Somers assay

Figure 1. Parallel Determination of Tannins and Polymeric Pigments. The procedure shown in the rectangle (left) is a Somers assay performed at pH 4.9 and provides a measure of total pigments and total polymeric pigments (LPP + SPP) at that pH. The procedure shown in the polygon (right) is the Hagerman Butler assay for tannin. 2003 American Society for Enology and Viticulture.

Figure 2. Absence of a correlation between the background absorbance in the tannin assay and the amount of polymeric pigment in 12 wines as determined in the standard Somers assay at wine pH. The analysis was performed twice about eight months apart. 2003 American Society for Enology and Viticulture.

The wine was a Syrah obtained soon after pressing and we could attribute over 30 percent of the total color at pH 4.9 to polymeric pigments that did not precipitate with protein. When the wines shown in Figure 2 were re-examined and the amount of LPP and SPP were added together, the correlation with polymeric pigments by the Somers assay was found to be quite good (Figure 4). 

This is not surprising since the way in which total pigments and total polymeric pigments (LPP +SPP) are determined is nothing more than a Somers assay at pH 4.9 (Figure 1). From these results (Figures 3,4) we concluded that protein precipitation fractionates polymeric pigments into two classes, those that precipitate with protein and those that do not. 

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