Chemical changes during coffee roasting

Scientific knowledge - Written: Tấn Huy


5:44 16/09/2022

CFRR – The process of roasting coffee creates a chemical change in the compounds found in the green kernel and determines much of the flavor of the coffee.

Coffee is a bean of the coffee plant, which after harvesting, people can create a good cup of coffee. “Seed to cup” is a routine in that each stage has chemical compounds. They have the most pronounced transformation, so it is possible to recognize them by sense during roasting.

Chemical compounds present in green bean 

Compounds in green bean coffee. Photo: Andrade and coworkers (2022)


As the major components of free sugars and polysaccharides that account for more than 50-60% of the weight of green coffee beans (Trugo and coworkers, 1985), the importance of carbohydrates due to complex changes in the roasting process contributes to the flavors in coffee drinks. Carbohydrates develop during fruit growth. Free sugar is a precursor to the aromas and flavors of roasted coffee beans, including glucose and fructose, but the most important is sucrose (about 6-7%).

At the fruit ripening stage, an abundance of sucrose was found (Garrett and coworkers, 2016), which contributed to the formation of flavor molecules in roasted coffee. The amount of this sugar depends on the degree of ripening of the fruit and the cultivar in arabica beans, where the concentration will be higher than in robusta beans (Campa and coworkers, 2004; Clarke and coworkers, 2008). Sucrose is concentrated in the endosperm and evenly distributed, while glucose and fructose are located mainly in the silver shell (Redgwell and coworkers, 2006).

Polysaccharides include galactomannan, arabinogalactan-protein, cellulose, a small amount of pectic polysaccharide, and xyloglucan (Redgwell and coworkers, 2002; Oosterveld and coworkers, 2003).


The main alkaloids to mention are caffeine and trigonelline.

Caffeine accounts for 1-4%, depending on the type of coffee beans and plant variety (Belitz and coworkers, 2009; Mazzafera and coworkers, 2010). In arabica beans, caffeine is from 0.9-1.7%, while in robusta beans, the caffeine content is double as 1.8-4%. Caffeine contributes about 10% bitterness and is a stimulating property of the central nervous system, increases blood circulation, and is well known for its effects on increasing alertness Protein makes up about 13-16% by weight in green coffee beans with the form of a- and B- legumin (Bau and coworkers, 2001), enzymes, and amino acids. These amino acids react with free sugars participating in the Maillard chain reaction that occurs during roasting, contributing to the browning of the beans, aroma, and flavor of brewed coffee (Kitzberger and coworkers, 2016). Amino acids participate in Strecker decomposition in roasting to form aldehydes (Rizzi, 2008). 2006, Belitz and coworkers, 2009).

Structure of caffeine. Photo:

Caffeine also acts as a protectant for the plant against harmful diseases. The caffeine content in its pods is two to ten times lower than in beans (Koshiro and coworkers, 2006), the variation of which depends on the genotype of each type and the stage of fruit development.

Trigonellin (N-methylnicotinic acid C7H7NO2) up to 0.8% in green coffee beans, melting point is 218 degrees Celsius. Under the effect of heat, Trigonellin will be hydrolyzed 50% to form Nicotinic acid (precursor of vitamin PP has a cholesterol-reducing effect), pyridine, 3-methyl pyridine, methyl ester… In the chemical composition, there will be no nicotinic acid, it is only formed only in trigonelline hydrolysis (Lang and coworkers, 2008).

Chemical structure of trigonelline. Photo: dreamstime

Organic acids

Major acid in coffee includes citric, acetic, lactic, malic, and y-aminobutyric acids (GABA) (Kramer and coworkers, 2010). Other acids exist in lipid-soluble compounds called fatty acids, chlorogenic acids in phenolic compounds that contribute to sour and bitter flavors in coffee, and succinic, gluconic, and fumaric acids with small amounts (Bahre, 1996).

Chemical structure of 5-caffeoylquinic acid, 5-feruloylquinic acid and 3,4-dicaffeoylquinic acid. Photo: Sara and coworkers, 2021.
Chemical structure of chlorogenic acid. Photo:


The lipid-soluble content of arabica beans is about 15-17%, and robusta beans are about 7-10% (Farah, 2012). Triacylglycerols (TAGs) consist of linoleic, oleic, and linolenic acids about 75%, diterpenes including cafestol and kahweo about 20%, sterols 5.5%, free fatty acids 1%, phospholipids 0.5%, tocopherols 0.05% (Kolling and coworkers associates, 2005). The content of lipid-soluble compounds is lower than that of other compounds, so it is suggested that fat composition is less related to coffee quality (Caporaso and coworkers, 2021).

