Rapid quantification of proanthocyanidins (condensed tannins) with a continuous flow analyzer

Proanthocyanidins (condensed tannins) frequently need to be quantified in large numbers of samples in food, plant, and environmental studies. An automated colorimetric method to quantify proanthocyanidins with sulfuric acid (H2SO4) was therefore developed for use in a continuous flow analyzer. Assay conditions were optimized using 50% methanol extracts of paper birch, sugar maple, and quaking aspen leaves. Short extraction times and centrifugation of samples prevented proanthocyanidin degradation that otherwise occurred in 50% methanol extracts of aspen leaves. Extraction of birch and maple proanthocyanidins with 50% methanol was comparable to or better than that with 70% acetone. Proanthocyanidin levels in aspen were lower when extracted with aqueous methanol, but relative differences among samples were consistent with those found in aqueous acetone extracts. Results from the automated sulfuric acid assay were highly correlated with those of the conventional BuOH−HCl method for proanthocyanidins and, except for birch, with the Folin−Denis assay for total phenolics. This new technique significantly improves assay processing rate and repeatability compared to conventional colorimetric proanthocyanidin assays