The poor precision of the Adams-Harbertson wine tannin assay which was proposed for commercial winemaking, thereby creating the real possibility of quality control problems, is documented. The method is a version of the Hagerman and Butler protein precipitation-based tannin method. An extensive invalidation of the assay results with luxury wine data shows that the assay cannot distinguish bottled wine with reasonable accuracy. Five laboratories used Adams-Harbertson to assay 9 replicates each, of 3 bottled wines (n 135) found in California supermarkets, with tannin concentrations of nominally 500 and 1000 ppm by high-performance liquid chromatography (HPLC). Reliability exceeded the 5 industry requirement by nominally 5 times (z-score based on 5 distribution). Coefficient of variation was 27, making the standard deviation range 54 for Pinot Noir, 34 for Merlot, and 44 for Cabernet Sauvignon. Validity exceeded the 100 requirement. Intralaboratory validity recovery was 5563. Interwinery validity was 71178 of the mean for Pinot Noir, 81144 for Merlot, and 83164 for Cabernet Sauvignon. Range as a function of the mean was 89 for Pinot Noir, 55 for Merlot, and 67 for Cabernet Sauvignon. Expect intermethod validity to be nominally 50, i.e., percent recovery to HPLC. These statistically significant errors were predicted by the literature. First-order error is related to the tannin-protein equilibrium constant (Ka), as suggested by the original author, Hagerman, and the protein equivalence point error as suggested by Silber. This does not obviate second-order errors for tannin-protein analytical chemistry. Winemakers using the measurements risk making wines that are relatively more tannic than the measurements report.
The poor interoperability of anthocyanin glycosides measurements by two pH differential methods is documented. Adams-Harbertson, which was proposed for commercial winemaking, was compared to AOAC Official Method 2005.02 for wine. California bottled wines (Pinot Noir, Merlot, and Cabernet Sauvignon) were assayed in a collaborative study (n=105), which found mean precision of Adams-Harbertson winery versus reference measurements to be 77 +/- 20%. Maximum error is expected to be 48% for Pinot Noir, 42% for Merlot, and 34% for Cabernet Sauvignon from reproducibility RSD. Range of measurements was actually 30 to 91% for Pinot Noir. An interoperability study (n=30) found Adams-Harbertson produces measurements that are nominally 150% of the AOAC pH differential method. Large analytical chemistry differences are: AOAC method uses Beer-Lambert equation and measures absorbance at pH 1.0 and 4.5, proposed a priori by Flueki and Francis; whereas Adams-Harbertson uses "universal" standard curve and measures absorbance ad hoc at pH 1.8 and 4.9 to reduce the effects of so-called co-pigmentation. Errors relative to AOAC are produced by Adams-Harbertson standard curve over Beer-Lambert and pH 1.8 over pH 1.0. The study recommends using AOAC Official Method 2005.02 for analysis of wine anthocyanin glycosides.
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