Proteins and amino acids 

Protein makes up about 13-16% by weight in green coffee beans with the form of a- and B- legumin (Bau and coworkers, 2001), enzymes, and amino acids. These amino acids react with free sugars participating in the Maillard chain reaction that occurs during roasting, contributing to the browning of the beans, aroma, and flavor of brewed coffee (Kitzberger and coworkers, 2016). Amino acids participate in Strecker decomposition in roasting to form aldehydes (Rizzi, 2008).

The content and composition of proteins in the two types of arabica and robusta were not significantly different overall (Folstar, 1985). However, there is still controversy about the protein ratio related to the organoleptic property, Franca and coworkers reported that higher protein content is associated with higher quality (Franca and coworkers, 2005, but there is an opposite report that lower protein intake resulted in higher. (Barbosa and coworkers, 2019)

The average free amino acid content of robusta is higher than that of arabica beans (Arnold and coworkers, 1994). That accords with the quality of arabica and robusta beans. When the content of free amino acids is higher, they inhibit some compounds, such as 2-furaldehyde and furanone. They affect the flavor and associate melanoidin formation with roasted coffee color. Excessively high levels of free amino acids can reduce the quality of brewed coffee (Poisson and coworkers, 2009), the same study found the presence of cysteine as one of two amino acids with sensation-related death.

Although free cysteine is not present in green coffee beans (Arnold et al., 1996), cyteine derivatives such as glutathione or the cysteine-rich protein are precursors of this flavor.

green coffee beans
Green coffee beans. Photo: Intercontinental Coffee Trading

Basic roasting process 

Physical change

 Roasting is the physical and chemical transformation from green coffee beans into roasted coffee beans. Roasters can experiment and select the right roasting profile for each type of coffee bean according to their desired purpose. According to John (2009), the coffee roasting process takes place with the following physical changes:

  • Change in color

When green coffee loses moisture, it will turn from green to light yellow, then to continue roasting, the beans will turn from light – yellow to light – brown to dark brown

  • The first crack

When the temperature reaches 193-204°C, the particle size will increase, and the moisture and the generated gases are released outside by the crack with the first explosion.

  • Caramelization

After the start of the first explosion, at this stage, the coffee beans will turn dark brown very quickly. It takes thorough observation and time to achieve the correct level of roasting.

  • The second crack

The second crack will be hard to identify than the first at this step, the bean’s color can change in such a fraction moment.

  • End of roasting

As reaching the desired roastery, the coffee bean is brought to cool quickly.

Chemical change 

Parallel to physical change is chemical reactions that form long aromas, sourness, sweetness, balance, and aftertaste. ACS – American Chemical Society has concluded the following:concluded on the transformation of chemical compounds during roasting as follows:

  • Maillard reaction

The reaction occurs at temperatures between 150-200°C, carbonyl groups and amino groups in proteins participate in the reaction to form flavor compounds, there are hundreds of coffee flavor compounds formed in this reaction.

  • Caramelization

The reaction occurs when the temperature reaches 170-200°C which stage, the sugar in coffee participates in the caramelization reaction, contributing to the release of aromatic compounds, and most of the sucrose converted into caramel compounds.

  • The first crack

If water inside the bean drained out, the particle size increased by about 50%, and the weight decreased by 5% due to dehydration.

  • Pyrolysis:

At about 22°C, the temperature changes the chemical substances in the beans, releasing CO2 gas. The weight is reduced by about 13%, and the color of the beans turns to medium brown.

  • Second Crack

The pyrolysis process continues when the roasting temperature reaches 225-230°C, and the cellulose in the bean cell wall is broken, forming a second crack. The beans turn dark brown and begin to appear with a glossy oil coating on the outside of the roasted coffee beans.

Roasting time – Roasting temperature

The roasting temperature and time will depend on the roaster, which is determined by the degree of roasting based on the actual color of the beans when viewed visually. The roaster will divide by color from green coffee beans, yellow beans, light brown, and dark brown to identify light, medium and dark roasting. (Herawati and coworkers, 2019).

Some compounds, such as phenolic, alkaloid, and terpenoid, were beneficial to health (Ding and coworkers, 2014; Ludwing et al., 2012). However, the phenolic and chlorogenic acid compounds degraded during roasting. (Herawati and coworkers, 2019).

The drying phase – endothermic reaction humidity of 10-12% decreases to about 2.5%. The mass of grain decreases significantly from the early stages of roasting from the golden transition, and the smell of bread begins to appear (Herawati and coworkers, 2019).

