title: Evaluation of direct phloroglucinolysis and colorimetric depolymerization assays and their applicability for determining condensed tannins in grape marc.
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.
authors.affiliation: The Australian Wine Research Institute, P.O. Box 197, Glen Osmond, Adelaide, South Australia 5064, Australia.
authors.forename: Josh L
authors.initials: JL
authors.lastname: Hixson
authors.affiliation: The Australian Wine Research Institute, P.O. Box 197, Glen Osmond, Adelaide, South Australia 5064, Australia.
authors.forename: Keren A
authors.initials: KA
authors.lastname: Bindon
authors.affiliation: The Australian Wine Research Institute, P.O. Box 197, Glen Osmond, Adelaide, South Australia 5064, Australia.|Flinders Centre for Marine Bioproducts Development (CMBD) and Department of Medical Biotechnology, School of Medicine, Flinders University , Bedford Park, South Australia 5042, Australia.
authors.forename: Paul A
authors.initials: PA
authors.lastname: Smith
mesh_terms: D002623:Chemistry Techniques, Analytical / Q000379:methods*; D010696:Phloroglucinol / Q000737:chemistry*; D010936:Plant Extracts / Q000737:chemistry*; D058105:Polymerization; D044945:Proanthocyanidins / Q000737:chemistry*; D012639:Seeds / Q000737:chemistry; D027843:Vitis / Q000737:chemistry*; D014866:Waste Products / Q000032:analysis*
chemical_list: D010936:Plant Extracts; D044945:Proanthocyanidins; D014866:Waste Products; D010696:Phloroglucinol
keywords: butanol-HCl; condensed tannins; grape marc; phloroglucinolysis; proanthocyanidin