer is placed near a hot body, the particles of the hot body (in motion) transmit this energy to the particles of the liquid present in the thermometer, which then begin to move until reaching a state of thermal equilibrium with the body they were put in contact with.

The greater the energy transmitted, the greater the movement of the particles that make up the thermometer’s liquid. At a macroscopic level, this increase in movement is detected with an increase in the body’s volume, which is then measured with an appropriate scale.

Inside the roaster, temperature is measured using one or more thermocouples.

Each thermocouple consists of two different metal wires (the conductors) joined at one end at a point called the hot junction and separately connected at the other end to a point called the cold junction, forming a circuit.

The hot junction is the point where the temperature to be measured is applied, while the other end (cold junction) is connected to a body whose temperature is known.

The temperature difference between the two junctions creates what is called the Seebeck effect, meaning two different metals, when heated, can generate current within the circuit due to a potential difference, which can be used to measure (derive) the temperature of the measured body through the use of a voltmeter.

Conduction, Convection, and Radiation

During coffee roasting, heat transfer occurs from the surrounding system to the coffee beans.

As we have seen, bodies can transfer heat between them. There are three modes of heat transmission:

• conduction;

• convection;

• radiation.

Conduction is the method by which heat is transmitted in solid objects and objects in contact with each other. Through this mechanism, the atoms of the hotter body (excited) transmit their vibrations to the atoms of the cold body they are in contact with, which in turn transmit this vibration to nearby atoms, thus transmitting heat.

This is the case of the roaster’s hot drum which, when in contact with coffee beans, transfers heat to them through conduction. Within the coffee beans themselves, heat is transmitted from outer particles to inner ones through conduction. It should be noted that the atoms within the object don’t move freely but “vibrate in place.”

Thermal convection occurs when at least one of the two bodies exchanging heat is a fluid (such as air). This fluid must be in relative motion to the other body with which it exchanges heat. This movement can be given naturally by “convective motions” or forcibly (for example through the use of a fan). The modes of heat transmission are similar to those of conduction, but at a macroscopic level, there is also the motion of the fluid that carries such energy.

During roasting, hot air flows over the coffee bean transmitting heat to its surface through convection (with excited particles moving toward the bean’s surface with subsequent transmission of such energy), from the bean’s surface to the inside of the bean itself (at least regarding solid materials) heat is then transmitted by conduction.

Regarding radiation, it should be remembered that any body emits electromagnetic radiation; When energy is emitted from the hot body in the form of electromagnetic radiation and hits a second body, part is absorbed and part reflected. Only the absorbed component of radiant energy leads to actual heating of the body and thus heat transmission by “radiation.”

Endothermic and Exothermic Reactions

During coffee roasting, chemical reactions occur inside the bean. A chemical reaction that develops heat while occurring is called “exothermic” (releases energy), while a chemical reaction that absorbs heat from the outside during its occurrence is called “endothermic” (requires energy).

An “exothermic” reaction may still need heat to be activated and then develop a greater amount of heat than that needed for activation.

It’s often mistakenly thought that exothermic reactions in the bean begin after the first crack (the moment during roasting when the coffee bean crackles with a characteristic sound similar to that of popcorn).

In coffee heating, calorimetric curves show an endothermic peak above 100°C (aqueous phase transition) and at higher temperatures, an exothermic trend. These exothermic reactions begin at about 140°C for green coffee beans. These exothermic phenomena are mainly attributed to reactions occurring at the level of carbohydrates contained in the beans⁴⁶.

Physical Changes in Beans During Coffee Roasting

During roasting, coffee beans undergo physical changes that briefly affect volume, mass, relative density, and coloration.

Regarding volume, it increases approximately 40-65% depending on factors such as roasting time and temperature⁴⁵.

The mass lost concerns water evaporation, loss of organic matter, degradation, and evaporation of substances. According to Oosterveld et al., weight losses of 11%, 15%, and 22% represent light, medium, and dark roasted coffee beans respectively⁴⁵.