After the endothermic reaction takes place, the actual roasting phase begins to continue the chemically changed compounds dramatically, the pyrolysis of important flavor compounds. Starting around 190°C – 210°C several reactions such as lipid oxidation, maillard, and caramelization occur simultaneously in this process, and volatile compounds are also released (Kocadagli and coworkers, 2012). The final rapid cooling stage is intended to stop the ongoing reactions, limiting unwanted compounds.

The roasting levels that determined by correlation based on color, time, and temperature. Roasting levels that describe as light, medium, or dark. Where time is a factor that can affect the reactions in the beans, a long roasting time will produce a bitter taste and lack of aroma short roasting time with a high temperature doesn’t have enough time to complete the reaction, which leads to underdeveloped flavors. (Roberto and coworkers, 2003)

Roast Levels

According to Kim and coworkers (2018), roasting levels have divided into several steps based on the color change during roasting. The several levels of roasting based on the sound of the separation process when the coffee beans crack can be determined as follows:

  • Light (light roast) – the start of the first sound
  • Medium (medium roast) – the end of the first one.
  • Medium-dark – the start of the second sound
  • Dark roast – the end of the second one.
ArabicaAgtron Spectrophotometer Light-moderate light: 80.8
Moderate light-medium light: 71.8
Medium light-medium: 62.3
Medium-moderate dark: 48.5
Moderate dark-dark: 40.5
Dark-very dark: 32.3
Pires and coworkers, 2021
ArabicaReduced amount (%)Light: 14%
Medium: 15%
Dark: 19%
Rao and coworkers, 2020
RobustaAcidity (pH)Light: 5.27
Medium: 5.53
Dark: 5.66
Schenker and coworkers, 2017
RobustaAcidity (titration method)Light: 0.31%
Medium: 0.24%
Dark: 0.22%
Lee and coworkers, 2013
Reference table for roasting levels. Photo: CFRR

The weight loss reduction rate of coffee beans that light roasted is about 14%, medium roast about 15%, and dark roast 19% (Franca and coworkers, 2009; Hecimovic and coworkers, 2011). The roast level is assessed based on the pH meter and titration method. Light roast coffee will have the highest acidity, and dark roast will have the lowest rate. (Lee and coworkers, 2013).

The color change in coffee is also based on the agtron value, the intensity of coffee from light roast to dark roast can be reduced from 90.8 to 32.3 (Budiastra and coworkers, 2020). The regulation of temperature and color of roasted coffee beans is closely related to the aroma and flavor of brewed coffee.

Advances in measurement technology contribute to the determination of the desired roast level. The two accepted industry standards for the quality of coffee include scores of sensory attributes in brewed coffee and the agtron scale for classifying roasting levels.

According to SCA (2018), an agtron analyzer with the ability to use near-infrared energy outside the spectrum to evaluate changes to the quinon group of compounds. Since quinone variability is predictable and quantifiable directly related to aroma and flavor in brewed coffee, quinon becomes a target for spectroscopic measurements. The lower the score in the analyzer, the darker the roast. Agtron, 2004 analyzes the roast level and scores according to the SCA standards as follows:

Roast LevelsVery LightLightMedium lightMediumMedium darkDarkVery dark
Agtron “Gourmet”85756555453525
Agtron “commercial”64.356.949.44234.627.119.7
Roast level and score on the agtron scale. Source: Agtron, 2004

Agtron has two scales used in the coffee industry, gourmet agtron and commercial agtron. In which the gourmet scale is provided with higher resolution, it shows a more pronounced difference in the scale between the roast levels.

The change of precursors

Main reactions

Coffee roasting
Cool the coffee beans after roasting. Photo: Driven coffee

The study by Illy and coworkers, 1995; Reineccius, 1995; SemmeIroch and coworkers, 1995, presented complex reactions during roasting.

Maillard that occurs during roasting is a complex series of reactions including sugar breakdown, reactions between nitrogenous substances (proteins, peptides, amino acids, seroto-nine, trigonelline), and pyrolysis (Kocadagli and coworkers, 2012), to create aminoaldose and aminoketone by condensation.

  • The reaction between free sugars and amino acids produces a Schiff base, which produces low molecules in volatile and non-volatile compounds.
  • The reaction produces melanoidin that contributes to the yellow-brown color in coffee from the polymerization of the compound ketamine (Herawati and coworkers, 2019).
  • The maillard and pyrolysis reactions produce CO2 a gas that is released when the coffee bean cracks, most of which is still inside the bean and is released slowly after the end of the roasting process. The release of air when roasting increases the pressure of the coffee beans to more than 10 bar, and the volume of roasted coffee beans gradually expands (Schenker and coworkers, 2017).

