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Anthocyanins

The glycosylated flavonoid responsible for the color of red wine. Carl Formaker, 2015

Color is one of the most important attributes in the determination of quality in red wines, and anthocyanins are the principal source of the red color in wine. Anthocyanins are water soluble flavonoid pigments that accumulate in grape cell vacuoles which contribute color to wine depending on pH and complexing agents available.  They are widely spread throughout the plant kingdom, and can occur in almost all tissues of higher plants, including roots, stems, leaves, flowers, and fruits. Thus they are considered to be the major natural pigments in the plant-derived food, including red wines.

Structurally, anthocyanins are glycosides and acylglycosides of anthocyanidins, the term for a simple flavonoid ring system.  The core of the anthocyanidin, the flavylium, has a C6-C3-C6 flavonoid skeleton which contains one heterocyclic benzopyran ring (C ring), one fused aromatic ring (A ring) and one phenyl constituent (B ring) (He et al., 2012). There are five prominent aglycones in red grapes which differ in their hydroxyl and methoxyl substitutions on the B-ring. The common anthocyanidins found in grapes are cyanidin, delphinidin, peonidin, petunidin, and malvidin.  In red grape varieties, Malvidin-3-glucoside is the most populous representing about 40% of the anthocyanins and makes up the majority of the red pigments that are acylated (He et al., 2010). Due to presence of eight conjugated double bonds carrying a positive charge, anthocyanins are intensely red or orange when the solution’s pH is below 2, but at a higher pH they are colorless and in alkaline conditions change their color into blue.  Due to the pH dependence of anthocyanin equilibria, the color of a wine will be dependent on the anthocyanin concentration as well as the pH.

During fermentation and in the first couple years of maturation, the monomeric anthocyanins in wines undergo a wide variety of reactions and associations.  Although the concentration of monomeric anthocyanins declines during aging, red wines can still maintain their red color.  During vinification, various anthocyanin-derived new pigments are formed, which are crucial for wine color stability (Kelebek et al., 2009). The processes are composed of complicated short term mechanisms and relatively long-term reactions.  The transient short term reactions include copigmentation from anthocyanin self-association or with other flavonoids.  The formation of polymeric anthocyanins with flavan-3-ols and proanthocyanidins and the development of pigmented tannin, such as pyranoanthocyanins and pinotins increase in concentration as the wine ages thus creating stable pigment (He et al., 2010).

References

  1. He, F., Liang, N. N., Mu, L., Pan, Q. H., Wang, J., Reeves, M. J., & Duan, C. Q. 2012. Anthocyanins and their variation in red wines I. Monomeric anthocyanins and their color expression. Molecules17(2), 1571-1601.
  2. He, F., Mu, L., Yan, G. L., Liang, N. N., Pan, Q. H., Wang, J., & Duan, C. Q. 2010. Biosynthesis of anthocyanins and their regulation in colored grapes. Molecules15(12), 9057-9091.
  3. Kelebek, H.; Canbas, A.; Selli, S. 2009. Effects of different maceration times and pectolytic enzyme addition on the anthocyanin composition of Vitis vinifera cv. Kalecik Karasi wines. J. FoodProcess. Preserv. 2009, 33, 296–311.

 

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