The coloration of coffee beans varies with the varying intensity of roasting (time/temperature) involved.

Effect of Roasting on Coffee’s Chemical Composition

Coffee beans, besides water and minerals, contain a large quantity of substances whose concentrations can vary depending on the type of coffee.

In general, the main components of coffee beans are carbohydrates, but other substances containing nitrogen, both protein and non-protein, lipids, organic acids, volatile substances, oils, and phenolic substances are also present.

The reactions that take place during roasting still present knowledge gaps today due to the difficulty of reproducing results in laboratories of all reactions that take place inside the coffee bean. What is certain, and several studies demonstrate this, is that roasting modifies the initial chemical composition of coffee beans.

Obviously, a “dark” roast leads to a much more significant variation in chemical composition compared to a “light” roast. Initially, the roasting process leads to the formation of carbon dioxide and water.

Carbon dioxide is quantitatively the most important volatile component that originates during roasting, but it doesn’t contribute to coffee aroma. This originates from pyrolysis reactions and from the Strecker degradation reaction.¹ Subsequently, there is degradation of carbohydrates, proteins, and chlorogenic acids, with a relative increase in organic acids, while trigonelline and caffeine levels remain roughly unchanged.²

Whatever the degree of roasting, a roasted coffee bean will always have a volatile component, meaning a set of chemical compounds that tend to evaporate, and a non-volatile one.

Roasting plays a fundamental role for a coffee producer as it allows obtaining a non-volatile component, responsible for coffee aroma, which would otherwise exist only partially.

Non-volatile Component after Coffee Roasting

From a chemical point of view, the non-volatile component of a roasted coffee bean essentially consists of:³

1. Alkaloids such as caffeine and trigonelline, which give coffee robustness and body;
2. Macromolecules such as proteins and polysaccharides, like cellulose and hemicellulose, which play a fundamental role in retaining all the volatile components after roasting;
3. Humic acids and melanoidins, derived from the reaction between amino acids and monosaccharides during roasting and which give the classic dark color to the bean;
4. Carboxylic acids responsible for harshness;
5. Chlorogenic acids, mainly cinnamic, caffeic, and ferulic;
6. Lipids (terpenes, triglycerides, sterols) that confer a certain viscosity;
7. Minerals such as potassium, manganese, iron, and copper, which play an important role during the reactions that occur in roasting.

Many of the listed chemical species have evaporation temperatures that are reached and exceeded during the roasting process. Therefore, many substances should leave the roasted bean, which however doesn’t happen probably due to the conformation of the micropores inside the coffee beans.

The internal micropore structure of coffee beans hasn’t been completely revealed yet, a “plasmodesma” structure or a more chaotic three-dimensional permeable structure of polysaccharides is suspected. In this latter case, the permeable network of polysaccharides would prevent bean degassing and oil surfacing⁴⁷.

An increase in polysaccharide degradation at higher temperatures could be the cause of the wider cell wall micropores found in high-temperature roasted coffee samples that cause increased degassing and oil migration to the surface⁴⁷.

Indeed, the ability to retain gases formed during roasting represents one of the most remarkable properties of coffee beans. Based on total CO2 content and bean porosity, maximum internal pressures (depending on the scientific research consulted) of 4.4 atm and 7.9 atm have been reported in coffee beans⁴⁸.

It is known that the increase in internal pressure in coffee beans leads, obviously, to an increase in the temperature necessary for the evaporation of many chemical substances.

But what happens when the internal pressure in the coffee bean becomes too high? First of all, it must be remembered that during roasting, water evaporates from the coffee bean. When its content falls below 2 – 1.5%, it causes a change in the bean’s consistency from plastic and viscous to brittle and easily breakable⁴.

The increase in internal pressure caused by gases and the increase in bean brittleness lead to the formation of internal and external cracks in the bean and to the famous phenomenon of “cracking.”

