Condensation with anthocyanins

Bisulfite ions, HS03", condense with anthocyanins. This reversible reaction decreases the color by forming a colorless compound (12) (16). This effect is less evident in strongly acid media because the bisulfite ions are not as numerous since they are being converted to the undissociated acid. This property explains the decolorization of red wines following sulfite treatment but, since it is reversible, the color gradually reappears as the free S02 (bisulfite ions) disappears. The major role of tannins in the color of old wines explains their insensitivity to color change with SOo. 

In general, the effects of ascorbic acid on anthocyanins are complex and not easily predictable. In the absence of oxygen, ascorbic acid may condense with anthocyanins to form unstable products that degrade into colorless compounds (Markakis, 1982). It is assumed that condensation of anthocyanins with flavonols prevents formation of complexes between anthocyanins and ascorbic acid so that the deteriorating effect of ascorbic acid is diminished (Jackman et al., 1987a). 

The procyanidin molecules from the grapes tend to polymerize, condense with anthocyanins and combine with plant polymers such as proteins and polysaccharides. Several reactions are involved (Section 6.3) ... 

Francia-Aricha, E.M. et al.. New anthocyanin pigments formed after condensation with flavanols, J. Agric. Food Chem., 45, 2262, 1997. 

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. 

Cinnamic acids may condense with molecules other than quinic acid, including rosmarinic, malic and tartaric acid, aromatic amino acids, choline, mono- and polysaccharides, glycerol, myo-inositol, and different glycosides (anthocyanins, flavonols and diterpenes) [13]. 

Figure 7. Condensation of anthocyanins with flavans. The dimer obtained is red, but as the 4 position of the anthocyanin is substituted, it does not react with bisulfite and its color does not vary with changes in pH.

During the weeks that follow vinification several things happen simultaneously (a) The reduced anthocyanins are reoxidized, producing a color increase (b) The anthocyanins, in reduced or oxidized form, are partially destroyed by various chemical reactions or by condensation with tannins. One may thus explain why, during that period the color of some wines increases while that of others decreases, depending on the relative rates of the two reactions occurring at this time. 

During aging the anthocyanins continue to disappear by condensing with tannins which themselves undergo an oxidative condensation resulting in a color shift from yellow to orange-brown. Finally it is these tannins which play the most important role in the characteristic color of old wines. 

Acetaldehyde plays an important role during red wine fermentation in which it can react directly with anthocyanins pigments to form vinyl adducts (pyroan-thocyanins) (Fulcrand et al. 1998), and can participate in the condensation of proanthocyanidins (tannins) and anthocyanins to form ethyl-bridge linked dimers... 

Precursors. Precursors for this reaction are compounds exhibiting keto-enol tau-tomerism. These compounds are usually secondary metabolites derived from the glycolysis cycle of yeast metabolism during fermentation. Pyruvic acid is one of the main precursor compounds involved in this type of reaction. During yeast fermentation it is decarboxylated to acetaldehyde and then reduced to ethanol. Acetone, ace-toin (3-hydroxybutan-2-one), oxalacetic acid, acetoacetic acid and diacetyl, among others, are also secondary metabolites likely to participate in this kind of condensation reaction with anthocyanins. 

Since it appears that anthocyanins and tannins condense in wine to form secondary wine pigments, the nature of the linkage between these compounds has been the focus of many investigations. There are a large number of possibilities but this chapter will discuss only two for historical purposes. Others will be discussed by other authors in this book. Based upon the structure of a natural compound, Jurd synthesized a dimer from catechin and a synthetic anthocyanidin (Figure 1). The flavylium form of the anthocyanidin has an electrophilic carbon at C-4 that will condense with the phloroglucinol ring of catechin. The new product, a flavene, will be oxidized by a second flavylium... 

Other studies have described similar results to those found in our research, that is, that ellagitannins are more abundant in French oak (26) and therefore, in wines matured in French oak barrels. So, the presence of ellagitannins enhances the color of wine and increases absorbance at 620 nm by fevoring anthocyanin-procyanidin type tannin condensations with acetaldehyde (purple pigments) and that foct also helps to explain the higher PVPP index values of wines fix)m new French oak barrels. They also prevent the development of brick-yellow color by preventing the oxidation of phenolic compounds (24). 

The rest of the malvidin-3-glucoside in Remy et al. could not be recovered by thiolysis, probably because it is in a flavene-type structure or there are additional linkages to the pigment such as vinyl condensation (9) (10). It has been reported that enzymatic oxidation of caffeic yields an o-quinone that will condense with malvidin-3-glucoside (11) and overall 520 nm absorbance decreases. Similarly, fIavan-3-ols could also react with anthocyanins through an oxidized catechol ring (non-enzymatic formation). Such products would have bonds between the tannin and anthocyanins that cannot be cleaved by mild acid. 

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