Trace levels of copper can impact the flavor stability of beer. The main source of copper is malt, and the wort copper levels are established during mashing and lautering. This study focuses on sweet worts made from experimental roasted and caramel malts. Potentiometric titrations using ion-selective electrodes combined with electron paramagnetic resonance spectroscopy have been used to investigate Cu(II) binding in worts as well as the impact of Cu(II) ions on the wort oxidative stability. High-temperature treatment during malting decreased Cu(II) binding affinities in the worts, with roasted malt worts having lower affinities than caramel malt worts of similar color and pH. Electron paramagnetic resonance spectra indicated dipeptides as the main Cu(II) chelators. A positive correlation between Cu and free amino nitrogen levels in worts is demonstrated. In dark worts with high rates of radical formation, Cu(II) had pronounced antioxidative effects. In contrast, moderate prooxidative effects were observed when adding Cu(II) to pale worts with inherently low rates of oxidation.
Keywords: beer; caramel malt; copper; electron spin resonance spectroscopy; ion selective electrodes; oxidation; roasted malt.