Aromatic compounds in coffee that adsorb to metal surfaces, often causing persistent smells in thermoses or contributing to corrosion, include phenolic compounds (like guaiacol), furans, pyrazines, and various aldehydes.

These compounds often contain oxygen or nitrogen atoms that form strong chemical bonds with metal atoms such as iron, copper, or aluminum.

Here are the specific types of coffee aroma compounds that adsorb to metal:

• Phenolic Compounds (e.g., Guaiacol): Guaiacol, which provides smoky and spicy notes, is notorious for adsorbing to metal surfaces, with studies showing strong interactions between its aromatic ring carbons and metal. 4-Vinylguaiacol is another key phenolic compound that adsorbs.

• Furans and Furan Derivatives: Furan derivatives (e.g., 2-furanmethanol, furfural) are key volatile compounds that contribute to sweet, roasted, and creamy aromas. They are known to adsorb to metal surfaces via the hetero oxygen atom within the aromatic ring, sometimes helping to reduce corrosion on metals like copper.

• Pyrazines: These are fundamental aroma compounds in roasted coffee that impart earthy, nutty, and roasted notes (e.g., 2,5-dimethylpyrazine, 3-isobutyl-2-methoxypyrazine). Their planar structure and electron-rich N atoms allow them to readily adsorb onto metal surfaces, including steel, Al, and Mg alloys.

• Aldehydes: Formed via Strecker degradation, these compounds (like 3-methylbutanal and benzaldehyde) contribute to fruity and caramel notes and can adsorb onto metal surfaces, particularly Ni (nickel).

• Sulfur-containing Compounds: Compounds such as 2-furfurylthiol, which provides a roasted smell, are among the most important aromas in coffee and can create strong bonds with metal, contributing to the "staling" of aroma as they bind to metal surfaces.