A Strecker decomposition reaction follows that consists of a reaction between amino acids and dicarbonyls to form aminoketones, pyruvaldehyde, and diacetyl. These newly formed compounds further react with ammonia and hydrogen sulfide to develop pyrazine, pyridine, pyrrole, and furan compounds that contribute to the flavor of brewed coffee (Hustiany, 2013).

The degradation of CGA liberates caffeic acid, which enhances bitterness, and also forms lactones and phenol derivatives responsible for taste and aroma (Ginz and coworkers, 1995; Variyar and coworkers, 2003).

Formic, acetic, glycolic, and lactic acids increase during roasting, where sucrose acts as a precursor of these acids, so different amounts of sucrose will result in veriety amounts of final acid (Ginz and coworkers, 2000). The citric and malic acids in green coffee beans degrade to succinic, fumaric, maleic, and others (Balzer, 2008).

The decomposition reaction of sulfur amino acids such as cysteine and methionine are converted to mercaptan, which continues to react with intermediates of the maillard reaction.

The breakdown of hydroxy amino acids such as serine and threonine can react with sucrose to form alkyl pyrazine.

The breakdown of trigonelline to alkyl pyridines and pyrroles.

The breakdown of quinic acid radicals to form phenols.

Most of the carotenoid pigments will disappear.

Flavors are created from ingredients

DSC 8126
Coffee beans after roasting. Photo: House of coffee beans

Lipids are only partially involved in the roasting process to create coffee flavor. Volatile compounds related to the senses include aldehydes, ketones, etc. (Sunarharum and coworkers, 2014). The free amino acids had almost broken down during roasting. Trigonelline degraded partially during roasting and converted to nicotinic acid and volatile compounds such as pyridine (Viani and coworkers, 1974).

After the roasting process, substances change and no longer resemble the aroma and flavor compounds in the previously identified green coffee. The roasting process loses some water, pyrolysis volatiles, polysaccharides, sugars, amino acids, and chlorogenic acids are significantly reduced, organic acids and lipids are relatively increased, caffeine and trigonelline (N-methyl nicotinic acid) concentrations were almost constant (Roberto and coworkers, 2003).

Thus, the chemical components remaining after the roasting process are volatile and non-volatile, and the aroma components in coffee beans after roasting have formed basic taste sensations such as sourness, bitterness, and astringent tongue.

Research from Illy and coworkers, 1995; Clarke and coworkers, 1987; Illy and coworkers, 1997, determined the density of non-volatile substances in roasted coffee including:

  • Caffeine contributes to the strength, body, and bitterness of brewed coffee.
  • Trigonelline, nicotinic acid, and N-methyl nicotinamide provide values that define the coffee roaster flavor.
  • Proteins and peptides do not undergo the maillard reaction.
  • Polysaccharides: cellulose, hemicellulose, arabinogalactan, and pectins that hold volatiles
  • Humic acid and melanoidin are the end product of the maillard reaction, which gives roasted coffee its characteristic brown color.
  • Chlorogenic acid after pyrolysis forms cinnamic, caffeic, ferulic, ferulic, sinapic, and quinic acids that all have the effect of bitter and astringent taste.
  • Minerals: potassium (about 40%), manganese metal (10-50ppm), iron (15-40ppm), copper (2-5ppm).
  • Increased formic, acetic, glycolic, and lactic acids are responsible for the sour taste in coffee.

De Morais and coworkers (2008) analyzed three levels of light-medium-dark roasting for robusta coffee beans and analyzed the chemical compositions as follows:

  • The total phenol content decreased as the degree of roasting increased.
  • The concentration of proanthocyanidins increased as the level of roasting increased.
  • Chlorogenic acid decreases as the level of roasting increases.
  • The 5-caffeoylquinic acid (5-ACQ) was determined to be higher in light and medium roasts and lower in dark roasts compared to arabica beans.

Nascimento and coworkers (2007) analyzed the compounds of volatile and aroma found to be highest at the light roast.


The roasting process is considered the time to flavor the coffee beans according to the roaster’s wishes based on the existing fresh bean qualities. The chemical changes of compounds with temperature in this process are not exactly the same between different types of coffee. Therefore, roasters need to understand the general principles of chemical and physical changes in coffee beans during roasting. From there, conduct different tests with the beans you have to find the most reasonable roasting profile, in order to produce the desired coffee flavor.


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