Assay-Dependent Variability of Antioxidant Responses in Hop Extracts: Implications for Cross-Study Comparability and Antioxidant Interpretation

Antioxidant activity of plant extracts is commonly interpreted as a directly comparable property despite substantial methodological differences among analytical assays and extraction systems. This study investigated how extraction selectivity and assay chemistry influence antioxidant-associated responses in hop (Humulus lupulus L.) extracts by integrating experimental and literature-derived datasets. Extracts obtained using different solvents and extraction techniques were evaluated using ABTS, DPPH, and Folin–Ciocalteu (TPC) systems. Multivariate statistical analyses, including principal component analysis (PCA), correlation analysis, and non-parametric comparisons, were applied to normalized datasets to assess assay-dependent variability and cross-study comparability. The results suggested substantial divergence between ABTS- and DPPH-associated responses, including a statistically significant negative correlation between both assay systems. PCA indicated assay-selective separation patterns, while TPC values did not consistently correlate with antioxidant-associated responses. Different extraction conditions were associated with distinct antioxidant response profiles, suggesting selective redistribution of analytically detectable antioxidant fractions rather than uniform changes in antioxidant capacity. Based on these observations, this study proposes the Assay–Extraction Interaction Framework (AEIF), an interpretative framework that views antioxidant activity as a context-dependent analytical response rather than a universal intrinsic property of the extract.