Potential impact of medium characteristics on the isomerisation of hop α-acids in wort and buffer model systems

Many brewing variables, e.g., pH, boiling time, temperature, and the presence of metal catalysts, are essential in view of the isomerisation of hop α-acids into iso-α-acids. In this work, a comparative study on the impact of several selected factors on α-acids isomerisation was performed in both buffer model solutions and wort, in order to find out enhanced boiling conditions for higher isomerisation yields.Increase of the pH of the reaction mixture has a positive effect on the isomerisation yield, in particular in wort, whereas wort gravity only becomes a limiting factor for α-acids conversion at high extract levels. Of the metal catalysts tested, in particular Mg2+, Al3+, and especially Fe3+ showed to have a pronounced positive impact on iso-α-acids formation in wort. However, due to precipitation phenomena, this effect was not observed for Al3+ and Fe3+ in the aqueous buffer model system. Neither the presence of glucose or polyphenols, nor ionic strength seem to exert any influence on α-acids conversion.