Anthocyanins breakdown reactions

However, if the wine oxidizes too rapidly, anthocyanin breakdown reactions may also occur, causing loss of color, possibly accompanied by the formation of glyoxylic acid and yellow xanthylium. The end result depends on the relative quantities of anthocyanins and tannins in the wine. 

In addition to the molecnlar strnctnre, the tan-nin/anthocyanin ratio also affects wine development. Color has been observed to change rapidly due to anthocyanin breakdown reactions when the medinm has a low tannin content (molar ratio T/A tannin polymerization reactions when the tannin concentration is much higher than the anthocyanin content (molar ratio T j A < 4). If a wine is to develop harmoniously, this ratio should be between 1 and 4, i.e. 500 mg of anthocyanins and 1-3 g of tannins per liter of wine. 

Anthocyanin molecules are not very stable, so their concentration in wine drops sharply during the first few months of barrel aging. They disappear completely in a few years, although the wine remains red. This decrease is due to combination reactions with various other compounds in the wine, especially tannins, as well as breakdown reactions. 

Anthocyanins are extracted at the beginning of vatting, mainly in the aqueous phase, during maceration prior to fermentation and at the beginning of alcoholic fermentation. When the alcohol content reaches a certain level, a decrease is observed in the results of assays for these molecules. At this stage, extraction of anthocyanins from the grapes is almost completed and several mechanisms intervene to decrease concentrations. These include adsorption of anthocyanins on solids (yeast, pomace), modifications in their structure (formation of tannin-anthocyanin complexes) and, possibly, breakdown reactions (Volume 1, Section 12.5.2). 

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. 

The temperature depends on the winery. Low temperatures are useful for precipitating unstable colloids. On the one hand, temperatures above 20°C promote the formation of carbocations from procyanidins, and therefore the TA complex (red, orange), as well as homogeneous polymerization. On the other hand, they also facilitate combinations with polysaccharides as well as color breakdown reactions. Furthermore, it promotes the thermal degradation of some anthocyanins, particularly malvidin. Alternating a low temperature with a temperature around 20°C promotes development, while maintaining it within certain limits. 

The above transformations result in a reduced anthocyanin content, contrasting with the increase in color. The new condensed pigments formed are more intensely colored than anthocyanins. Other anthocyanin and tannin breakdown reactions may lead to a loss of color, generally accompanied by a tendency towards yellow-orange hues. This is characteristic of the normal development of bottle-aged red wines. The breakdown of anthocyanins involves a loss of molecular structure in the red coloring matter, possibly accompanied by the appearance of a yellowish hue. 

Rapid oxidation (Section 6.3.3) has an effect on all these molecules if they are not protected by a sufficient quantity of tannins. The molar ratio T/A must be at least 2, otherwise wine behaves like a pure authocyanin solution. There is a much lower risk of breakdown reactions under controlled oxidation conditions as malvidin, the main anthocyanin in wine, is not dihydroxylated and, consequently, is not very sensitive to slow oxidation. 

The factors that inhibit these breakdown reactions are a good molar balance between anthocyanins and tannins (T/A >2), temperature <20°C and controlled oxidation. 

In an aqueous acid medium containing acetone, anthocyanins produce orange-colored compounds (Glories, 1978a). Various mechanisms have been suggested to explain the formation of these orange compounds (Section 6.3.9) hydrolysis of the anthocyanins and conversion of the anthocyanidins into dihydroflavonols, breakdown of the heterocycle with formation of benzoic acids, or a reaction between acetone and anthocyanin via polarized double bonds. 

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