Phenolics: A Comparison of Diverse Analytical Methods

Abstract

A variety of phenolic assays were used to evaluate the phenolic content of Californian red and white wines. Conventional methods (Folin-Ciocalteu, RP-HPLC, NP-HPLC, and dimethylaminocinnamaldehyde [DAC] assays) and relatively new methods (tannin and polymeric pigment assay, cyclic voltammetry, and antioxidant assay) were compared. The total phenol contents measured using the Folin-Ciocalteu, RP-HPLC, and cyclic voltammetry methods were strongly correlated (p < 0.001, r > 0.90). Considering assays measuring the monomeric phenols, the flavan-3-ol content measured using the DAC, RP-HPLC, and NP-HPLC methods were correlated (p < 0.001). Anthocyanin content (RP-HPLC) and colored monomer content (NP-HPLC) were also correlated (r = 0.99, p < 0.001). Tannin and polymeric pigment assays were correlated (r > 0.9, p < 0.001) to the high molecular-weight polymer (HMWP) and total polymer content using the NP-HPLC method, but not to the LMWP. The small polymeric pigment (SPP) and large polymeric pigment (LPP) measured using the tannin and polymeric pigment assay were both correlated (r = 0.79, p < 0.001) to the high molecular-weight colored polymers (HMWCP) (NP-HPLC), but neither with the LMWCP. As the sum of SPP and LPP correlated even better with HMWCP (r = 0.89, p < 0.001), it seems that both these are included in the HMWCP measure. The antioxidant activity of wines correlated (p < 0.001) with the total phenol content (using all assays, r > 0.90). The identity of the substances quantified in the pigmented tannin assays need clarification.