Anthocyanin condensation reactions

Aldehyde-tannin and aldehyde-anthocyanin condensation reactions result in polymer formation (Figure 1). These polymers may be responsible for haze formation in wine and the polymers may eventually precipitate out of solution (26). The polymerized tannins have different flavor properties than the monomeric starting units (21-29) and formation of anthocyanin polymers affects wine color. In addition, these reactions may result in a reduction of aldehyde flavors in the wine. These condensation reactions are discussed more fully in other chapters of this volume. The formation of strong covalent bonds between the aldehyde and the tannin or anthocyanin makes recovery of the bound aldehydes difficult. 

Corrales, M., Butz, P, and Tauscher, B. 2008b. Anthocyanin condensation reactions under high hydrostatic pressure. Food Chemistry 110 627-635. 

Since the early contributions of Willstatter and Robinson, several alternative approaches following mainly two routes have been considered for synthesis of anthocyanins.One of the routes includes condensation reactions of 2-hydroxybenzaldehydes with acetophenones, while the other uses transformations of anthocyanidin-related compounds like flavonols, flavanones, and dihydroflavonols to yield flavylium salts. The urge for plausible sequences of biosynthetic significance has sometimes motivated this latter approach. In the period of this review, new synthetically approaches in the field have also predominantly been following the same general routes however, some new features have been shown in synthesis of pyranoanthocyanidins. 

A.2.2 Anthocyanin-Flavanol and Anthocyanin-Anthocyanin Direct Condensation Reactions... 

Studies in model solution containing an anthocyanin and flavanol oligomers (up to tetramers) at pH 3 carried out at 50 °C demonstrated that temperature is another factor that affects the progress of direct condensation reactions (Malien-Aubert et al. 2002). At acidic pH and high temperature, the anthocyanin is in equilibrium with the colorless chalcone. Although breakage of the flavanol C-C bond occurred under these conditions, the formation of the chalcone impeded the synthesis of F-A products and only A-F adducts were formed (Malien-Aubert et al. 2002). 

A.2.4.1 Anthocyanin-Vinylphenol/Anthocyanin-Hydroxycinnamic Acid Condensation Reactions Hydroxyphenyl-Pyranoanthocyanins... 

A.2.4.2 Anthocyanin-Vinylflavanol Condensation Reactions Flavanyl-Pyranoanthocyanins... 

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. 

Mechanism of reaction. The adduct of malvidin-3-glucoside with pyruvic acid, also known as vitisin A(Fig. 9A.3h), was firstly detected in fortified red wines (Bakker et al. 1997) and in a grape marc (Fulcrand et al. 1998) and further isolated and characterized by NMR (Bakker et al. 1997 Fulcrand et al. 1998). According to Fulcrand et al. (1998), the reaction between pyruvic acid and grape anthocyanins occurs through a series of steps similar to those previously described for the hydroxyphenyl-pyranoanthocyanins (Sect. 9A.2.4.1 Fig. 9A.3f). Later studies performed by NMR (Mateus et al. 2001b) and mass spectrometry (Asenstorfer et al. 2001 Hayasaka and Asenstorfer 2002) have confirmed the structure proposed by Fulcrand et al. (1998). This mechanism is extended to the condensation reaction between anthocyanins and other enolizable precursors found in wine (Benabdeljalil et al. 2000). 

Changes in the color of red wines that occur during aging are due to the anthocyanins undergoing chemical reactions and polymerization with the other wine compounds. More than 100 structures belong to the pigment families of anthocyanins, pyranoanthocyanins, direct flava-nol-anthocyanin condensation products, and acetaldehyde-mediated... 

The reaction mechanism proposed by Timberlake and Bridle (1976), suggests that acetaldehyde, in the form of a carbo-cation, reacts with flavanol (tarmin) at position C-6 or C-8 (Figure 1 (1)). This, via several condensation reactions, gives rise to tannin-ethyl-anthocyanin derivatives (Figure 1 (2)) such as malvidin-3-glucoside-ethyl-(epi)catechin and malvidin-3-(6-/7-coumaroyl)-glucoside-ethyl-(epi)catechin. 

Jurd, L. Anthocyanins and related compounds XI. Catechin-flavylium salt condensation reaction. Tetrahedron 1967,22,1057-1064 Somers, T.C. The polymeric nature of wine pigments. Phytochemistry 1971, 70,2175-2186... 

The incorporation of acetaldehyde derived bridges between anthocyanins and flavan-3-ols via acetaldehyde condensation reactions has been well described in fermented beverages such as red wine. The presence of acetaldehyde in alcoholic solutions is attributed to either oxidatative products of ethanol or microbial byproducts. In the case of cranbeny fruit and spray dried juice neither product was subjected to yeast fermentation. It becomes a concern then that die observed anthocyanin-pigments may be an arti ct of harvest, storage, juice processing or analytic techniques. 

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. 

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