Direct reaction with anthocyanins

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... 

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). 

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 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. 

The first phenomenon that should be mentioned is the direct reaction of red anthocyanins, in the form of positive flavylium cations, with flavanol molecules (catechins, procyanidins, etc.). This results in the formation of a colorless complex (flavene) that produces a red pigment when oxidized (Section 6.3.10). This reaction is stimulated by an acid pH (<3.5) and aT/A molar ratio <5. This mechanism causes wine to deepen in color following running-off. 

Along these lines, We have explored the possibility that red wine and red wine phenolics (e.g., anthocyanin fraction) promoted the formation of NO from nitrite in a pH-dependent and concentration-dependent way. This has been substantiated in vivo in healthy volunteers by measuring NO in the air expelled from the stomach, following consumption of wine and nitrate, as measured by chemiluminescence (Gago et al, 2007). Structure-activity studies revealed that the formation of NO from nitrite directly correlates with the reduction potential of the several phenols tested, including dimers B2, B5, B8, catechin, epicatechin, and quercetin, among others (results not published). EPR studies showed that, mechanistically, the reaction involves the one-electron reduction of nitrite to NO by the polyphenols and the concomitant formation of phenol-derived phenoxyl radicals (Gago et al, 2007) (Fig. 11.1). 

SO2 can also bind with phenolic compounds. In the case of proanthocyanic tannins, a solution of 1 g/1 binds with 20 mg/I of SO2 per liter. The combinations are significant with anthocyanins. These reactions are directly visible by the decoloration produced. The combination is reversible the color reappears when the free sulfur dioxide disappears. This reaction is related to temperature (Section 8.5.2) and acidity (Section 8.5.1), which affect the quantity of free SO2. The SO2 involved in these combinations is probably titrated by iodine along with the free SO2. In fact, due to their low stability, they are progressively dissociated to reestablish the equilibrium as the free SO2 is oxidized by iodine. 

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). 

The overall organoleptic impression is based on a harmonious balance between these two types of sensations, directly related to the type and concentration of the various molecules, such as anthocyanins, and especially tannins. One of their properties is to react with glycoproteins in saliva (mucine) and proteins in the mouth wall, modifying their condition and lubricant properties. A study of the reaction of the B3 procyanidins with synthetic, proline-rich proteins showed that three dimers were strongly bonded to the protein chains (Simon et al., 2003). According to the type and concentration of tannins, they may produce a soft, balanced impression or, on the contrary, a certain aggressiveness that is either perceptible as... 

The decrease in the anthocyanin concentration results from both breakdown reactions and stabilization reactions. In breakdown reactions (Section 6.3.3), free anthocyanins are broken down by heat into phenolic acids (mainly malvidin) and by violent oxidation, mainly delphinidin, petuni-din and cyanidin. They are highly sensitive to quinones and the action of oxidases, either directly or in combination with caftaric acid. This acid may even react in the (nucleophilic) quinone form and bond to anthocyanin s (electrophilic) node 8 as a carbinol base. 

---