Flavanols polymerized

The procyanidinic tannins, derived from flavanol polymerization, attain a maximum couceutration in the seeds before veraison. This then strongly decreases to a lower and relatively stable value... 

Flavan-3-ols orflavanols have a saturated three-carbon chain with a hydroxyl group in the C3 position. In foods they are present as monomers or as proanthocyanidins, which are polymeric flavanols (4 to 11 units) known also as condensed tannins. In foods they are never glycosylated. 

Flavanols and procyanidins Flavanols, or flavan-3-ols, are synthesized via two routes, with (+) catechins formed from flavan-3,4-diols via leucoanthocyanidin reductase (LAR), and (—) epicatechins from anthocyanidins via anthocyanidin reductase (ANR) (see Fig. 5.4). These flavan-3-ol molecules are then polymerized to condensed tannins (proanthocyanidins or procyanidins), widely varying in the number and nature of their component monomers and linkages (Aron and Kennedy 2008 Deluc and others 2008). It is still not known whether these polymerization reactions happen spontaneously, are enzyme catalyzed, or result from a mixture of both. 

Monitoring of acetaldehyde-induced polymerization of catechin and epicatechin by HPLC-MS demonstrated the formation of several methylmethine-linked flavanol dimers, trimers, and tetramers. Detection of the intermediate ethanol adducts confirmed the mechanism postulated by Timberlake and Bridle, which involves protonation of acetaldehyde in the acidic medium, followed by nucleophilic attack of the resulting carbocation by the flavan unit. The ethanol adduct then loses a water molecule and gives a new carbocation that undergoes nucleophilic attack by another flavanol molecule. Four dimers (C6-C6, C8-C8, and C6-C8, R and S) were formed from each monomeric flavanol. When both epicatechin and catechin units were present, additional isomers containing both types of units were... 

Es-Safi, N.E. et al.. Competition between (+)-catechin and (—)-epicatechin in acetaldehyde-induced polymerization of flavanols. J. Agric. Food Chem. 47, 2088, 1999. 

The flavan-3-ols most occurring in nature are (+)-catechin and (-)-epicatechin (EC), although gallocatechin and epigallocatechin have also been identified [42]. Proanthocyanidins (or condensed tannins) include oligo- and polymeric forms of the monomeric flavanols and will be examined later. Polymerization of monomeric flavanols can occur as a result of auto-oxidation, but more often it is catalyzed by polyphenoloxidase (PPO), an enzyme that is present in most plant tissues [43]. 

During fermentation in the preparation of black tea, oxidative polymerization of flavanols occurs with the formation of theaflavin, theaflavingallates, thearubigins, and epitheaflavic acid [44]. 

Condensed Tannins. The tannins found in grapes and wines are condensed polymers from 3-flavanols (catechins) (17, 18, 19) and from 3,4-flavandiols (leucoanthocyanidins) (20, 21, 22). The monomeric leucoanthocyanidins, like their polymerized forms, display the characteristic, which differentiates them from the catechins, of trans-... 

Polyphenols (PP) represent a very wide variety of about 6000 compounds divided into I I different classes represented by hydroxybenzoic acids, hydroxycinnamic acid, anthocyanidins, flavonols, flavanones, flavanols (divided into monomeric cathechins and polymeric proanthocyanidins3), flavones, isoflavones, stilbenes, and lignans (16). 

Total antioxidant activities in 13 typical Italian wines were determined with an average of 12.3 pM Trolox equivalents for red wines and 1.6 pM for white wines (Simonetti et al., 1997). The values correlated well with total phenols (r = 0.9902) and flavanols (r = 0.9270) and clearly demonstrated that red wine polyphenols are significant in vitro antioxidants. The antioxidant capacity of 16 red wines from several countries was tested using electron spin resonance (ESR) spectroscopy (Bums et al., 2000). The antioxidant activity ranged from 4.13 x 1021 to 9.29 x 1021 number of Fremy s radicals reduced by 1 L of wine. The antioxidant capacity was associated with the phenolic content of the wines, determined either by Folin-Ciocalteau or by summation of the individual phenolics determined by HPLC or by spectroscopy. Total phenolics as measured by the Folin-Ciocalteau method ranged from 6.47 to 18.6 mM GAE per liter. The anthocyanin levels ranged from 101.5 to 325.7 pM. There were substantial differences in the proportion of polymeric pigments in the wines. 

Torres, J. L. and Bobet, R. (2001). New flavanol derivatives from grape (Vitis vinifera) byproducts. Antioxidant aminoethylthio-flavan-3-ol conjugates from a polymeric waste fraction used as a source of flavanols. J. Agri. Food Chem. 49, 4627-4634. 

Finally, the relative amounts of anthocyanin-flavanol pigments and anthocyanin-anthocyanin pigments obviously depend on the anthocyanin to flavanol ratio. However, the kinetics of anthocyanin polymerization and flavanol-anthocyanin reactions as well as the quantities of the resulting pigments in wine remain to be established. Consequently, the importance of these processes with respect to other anthocyanin reactions (hydration and subsequent degradation, cycloadditon with yeast metabolites) and the impact of resulting polymeric anthocyanins on wine quality are speculative. 

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