Pyranoanthocyanins between

Fig. 2.102. Structures of all the compounds found in fraction A (a) anthocyanins (b) A-type vitisins (c) pyranoanthocyanins originated by reaction between anthocyanins and vynilphenol, vynilcatechol or vynilguaiacol (d) pyranoanthocyanins originated by reaction between anthocyanins and vynil(epi)catechin. Reprinted with permission from C. Alcalde-Eon et al. [236],...

Mechanism of reaction. The mechanism of the reaction between anthocyanins and 4-vinylphenols was first proposed by Fulcrand et al. (1996). Hydroxyphenyl-pyranoanthocyanins result from the cycloaddition of the ethylenic bond of the 4-vinylphenol molecule at positions C-4 and C-5 of the anthocyanin followed by an oxidation process, resulting in a pyrane ring (Fulcrand et al. 1996) for which this type of compound receives the name of pyranoanthocyanins (Fig. 9A.3f). The vinylphenol addition at C-4 protects the anthocyanin of being hydrated. 

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). 

The reactions involved in these color changes and the oxidative transformations of phenols in wine mainly involve ethanal. They either result in the formation of an ethyl cross-bond between anthocyanin and tannin molecules (Section 6.3.10), or a cycloaddition to the anthocyanins, producing tannin-pyranoanthocyanins (Atanasova et al.,... 

Previous studies regarding the composition and amount of several pyranomv3glc-flavanol compounds in 3-, 4-, and 6-year-old Port wines have shown that pyranoan-thocyanin-procyanidin dimers were more abundant than pyranoanthocyanin-catechin monomers (He et al, 2006b). No relationship between the concentration of these compounds and the wine age was ascribed and their contribution to the red wine color remains unclear. 

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). 

Fig. 3.12 Reaction between anthocyanins and carbonyl compounds (enolic forms) leading to the formation of pyranoanthocyanins in red wine.

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