The biosynthesis of anthocyanins

The quantity and quality of phenolic compounds (especially tannins, anthocyanins, and co-factors) in the wine post-fermentation, and the interaction of those compounds, are critical elements in the postmodern winemaking process as laid out by Clark Smith. Before delving into these phenolic interactions, I sought to provide some background, beginning with tannins in a prior post and anthocyanins in the current.

The most important source of color in red wines is anthocyanin, a class of phenolic compound resident in grape skins. As is the case for all phenolic compounds, anthocyanin is synthesized from the amino acid phenylalanin through the phenylpropanol and flavonoid pathways (graphically illustrated below).

The flavonoid pathway. Abbreviations:
ANS -- anthocyanidine synthase;
CHI -- chalcone isomerase;
CHS -- chalcone synthase;
DFR -- Dihydroflavonol reductase;
F3H -- flavanone 3-hydroxylase;
GT -- glucoysltransferase;
Source: http://www.hindawi.com/

Grape anthocyanidins. Source: www.intechopen.com/books/

Anthocyanins are synthesized directly from anthocyanidins by glycosylation (adding of a sugar to a protein) at the 3 and 5 positions of the C ring and are accumulated in the berry skins from veraison until full maturity. After synthesis in the cytosol (fluid portion of the cell cytoplasm), anthocyanins are transported into the vacuole (cell organelle responsible for a number of functions including nutrient storage) where they are stored as colored coalescences called anthocyanic vascular inclusions (Flamini, et al.)

Environmental effects can influence anthocyanin content in the fruit but its composition is most closely associated with grape variety. In the fruit, anthocyanin has the following functions (Flamini, et al.):

• Protection against solar exposure and and UV radiation
• Protection against pathogen attacks
• Protection against oxidation attacks
• Protection against attacks by free radicals
• Attracting animals for seed dispersal after the fruit has attained maturity

Each anthocyanin has a particular hue ranging from red to blue (shown below) and the combination of quantities and profiles will determine the color intensity of fruit and wine.

Grape anthocyanins. Source: gopixpic.com

In a follow-up post I will describe anthocyanin interactions and reactions in young wines.


Bibliography
Flamini, et al., Advanced knowledge of three important classes of grape phenolics: Anthocyanins, Stilbenes, and Flavonols, Int J Mol Sci. 2013, Oct; 14910). Accessed online at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3821578/
Enology Note #120


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