Coffee Roasting: Alkaloids

It’s not simple to briefly define an alkaloid. Alkaloids can be defined as cyclic molecules, meaning closed on themselves, containing at least one nitrogen atom and of plant origin.

They are highly reactive substances and have been the subject of pharmacological study since the 1940s. Some alkaloids like those belonging to the taxane class, have been used in chemotherapy due to their excellent antitumor properties; their chemical structure has represented the starting point for many researchers in the development of other antitumor drugs like docetaxel.⁵

Caffeine and trigonelline belong to the alkaloid family and are part of those non-volatile chemical compounds contained in a coffee bean.

Caffeine (1,3,7-trimethylxanthine) was isolated from coffee beans by chemist Friedlieb Ferdinand Runge in 1819. Most studies concern its physiological effects on humans that influence various systems: urinary, nervous, circulatory, etc. Regarding its role within the bean, it is hypothesized to act as a deterrent for parasites and pathogens.

Its concentration within a green bean can vary between 0.9 and 2.5%. Although coffee roasting causes a total reduction in caffeine content of up to 30%, in darker roasted coffees a higher caffeine content is often detectable⁴⁴.

This happens because during the coffee roasting process, in addition to caffeine, the beans lose weight due to the evaporation of water and other compounds. For this reason, the percentage of caffeine appears increased in dark roasted coffees compared to light roasted coffees even though in reality the total level has decreased.

Data has shown that roasting temperature, rather than time, has a greater influence on the final caffeine content⁴⁴.

Another alkaloid present in coffee is trigonelline (about 0.6%), derived from the methylation of nicotinic acid. Several studies show that this alkaloid has antitumor properties, while there are no studies clarifying its physiological function within the bean.

It plays an important role in the roasting process as it leads to the formation of pyridines, acids, and pyrroles. In fact, a variable amount of trigonelline, between 50–80%, is degraded during roasting (the degraded amount depends on the roast level).⁷

Macromolecules: Proteins and Polysaccharides

Green coffee has a protein content that can vary between 10% and 15%, of which roughly 1% consists of free amino acids that disappear during roasting, where they are precursors for volatile compounds such as furans, pyridines, pyrazines, and pyrroles.⁸

A large portion of the dry weight of a coffee bean, however, is due to polysaccharides (around 50%), mainly cellulose and arabinogalactans, which make up 25% of a bean’s cell wall.⁹

Due to thermal degradation, during roasting there can be a loss of carbohydrates between 10% and 40%, where polysaccharides release monosaccharides that then take part in Maillard reactions with the bean’s proteins.¹⁰

In addition to polysaccharides, a substantial portion of the carbohydrates in coffee beans is sucrose, but very little remains after roasting. It undergoes a degradation process, called hydrolysis, which converts it into glucose and fructose.

Glucose and fructose can be converted into aliphatic acids or take part in Maillard reactions with proteins and amino acids. By the end of a “dark” roast, 96–98% of sucrose is degraded.

The acid content of coffee increases with darker roasting because more sucrose is degraded.¹¹

The formation of lactic, formic, and acetic acid is described by Lobry-de Bruyn-van Eckenstein, who explain how glucose and fructose, resulting from sucrose hydrolysis, lead to molecules (1,2-enediol and 2,3-enediol) that then evolve into acids. The acids formed during roasting, however, do not have a significant effect on the coffee’s final pH.

This effect would seem to contradict what has been said so far (higher acid concentration = lower pH = more acidic coffee). In reality, the salts present in coffee buffer this effect. However, the coffee’s final taste will depend greatly on the quantity and type of acids present in the finished beverage. ¹²

Coffee Roasting: Humic Acids and Melanoidins

Humic acids are natural chemical compounds formed by the microbial action of bacteria on organic matter (plant or animal) or by complex reactions between amino acids and carbohydrates.

