Advancing quantification methods for polyphenols in brown seaweeds—applying a selective qNMR method compared with the TPC assay

INTRODUCTION: Brown seaweeds are a sustainable biomass with a potential for various industrial applications. Polyphenols are an important contributor to this potential. OBJECTIVE: The aim was total quantification of polyphenols in brown seaweeds from different tidal zones, using a selective (1)H quantitative NMR (qNMR) method, comparing the results with the colorimetric Folin–Ciocalteu total phenolic content (TPC) assay. METHOD: qNMR was performed with integration of selected peaks in the aromatic region (7–5.5 ppm). Deselection of non‐polyphenolic (1)H signals was based on information from 2D ((1)H‐(13)C, (1)H‐(15)N) NMR spectra. (13)C NMR phlorotannin characterisation facilitated the average number of protons expected to be found per aromatic ring used for the (1)H quantification. RESULTS: Selective qNMR and the TPC assay showed similar results for the three sublittoral growing species from the Laminariaceae; lower amounts for Laminaria hyperborea and Laminaria digitata (qNMR: 0.4%–0.6%; TPC: 0.6%–0.8%, phloroglucinol equivalents (PGE), dry weight (DW)) and higher amounts for Saccharina latissima (qNMR: 1.2%; TPC: 1.5%, PGE, DW). For the eulittoral Fucaceae, Fucus vesiculosus (qNMR: 1.1%; TPC: 4.1%; PGE, DW) and Ascophyllum nodosum (qNMR: 0.9%; TPC: 2.0%; PGE, DW), the TPC results were found to be up to three times higher than the qNMR results. The (13)C NMR characterisation showed the highest phlorotannin polymerisation degree for F. vesiculosus . CONCLUSION: The TPC assay provided similar polyphenolic amounts to the selective qNMR method for sublittoral species. For eulittoral growing species, the TPC method showed amounts up to three times higher than the qNMR method—most likely illustrating the lack of selectivity in the TPC assay.