Analytical determination of antioxidant capacity of hop-derived compounds in beer using specific rapid assays (ORAC, FRAP) and ESR-spectroscopy

There is a relationship between antioxidant activity and ageing stability of beer. The high-throughput antioxidant capacity assays ORAC and FRAP, which rely on specific reaction mechanisms, are validated as an addition to the established ESR-ST method with a focus on hop-derived antioxidants. Beers were brewed with systematic variations in hop variety, hop product, and the hopping regime (late hopping) to achieve sample beers varying widely in the concentrations of hop-derived antioxidants (α- and iso-α-acids, phenolic compounds). A significant positive correlation between phenolic compounds and the ORAC- and FRAP-values (p < 0.01) was found. The effect of individual resinous substances and free phenolic acids and flavonoids on the antioxidant capacity of beer (ORAC, FRAP, and ESR-ST) was proven in spiking trials. We found a correlation between the occurrence of the o-di-OH-group and the ORAC- and FRAP-values of phenolic compounds. Phenolic compounds did not react as anti- or prooxidants in the radical generation (ESR-ST). Higher concentrations of unisomerized α-acids significantly reduced ESR-signal intensity but had no significant effect on ORAC- or FRAP-values. Beers brewed with late hop addition, which yielded higher concentrations of unisomerized α-acids and phenolic compounds, had higher ORAC, FRAP-values and a reduced ESR-signal intensity. These three methods rely on different reaction principles, and therefore, different groups of hop-derived compounds act as antioxidants in these assays. A combination of the two high-throughput methods (ORAC, FRAP) and ESR-ST is advantageous for the evaluation of the antioxidant capacity of beers varying in hop-derived compounds.