We are not speaking of a single acid but rather a complex mixture of acids whose composition changes depending on the organic matter from which it is generated.¹³

Melanoidins, on the other hand, are complex dark-colored substances and are responsible for the brown color of roasted coffee. They are generated during roasting and belong to that category of molecules that boast antioxidant power both on the food and on the body’s cells that absorb them.¹⁴

Carboxylic and Chlorogenic Acids

Carboxylic acids are a class of molecules easily identifiable by the presence of a COOH sequence of atoms.

Carboxylic acids make up part of the non-volatile component of coffee beans after roasting, generated by the complex reactions that occur during roasting.

It is not easy to state precisely which class of substances they originate from (proteins, lipids, or polysaccharides), since a carboxylic acid can be released quite readily—thanks to the high roasting temperatures—from any macromolecule present in a coffee bean.

However, it has been shown that most of the acids released during roasting originate from sucrose. The most abundant acids found at the end of the roasting process are lactic, formic, acetic, citric, and malic acids, all water-soluble and therefore influential on the coffee’s final taste.

Carboxylic acids can form hydrogen bonds that cause the molecules to remain linked together in a sort of network, preventing them from evaporating. This, in addition to pore structure, explains why these substances remain trapped despite the high temperatures reached during roasting.¹⁵

Phenolic Compounds

Among the compounds that play a key role in coffee are phenolic compounds. These include chlorogenic acids and polyphenols, such as tannins.

Chlorogenic acids are derivatives of quinic acid whose concentration in the bean increases during growth and then drops sharply upon maturation. They amount to about 10% of the green Arabica bean weight and 4% for Robusta.¹⁶

During roasting the concentration of chlorogenic acids decreases and they degrade into melanoidins and other low-molecular-weight compounds, such as certain phenolic acids, but a significant amount remains trapped within the bean itself. They appear to have good antioxidant properties and thus play an important protective role against free radicals, which are responsible for aging.

The term chlorogenic acid is often used, but it actually refers only to the product obtained by combining caffeic acid with quinic acid.¹⁷

Polyphenols are a widely studied class of molecules thanks to their antioxidant properties and are reported to have positive effects both at the cardiovascular level and against tumor growth.¹⁸

Tannins (such as tannic acid) are water-soluble polyphenolic compounds that can be divided into two categories: condensed tannins (also called flavanols) and hydrolyzable tannins.

Unlike condensed tannins, hydrolyzable tannins are easily degraded by acids and enzymes, leading to the formation of sugars and phenolic acids such as gallic acid.¹⁹

The content of tannins in the final roasted coffee depends essentially, as with trigonelline and other substances, on the temperature reached during roasting. Concentrations range from about 51.60 milligrams to 3.10 mg per 10 grams of coffee, moving from a green coffee to a “dark” roast coffee.²⁰

Coffee Roasting and Lipids

Another important component of coffee beans is lipids. The lipid fraction consists mainly of triacylglycerols (about 75%), fatty acids (1%), sterols, tocopherols, and diterpenes.²¹

Of particular interest are cafestol and kahweol, since they have been shown to have a protective effect against the onset of certain types of tumors in humans, although their function within the bean is still unknown.²² The degradation of these two molecules during roasting is still under study.

16-O-methylcafestol is another substance of interest because, besides not being produced by Arabica beans but only by the Robusta variety, it does not degrade during roasting, which makes it useful for detecting: “How much Robusta is in Arabica blends?”.²³

The fatty acid fraction consists mainly of unsaturated fatty acids (oleic, linoleic, and linolenic acids), which are important for preserving coffee freshness.²⁴

Minerals

Potassium accounts for 40% of all minerals present in a green coffee bean (about 1–2 grams per 100 grams of green coffee), while the remaining portion consists mainly of magnesium, calcium, manganese, iron, and copper.

