To ensure high quality, safety, and desirable flavor in coffee, several volatile and nonvolatile chemical compounds, primarily associated with processing defects, contamination, or aging, should be minimized or kept out.
Volatile Compounds to Avoid
Undesirable volatile organic compounds (VOCs) generally result from poor post-harvest processing (e.g., over-fermentation), improper storage, or off-roasting conditions.
- Earthy/Musty/Green notes: Compounds such as 2-methoxy-3-isopropylpyrazine and 2-methoxy-3-isobutylpyrazine (associated with earthy, potato, or bell pepper notes), geosmin, and 2-methylisobutanol are markers of defective beans.
- Rancid/Fatty notes: Hexanal, (E)-2-nonenal, 2-methylbutanal, and various aldehydes/ketones can produce butter-rancid, fatty, or cheesy aromas when present in high concentrations, often a sign of oxidation or staling.
- Sulfurous/Onion notes: While some sulfur compounds are key to a good coffee aroma (e.g., 2-furfurylthiol), off-notes can be caused by compounds like methanethiol (rotten eggs) or dimethyl trisulfide (cabbage, onion), which indicate degradation.
- Phenolic/Burnt notes: High levels of compounds like guaiacol, 4-ethylguaiacol, or 4-vinylguaiacol can result in harsh, smoky, or clove-like phenolic off-flavors, often linked to dark roasting or poor bean quality.
- Solvent residues: In decaffeinated coffee, residues of chemical solvents like methylene chloride or ethyl acetate should be avoided, with the Swiss Water Process being a safer alternative.
Nonvolatile Compounds to Avoid
Undesirable nonvolatile compounds are often contaminants or natural compounds that are managed through proper agricultural and processing practices to ensure food safety.
- Mycotoxins (Mold Toxins): These are a major concern and include:
- Ochratoxin A (OTA): A potential carcinogen linked to kidney damage, produced by molds like Aspergillus and Penicillium during poor storage in humid conditions.
- Aflatoxin: A known liver carcinogen also produced by mold.
- Acrylamide: A natural byproduct of the Maillard reaction during high-temperature roasting, classified as a potential human carcinogen. Levels are lower in darker roasts and can be minimized through controlled roasting and processing.
- Heavy Metals: Conventional coffee may contain traces of metals like arsenic, cadmium, lead, and mercury through contaminated soil or processing equipment.
- Pesticides and Herbicides: Residues from chemicals like glyphosate (and its byproduct AMPA), chlorpyrifos, and malathion can remain in non-organic beans and are linked to various health problems.
- Polycyclic Aromatic Hydrocarbons (PAHs): These organic chemicals can form from incomplete combustion during the drying or roasting processes, with some like benzo[a]pyrene being carcinogenic.
To minimize exposure to these compounds, it is best to choose high-quality, specialty-grade coffee, ideally certified organic or “verified-clean” (tested for specific toxins and pesticides), which has been stored and roasted correctly.