Anthocyanins bleaching

Anthocyanins usually give a purple red colour. Anthocyanins are water soluble and amphoteric. There are four major pH dependent forms, the most important being the red flavylium cation and the blue quinodial base. At pHs up to 3.8 commercial anthocyanin colours are ruby red as the pH becomes less acid the colour shifts to blue. The colour also becomes less intense and the anthocyanin becomes less stable. The usual recommendation is that anthocyanins should only be used where the pH of the product is below 4.2. As these colours would be considered for use in fruit flavoured confectionery this is not too much of a problem. Anthocyanins are sufficiently heat resistant that they do not have a problem in confectionery. Colour loss and browning would only be a problem if the product was held at elevated temperatures for a long while. Sulfur dioxide can bleach anthocyanins - the monomeric anthocyanins the most susceptible. Anthocyanins that are polymeric or condensed with other flavonoids are more resistant. The reaction with sulfur dioxide is reversible. 

Colorimetric measurements have been applied to various analyses of red wines and model wine solutions. °° ° ° Accurate definitions of the wines have been achieved by the L (lightness) and a (redness) values, while the representation of AL against AC revealed the color differences between various wines. ° The color stability of wine and model wine solutions toward storage time and bleaching by sulfur dioxide has been extensively studied by Bakker et while the color stability of a range of anthocyanin-containing... 

The flavylium ions of direct flavanol-anthocyanin adducts (i.e., A" -F and F-A" ") and anthocyanin dimers (A+-AOH) have the same UV-visible spectra as anthocyanins. Species in which the anthocyanin is substituted in the 4-position (A" -F, A" -AOH) are expected to be resistant to sulfite bleaching and hydration whereas F-A is as susceptible to water addition as their anthocyanin precursor. ... 

Yamasaki, H., Uefuji, H., and Sakihama, Y., Bleaching of the red anthocyanin induced by superoxide radical. Arch. Biochem. Biophys., 332, 183, 1996. 

Identification Transfer 1 g of sample and 1 g of potassium metabisulfite to a 100-mL volumetric flask, dissolve in about 50 mL of pH 3.0 Citrate-Citric Acid Buffer (see Assay, below), and dilute to volume with the same buffer. The red color caused by anthocyanins is bleached. 

Jurd, L. 1964. Reactions involved in sulfite bleaching of anthocyanins. J. Food Sci. 29 16-19. 

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

The antioxidant activity of a compound depends upon which free radical or oxidant is used in the assay (Halliwell and Gutteridge, 1995), and a different order of antioxidant activity is therefore to be expected when analyses are performed using different methods. This has been demonstrated by Tsuda et al. (1994) in their study of antioxidative activity of an anthocyanin (cyanidin-3-O-p-D-glucosidc) and an anthocyanidin (cyanidin) in four different lipophilic assay systems. Both compounds had antioxidative activity in all four systems, but the relative activity between them and their activity, compared with Trolox, varied with the method used. Fukumoto and Mazza (2000) reported that antioxidant activity of compounds with similar structures gave the same trends, although not always the same results, when measured by P-carotene bleaching, DPPH and HPLC detection of malonaldehyde formation in linoleic acid emulsion. 

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