The mineral composition of a bean, however, depends greatly on the soil in which the plant producing the fruit is grown. There are few studies on their involvement in the reactions that occur during roasting, but it is hypothesized that they may act as catalysts, i.e., they increase the rate of a reaction.²⁵

Volatile Component after Roasting

The volatile component of a coffee bean at the end of roasting is even more complex than the non-volatile part. From the plant to the bean, several hundred volatile chemical substances have been identified in coffee, many with still-unknown functions, yet fully representing the complexity of plant chemistry.

A green coffee bean contains roughly 300 volatile compounds, many of which are degraded during roasting; others remain stable; others are transformed. Roasting itself is the key step for a good final coffee.

The roasting process increases the concentration of volatile compounds, reaching nearly a thousand species present in a bean and, through their combination, giving rise to coffee’s characteristic aroma. Volatile chemicals can be classified into:²⁶

• Thiols, thiophenes, and thiazoles;
• Pyrazines, pyridines, and pyrroles;
• Furans;
• Aldehydes and ketones;
• Phenols.

Gas chromatographic olfactometric analyses, such as AEDA (aroma dilution extraction analysis), have enabled the identification of certain key molecules responsible for coffee aroma, arising from reactions that occur during roasting.

Among these are: 3-mercapto-3-methylbutyl formate, 2-furfurylthiol, β-damascenone, guaiacol, 4-vinylguaiacol, 4-ethylguaiacol, and 5-ethyl-3-hydroxy-4-methyl-2(5H)-furanone. Some of coffee’s main positive notes (citrusy and fruity) are due to a class of compounds called monoterpenes (linalool, limonene, geraniol, etc.), which are released during roasting from polysaccharide chains.²⁷

During roasting, carbon dioxide develops—a volatile but odorless component. Quantitatively, it is the most abundant volatile component and is produced by pyrolysis reactions and by the Strecker degradation reaction (so named because the German chemist Adolph Strecker was the first to discover it in 1862).

The amount of carbon dioxide that develops certainly depends on the degree of roast and can exceed 10 milliliters per gram of roasted coffee. It has been shown that 45% of the carbon dioxide formed is released from the roasted bean within the first 5 minutes after roasting. ²⁹

Reactions During the Roasting Process

The reactions involved during roasting are extremely complex because they involve a large number of substances. Nevertheless, they are essential to coffee aroma. Among them are:³⁰

• Maillard reaction: reaction between nitrogen-containing compounds (proteins, peptides, and amino acids) and carbohydrates on one side, and acidic compounds and phenols on the other, to form aminoaldoses and aminoketones through condensation;
• Strecker degradation: reaction between an amino acid and a carbonyl compound to form aminoketones that condense to form aromatic heterocyclic compounds or react with formaldehyde to yield oxazolones;
• Formation of sulfur-containing compounds starting from amino acids and intermediates of the Maillard reaction;
• Trigonelline degradation with formation of pyridines, pyrazines, and pyrroles;
• Chlorogenic acid degradation with formation of phenols;
• Pigment degradation, especially carotenoids;
• Lipid degradation, especially diterpenes.

As noted, roasting is a crucial process for a coffee producer because it allows the chemical composition of a green coffee bean to be modified, enhancing its aroma and taste.

During roasting, the temperature inside the coffee bean exceeds 180 degrees Celsius. At this temperature, several reactions occur, including the Maillard reaction, which generates melanoidins—and more—that make up 29% of the weight of a roasted coffee bean.³¹

The reaction is named after the French chemist Louis Camille Maillard, who was among the first to study it. When we speak of the Maillard reaction, we do not refer to a single reaction but to a set of very complex reactions that occur between sugars and proteins.

For these reactions to occur, high temperatures are needed, typically reached when food is cooked. After cooking, the food appears brownish and reaches our nose with an aroma reminiscent of freshly baked bread.³²

All this indicates that the Maillard reaction has occurred. Reactant concentration, acidity, temperature, and cooking time are parameters that influence the reaction. Schematically, the reaction can be divided into three stages: ³³

• Reaction between a sugar and an amino acid found at the end of a protein chain. The reaction leads to the formation of a molecule belonging to the glycosamine family. These molecules are highly unstable and, through a series of reactions called isomerization, evolve into more stable compounds called ketosamines or Amadori compounds (named after the Italian chemist Mario Amadori, among the first to study them). This key step in the Maillard reaction is strongly influenced by acidity and does not produce odorant or colored molecules but only ketosamines, intermediate compounds that will subsequently transform in the second phase of the reaction.

