Evaluation of Direct Phloroglucinolysis and Colorimetric Depolymerization Assays and Their Applicability for Determining Condensed Tannins in Grape Marc

Josh Hixson, Keren Bindon, Paul Smith

    Research output: Contribution to journalArticlepeer-review

    17 Citations (Scopus)

    Abstract

    To determine the optimum methods for determining condensed tannin (CT) content in grape marc, butanol-hydrochloric acid assays and phloroglucinolysis were adapted for use, applied to a range of grape marc types, and the methods compared. Porter's assay (butanol-HCl) was found to give unreliable results due to nonlinear color responses to grape skin and seed tannin concentrations, whereas the modification to include acetone (Grabber's assay) overcame this. Differences between skin and seed tannin responses highlighted the need to adequately select the correct grape tannin standard, and the formation of pH-dependent color was accounted for through acidification of blank samples. For phloroglucinolysis, the inability to remove highly bound tannins from cell wall material was highlighted, although a measure of tannins remaining post-phloroglucinolysis (Grabber's assay) showed a trend with the level of exposure to oxidative storage or processing conditions. The comparison of CT concentrations from phloroglucinolysis and Grabber's assay gave poor correlation coefficients.

    Original languageEnglish
    Pages (from-to)9954-9962
    Number of pages9
    JournalJournal of Agricultural and Food Chemistry
    Volume63
    Issue number45
    DOIs
    Publication statusPublished - 2 Nov 2015

    Keywords

    • butanol-HCl
    • condensed tannins
    • grape marc
    • phloroglucinolysis
    • proanthocyanidin

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