Model reactions with anthocyanins

Figure 5. Correlation of loss of malvidin-3-glucoside (MSG) with pigmented polymer formation in model reactions with anthocyanins, tannins, acetaldef de...

All natural anthocyanins suffer from inherent instability, so they may be degraded to form colorless or brown-colored, often insoluble products. It is evident, that depending on the reaction type, different degradation products are formed however, only little analytical data is available. For example, after reaction of anthocyanins with hydrogen peroxide, substances of the benzofu-ran type were detected (139). Studies with grape must like model solutions indicated that anthocyanins are degraded by coupled oxidation and that they form adducts with caffeoyltartaric acid... 

Factors affecting the reaction. The extent of the reactions between anthocyanins and pyruvic acid in model solutions follows a first order kinetic with respect to the anthocyanin disappearance. This reaction is affected by several factors, such as anthocyanin composition, pH, pyruvic acid concentration, temperature and acetaldehyde concentration. The maximum formation took place at pH 2.7-3.0 due to requirement of the anthocyanin fiavylium form, at high pyruvic acid concentration, at low storage temperature (10-15 °C) and in the absence of acetaldehyde (Romero and Bakker 1999a,b, 2000a,b). 

While the identity of the relevant yeast metabolites in the fermented medium sampled at day 2 needs to be clarified, previously published data have provided some evidence about the role of acetaldehyde-mediated condensation of catechin with MSG (13-20). We therefore aimed to extend these model studies and to confirm chemical formation of pigmented polymers from condensed tannins, which are commercially used in red winemaking, and anthocyanins. The model reactions were conducted with vatying concentrations of acetaldehyde and SO2 as shown in Table 2 and analysed by HPLC after 2, 4 and 7 days. After 7 days visible precipitation of unidentified material started to occur in presence of acetaldehyde and the reactions were discontinued. 

Similarly, direct reactions between anthocyanins and a flavanol dimer (namely epicatechin-epicatechin 3-gallate) were investigated in model solutions at pH 2 and pH 3.8 (31). Compounds with mass values corresponding to epicatechin-anthocyanin adducts and to anthocyanin-epicatechin-epicatechin 3-gallate, were detected at pH 2 and pH 3.8, respectively. Given the structure of the flavanol precursor, the former can be interpreted as F-A+ arising from cleavage of the flavanol dimer followed by addition of the anthocyanin AOH form onto the epicatechin carbocation whereas the latter is presumably an A+-F species formed by nucleophilic addition of the flavanol dimer onto the flavylium cation. Anthocyanin dimers (A+-AOH) were detected in model solutions at pH... 

The probable involvement of xanthylium salts in red wine color change is supported by the fact that their absorption maxima match well with the increase in yellow color around 420 nm observed during ageing of red wine. Thus, Liao et al. (22) demonstrated in model systems that reactions between anthocyanins and flavan-3-ols give rise to orange-yellow (440 nm) pigmented products. Based on the earlier observations of Jurd Somers (24) and Hrazdina Borzell (48 such products have been postulated to be xanthylium salts proceeding from direct anthocyanin-flavanol adducts. 

Reactions of anthocyanins have been especially investigated in red wines and related model systems. In fact, the progressive change from the red-purple to tawny as the wine ages is believed to be due to conversion of grape anthocyanins to more stable polymeric forms, where a reaction with, for example, acetaldehyde is the first step leading the reaction. 

Vitisin B pigments are another minor group of pyranoanthocyanins that show no substituent on the pyranic D-ring (Bakker Timberlake, 1997). These pigments were described to result from the direct reaction between anthocyanins and acetaldehyde. However, there is no consensus on this matter because that reaction is very difficult to occur in model wine conditions and the mechanism involved is not fully understood. Morata et al. (2007) have observed that the addition of acetaldehyde (200 mg L ) to a young red wine (cv. Tempranillo) led to the production of fivefold amounts of vitisin B after 4 weeks with control wines (Morata et al, 2007). 

The thermal degradation of anthocyanins, both in extracts and model systems, was reported to follow first-order reaction kinetics in all studies. The stability of anthocyanins and all pigments found in foods decreased with increases in temperature. 

The influence of the stems, also studied in model solutions, is summarized in Table XIII (49). Stem extract and stems themselves are added to a solution of anthocyanins the anthocyanin concentration is the same in all cases. The stems decrease the anthocyanin levels and color intensity which is not observed with stem extract alone. These pieces exert their effects through absorption rather than through chemical reactions of one of their constituents. 

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). 

Numerous pigments have been characterised in wines and wine-like model solutions in the last decade. The structures newly identified can be classified into three groups with respect to their formation pathways (figure 3). Reactions involved in the conversion of grape anthocyanins to more stable pigments are... 

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