Anthocyanin bleaching reaction with bisulfite

Fig. 9A.2 Anthocyanin equilibrium forms and bleaching reaction with bisulfite...

The acetate buffer is set at pH 4.9 because 4.9 is the pi of BSA and thus affords maximum precipitation of the tannin/protein complexes formed during the precipitation reaction. Coincidentally pH 4.9 is particularly good for measuring absorbance due to polymeric pigments because the anthocyanins have their minimum absorbance at that pH (d). Since any remaining monomeric anthocyanins are bleached with bisulfite, this procedure assures that all of the remaining 520 nm absorbance is due to polymeric pigments (LPP + SPP). 

The procedure in the polygon in Figure 1 is the Hagerman Butler assay for tannin. The only modification is that we retain the supernatant after centrifugation of the precipitation reaction mixture and bleach the monomeric anthocyanins with bisulfite. The residual absorbance represents pigments that do not precipitate with protein and do not bleach in the presence of bisulfite. We have designated this material small polymeric pigment solely to denote these two characteristics. The LPP removed from the sample by protein precipitation... 

As red wines age, the monomeric forms of anthocyanins undergo condensation reactions with other anthocyanin or tannin molecules to form polymeric pigments. The polymeric pigments are less sensitive to bisulfite bleaching and changes in pH (10). The color analysis method of Somers and Evans (10) as described above takes advantage of these differences to follow color changes as wines age. 

Anthocyanin solutions are strongly bleached in the presence of sulfur dioxide. At a pH of 3.2, 96% of the sulfuric acid (SO2 + H2O) consists of HSOs" (bisulfite) anions that react with the flavylium cation, most probably on carbon 2 by analogy with the hydration reaction. The product formed is colorless ... 

---