Evaluation of Components Contributing to the International Bitterness Unit of Wort and Beer

Abstract

This study evaluated the components that contribute to the International Bitterness Unit (IBU) of wort and beer. A range of samples were analyzed in this study, including beer with 47.5 IBU containing 39.9 ppm iso-α-acid, and one with 49.9 IBU containing only 2.2 ppm iso-alpha acid. The IBU value of wort and beer is mainly contributed by iso-α-acid, non-isomerized α-acid, oxidative polar compounds, and malt-derived polar components. The isomerization of α-acid to iso-α-acid is initiated between 60 °C and 70 °C and the isomerization ratios accelerate rapidly at higher temperature. In the wort boiled below 60 °C, over 50% of the IBU value was contributed by non-isomerized α-acid. Most of the non-isomerized α-acid disappeared during fermentation, whereas the concentration of iso-α-acid was stable. The disappearance of α-acid resulted in a large decrease in the IBU value during fermentation, and the isomerization ratio influenced the degree of the decrease. Over 80% of the IBU value was contributed by the oxidative polar compounds in the beer hopped with the pellet stored at 40 °C for 90 days. Bitterness intensity was less in the beers with deteriorated hops than in those with non-deteriorated hops, despite both having the same IBU values.