• The second phase of the reaction is even more complex than the first, as multiple simultaneous reactions can occur, leading to a set of new substances. First, the ketosamines formed in the first phase can be destroyed through dehydration (a reaction followed by the loss of water) or cleavage, leading to the formation of carboxylic acids and aldehydes such as glyceraldehyde and pyruvic aldehyde. In addition to destruction, Amadori compounds can transform into new compounds called alpha-dicarbonyls. The alpha-dicarbonyl compounds can, in turn, cyclize, cleave, or react with free amino acids forming carbon dioxide (the Strecker degradation). From this second, complex phase, molecules responsible for roasted coffee’s aroma are obtained, such as furans, pyrazines, pyridines, pyrroles, etc. ³⁴

• The third phase consists in the formation of substances that will give the classic dark brown color to the roasted coffee bean, namely brown proteins and melanoidins. Melanoidins are formed via polymerization reactions between furans and/or pyrroles but also via polycondensations between aldehydes and ketones. ³⁵ This last phase, however, is very delicate because if the temperature of the roasting process is too high, acrylamide can be generated.³⁶

Coffee Roasting and Acrylamide

Acrylamide is a low molecular weight compound, highly soluble in water, that is formed from the reaction of the amino acid asparagine with sugars, normally present in some foods, starting from temperatures above 120 degrees Celsius. ³⁷

Why is it therefore easier to find higher quantities of acrylamide in light roasted coffee compared to dark roasted coffee? It should be known that, although acrylamide is formed at high temperatures, at higher temperatures it is largely removed; the higher the temperature reached in the bean, the less acrylamide we will find at the end of the process.

Studies show how acrylamide can increase the risk of developing cancer and, being present in many foods for daily use, this concern affects all consumers regardless of age group.

REGULATION (EU) 2017/2158 of the Commission of November 20, 2017, establishes mitigation measures and reference levels for the reduction of acrylamide presence in foods.

The control of acrylamide levels in the coffee market starts from the cultivation of coffee plants up to the final product. The reference level of acrylamide, that is the maximum possible quantity, is 400 μg per kilogram of roasted coffee and 850 μg per kilogram of instant coffee. ³⁸

It is not uncommon to find intermediate products in the roasted coffee bean, that is, products that have not completely degraded during the reaction, such as 5-hydroxymethylfurfural often called more simply by the abbreviation HMF. ³⁹

This is generated during roasting from the hydrolysis reaction of fructose that originates from sucrose (a reaction that is also currently used to produce it on an industrial scale) or from glucose. ⁴⁰

HMF has recently been recognized as a very versatile molecule for the production of various chemical products (fungicidal agents, pharmaceuticals and plastics). Currently there are many scientific studies regarding this compound as it could be used to replace some substances that are currently derived from petroleum. ⁴¹

If for a chemist HMF represents an interesting molecule, for a coffee consumer it would not be good to find it in high concentrations in their cup. Numerous studies demonstrate how HMF is a cytotoxic molecule, irritating to the eyes, skin and respiratory mucous membranes.

Recent studies conducted on rats demonstrate how it is also capable of promoting the onset of tumors in the gastrointestinal tract. ⁴² Therefore its levels in the roasted coffee bean must be periodically controlled even though the high temperatures of the process provide for the degradation of the molecule into aldehydes and ketones.

Consequently there is no real danger of finding significant quantities of HMF in the final coffee that could compromise our daily desire for this beverage. ⁴³

Do you want to have more information about coffee roasting through our courses or our products? Contact us!

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