Toxic compounds formed when coffee beans are roasted
What it found
Roasting coffee beans at 180 to 240 °C can create several unwanted substances, including acrylamide, furan, polycyclic aromatic hydrocarbons, 4-methylimidazole and chloropropanols.
Levels vary with the bean, the roast and the brewing method.
What they found
Animal and lab studies
Animal and lab findings
Mice given 170 mg of 4-MEI per kg of body weight developed lung tumours. In lab tests on human cells, glyoxal, methylglyoxal and diacetyl were toxic by blocking enzymes needed for gene regulation.
Other findings
Acrylamide in coffee
Acrylamide forms mainly from asparagine and reducing sugars during roasting, peaking at 160 to 180 °C. Coffee is among the foods with the highest acrylamide levels, averaging 522 μg/kg, and lighter roasts contain more than dark roasts.
Furan levels
Furan in coffee can range on average from 2300 to 5375 μg/kg as the roast goes from lighter to darker. Even though it is volatile, all brewing methods leave some furan in the final drink, with automated machines giving higher levels than household equipment.
PAHs and roast time
Polycyclic aromatic hydrocarbons form from organic matter during high-temperature roasting. Longer roasting times raise the levels of the heavier PAHs in roasted beans, so choosing a light roast can help lower them.
4-MEI and roast
4-methylimidazole levels are higher in Robusta than Arabica coffee and rise with higher roasting temperatures and longer times. Decaffeinated and soluble coffees did not differ significantly in their 4-MEI levels.
3-MCPD in coffee
3-monochloropropane-1,2-diol forms during roasting from fats and chlorine already present in the green bean. Levels are relatively low, highest in decaffeinated and long-roasted coffee, and it was not detected in the brewed drink because of dilution.
What the authors conclude
“It is important that the levels of these contaminants be continuously monitored in order to allow the assessment of the real risks that they pose to human health.”
Also in their conclusions
- They say that knowing the precursors of these contaminants and how they form is important for developing effective ways to reduce them.
- They note that many challenges remain, including finding ways to lower several contaminants at once, since one strategy can reduce one group while increasing another.
- They advise that most studies were done only on a laboratory scale and need testing on an industrial scale, and that contaminant levels should be monitored continuously to assess the real risks to human health.
How it was done
Coffee is drunk worldwide, and the high heat of roasting can form substances that may pose a long-term risk to consumers. Knowing how these compounds form and how to reduce them could help make coffee safer.
The authors gathered and reviewed 182 articles published between 1985 and 2022 on contaminants formed during coffee roasting and on ways to lower them. The articles came from six research databases and were written in English, Portuguese or Spanish.
What it can’t tell you
- This review pulls together other studies and does not run its own experiments, so it cannot show cause and effect.
- Most of the mitigation strategies described were tested only on a laboratory scale, so their results may not carry over to industrial production.
- The effects of these strategies on the taste, aroma and other sensory qualities of coffee are not well known.
Who paid
- Conflicts
- The authors declare no conflict of interest.
- Authors work at
- Cidade Universitária, Brazil; Instituto Nacional de Saúde Doutor Ricardo Jorge, Portugal
The paper
- Title
- Thermal Contaminants in Coffee Induced by Roasting: A Review
- Type
- ReviewThe authors read earlier studies and describe what they found. It isn’t a new study.
- Evidence
- Animals and lab studies
- Summarised from
- Full text
- Cite
- da Costa DS, Albuquerque TG, Costa HS, Bragotto APA (2023). Thermal Contaminants in Coffee Induced by Roasting: A Review. International journal of environmental research and public health. doi:10.3390/ijerph20085586Free full textPubMed 37107868DOI
Summary written 26 Sep 2026. Check it against the paper before it changes what you eat. How we summarise papers · Report an error