Metal Chelation Behavior of Hop Acids in Buffered Model Systems

Abstract

As part of the importance of metal ions for the brewing process and continuing interest in the hop acids? interactions with metal ions, the complexation behavior of hop acids towards different metal ions was investigated. Hop α-acids were shown to be capable of forming complexes with Cu2+ and Fe2+ ions and showed no complexation of Ca2+, K+, Mg2+, Mn2+, or Zn2+ as tested using UV-VIS spectroscopy in solutions at an ionic strength of 0.1 mol dm-3 and containing 10 % (v/v) ethanol. In separate trials, Fe2+ was found to be capable of binding 3 mol α-acid at pH 4.3 and pH 5.5, and 2 mol α-acid at pH 6.2 and pH 8.2 while α-acids, in turn, were only able to bind one mol Fe2+. Cu2+, Fe2+ and Fe3+ ions could be removed from solutions by complexation with hop α-acids, iso-α-acids, and β-acids and subsequent filtration using 0.45 ?m cellulose membrane filters, while Ca2+, Mg2+, Mn2+, or Zn2+ ions were not affected and remained in the solutions. The hop acids? capability to remove Cu2+ and Fe3+ was higher at higher pH values and was in the order α-acids > β-acids > iso-α-acids with the exception of Fe3+ and pH 4.3 where the iso-α-acids? effect was more pronounced than the β-acids?. For Fe2+, the order was α-acids > iso-α-acids > β-acids. A mixture of all hop acids had the highest effect. The hop acids? complexation characteristic can be considered beneficial for wort or beer quality as ?unwanted? metal ions are affected while vital metal ions are not, which e.g. favors beer flavor stability.