Low molecular weight ligands govern Cu(II) binding in beer.

Copper influences beer flavor stability by catalyzing oxidation reactions that can degrade aroma-active molecules in beer, including hop-derived polyfunctional thiols. We examined copper oxidation states, binding environments, and availability in finished beer using electron paramagnetic resonance spectroscopy and ion-selective electrode titrations. Approximately 36% of copper was present as Cu(II) (1.14μM), mostly bound by endogenous components, leaving trace free Cu(II) (<0.1μM). Fractionation and ligand-binding studies identified L-alanine, L-tyrosine, and oxalic acid as likely contributors to the Cu(II) coordination environment in beer. Coordination by these ligands may limit the ability of Cu(II) ions to react with thiols, potentially contributing to the preservation of volatile sulfur compounds important to beer flavor. Our findings demonstrate that copper exists in beer as a mixture of oxidation states and coordination environments that shape redox activity and can potentially affect flavor stability.