Formation of pyranoanthocyanin

Fig. 2.105. Formation of pyranoanthocyanins. Reprinted with permission from A. E. Hakansson et al. [239].

Fig. 2.107. The formation of pyranoanthocyanins 6-10 from anthocyanins 1-5 and acetone. Reprinted with permission from Y. Lu et al. [242].

Formation of pyranoanthocyanins through reaction of flavylium cations with compounds possessing a polarizable double bond, namely vinylphenol derivatives (including vinylflavanols and hydroxycinnamic acids) and enolizable aldehydes and ketones (e.g., acetaldehyde and pyruvic acid). 

Finally, reactions of flavonoid and nonflavonoid precursors are affected by other parameters like pH, temperature, presence of metal catalysts, etc. In particular, pH values determine the relative nucleophilic and electrophilic characters of both anthocyanins and flavanols. Studies performed in model solutions showed that acetaldehyde-mediated condensation is faster at pH 2.2 than at pH 4 and limited by the rate of aldehyde protonation. The formation of flavanol-anthocyanin adducts was also limited by the rate of proanthocyanidin cleavage, which was shown to take place at pH 3.2, but not at pH 3.8. Nucleophilic addition of anthocyanins was faster at pH 3.4 than at pH 1.7, but still took place at pH values much lower than those encountered in wine, as evidenced by the formation of anthocyanin-caffeoyltartaric acid adducts, methylmethine anthocyanin-flavanol adducts,and flavanol-anthocyanin adducts. The formation of pyranoanthocyanins requiring the flavylium cation was faster under more acidic conditions, as expected, but took place in the whole wine pH range. Thus, the availability of either the flavylium or the hemiketal form does not seem to limit any of the anthocyanin reactions. 

Morata, A., Gomez-Cordoves, M. C., Calderon, F, Suarez, J. A. (2006) Effects of pH, temperature and SO2 on the formation of pyranoanthocyanins during red wine fermentation with two species of Saccharomyces. International Journal of Food Microbiology, 106, 123-129. 

Formation of pyranoanthocyanins is faster at lower pH values since the mechanism requires anthocyanins to be under the flavylium cation form and ketones to be in the enol form, both of which are favoured in acidic media. Formation of flavanyl-pyranoanthocyanins should be further facilitated in acidic media due to enhanced cleavage of the ethyl bridges. However, even at higher pH values, pyranoanthocyanins are expected to accumulate progressively in wines because of their stability. 

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

Precursors. Both hydroxycinnamic acids and 4-vinylphenols can lead to the formation of hydroxyphenyl-pyranoanthocyanins. The main hydroxycinnamic acids present in wines are p-coumaric, caffeic, ferulic and sinapic acids. 4-Vinylphenol and 4-vinylguaiacol are volatile phenols associated with off flavors in wine (Eti6vant 1981) and arise from the decarboxylation of p-coumaric and ferulic acid, respectively, via the yeast cinnamate decarboxylase (CD) (Chatonnet et al. 1993). 

Schwarz et al. (2003b) proposed a different mechanism for the formation of hydroxyphenyl-pyranoanthocyanins in red wines, including the free... 

Mechanism of reaction. The mechanism proposed for the formation of flavanyl-pyranoanthocyanins (Francia-Aricha et al. 1997 Mateus et al. 2003a,b) (Fig. 9A.3g), is similar to that described above by Fulcrand et al. (1996) for the formation of hydroxyphenyl-pyranoanthocyanins (Sect. 9A.2.4.1 Fig. 9A.3f). 

Rentzsch, M., Schwarz, M., Winterhalter, R, Hermosm-Guitierrez, I. (2007). Formation of hydroxyphenyl-pyranoanthocyanins in Grenache wines precursor levels and evolution during aging. J. Agric. Food Chem. 55, 4883-4888. 

Another kind of reaction has been demonstrated for the cationic forms of anthocyanins, resulting in the formation of a second pyran ring. The mechanism involves both the electron deficient C-4 and the 5-hydroxyl group of the anthocyanin with compounds possessing a polarizable double bond (d). The new pigments thus formed are referred to as pyranoanthocyanins. 

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.,... 

The formation of large quantities of ethanal is also responsible for the development of orange tannin-pyranoanthocyanin complexes. 

Morata A, Calderon F, Gonzalez MC, Gomez-Cordoves MC, Suarez JA (2007) Formation of the highly stable pyranoanthocyanins (vitisins A and B) in red wines by the addition of pyruvic acid and acetaldehyde. Food Chem 100 1144—1152... 

The supply of oxygen to wine may improve the formation of many of those pigments such as pyranoanthocyanins, methylmethine-linked catechin-anthocyanins adducts (Atanasova et al, 2002a) however, excessive oxygenation may lead to wine browning and precipitation. 

Morata, A., Gonzilez, C. and Suirez-Lepe, J. A. 2007. Formation of vinylphenolic pyranoanthocyanins by selected yeasts fermenting red grape musts supplemented with hydroxycinnamic acids. International Journal of Food Microbiology. 116 144-152. 

Schwarz, M., Wabnitz, T. C. and Winterhalter, P. 2007. Pathway leading to the formation of vinylphenolic pyranoanthocyanins by selected yeasts. J. Agric. Food Chem. 51(12) 3682-3687. 

Rentzsch, M., Schwarz, M., Winterhalter, P. (2007b). Pyranoanthocyanins - an overview on structures, occurrence, and pathways of formation. Trends FoodSci. Technol., 18, 526-534. 

---