Tannins condensation reactions

According to some studies, the compounds form as reaction by-products between anthocyanins and flavan-3-ols, such as catechins and proanthocyanidins (condensed tannins). These reactions may also involve other molecules such as acetaldehyde, pyruvic acid, acetoacetic acid, vinylphenol, vinylguaiacol, vinylcatechol, and dimerization of anthocyanins (Asenstorfer et ah, 2001 Atanasova et al., 2002 Bakker and Timberlake, 1997 Brouillard and Dangles, 1994 Fulcrand et ah, 1996, 1998 He et al., 2006 Liao et ah, 1992 Remy et ah, 2000 Salas et ah, 2004 Schwarz et al., 2003 Timberlake and Bridle, 1976). 

Progress in the use of condensed tannins in adhesive formulations might be expected to be more rapid than is the case for lignins because of the impetus provided by the commercialization of wattle tannin-based adhesives and because of the extraordinarily high reactivity of tannins in reactions with formaldehyde. This reactivity offers an opportunity to substitute tannin for resorcinol (currently priced at about 1.80/lb) instead of phenol (about 0.40/lb). Now that wattle tannins have been successfully introduced, their application can be expected to continue to expand. The situation remains difficult, on the other hand, for use of conifer bark tannins in adhesives. Herb Hergert is certainly... 

Aldehyde-tannin and aldehyde-anthocyanin condensation reactions result in polymer formation (Figure 1). These polymers may be responsible for haze formation in wine and the polymers may eventually precipitate out of solution (26). The polymerized tannins have different flavor properties than the monomeric starting units (21-29) and formation of anthocyanin polymers affects wine color. In addition, these reactions may result in a reduction of aldehyde flavors in the wine. These condensation reactions are discussed more fully in other chapters of this volume. The formation of strong covalent bonds between the aldehyde and the tannin or anthocyanin makes recovery of the bound aldehydes difficult. 

The reaction mechanism proposed by Timberlake and Bridle (1976), suggests that acetaldehyde, in the form of a carbo-cation, reacts with flavanol (tarmin) at position C-6 or C-8 (Figure 1 (1)). This, via several condensation reactions, gives rise to tannin-ethyl-anthocyanin derivatives (Figure 1 (2)) such as malvidin-3-glucoside-ethyl-(epi)catechin and malvidin-3-(6-/7-coumaroyl)-glucoside-ethyl-(epi)catechin. 

As red wines age, the monomeric forms of anthocyanins undergo condensation reactions with other anthocyanin or tannin molecules to form polymeric pigments. The polymeric pigments are less sensitive to bisulfite bleaching and changes in pH (10). The color analysis method of Somers and Evans (10) as described above takes advantage of these differences to follow color changes as wines age. 

Humic substances represent a broad spectrum of different compounds. In the formation of humic substances saccharides, pectins, lignin, proteins, fats, waxes, resins, tannins, etc. participate. During humification, biochemical and chemical processes take place. Of these, the polymerization and condensation reactions of the products of the degradation of primary organic matter into products which are the result of the humification process are important. 

Direct condensation reaction anthocyanin tannin (A-T). In this reaction, anthocyanins act... 

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. 

Botha J J, Viviers P M, Ferreira D, Roux D G 1982 Condensed tannins Competing nucleophilic centers in biomimetic condensation reactions. Phytochemistry 21 1289-1294... 

Wine color modification always accompanies malolactic fermentation. Color intensity decreases and the brilliant red tint diminishes. This modification is due to the decolorizadon of anthocyanins when the pH increases, but condensation reactions between anthocyanins and tannins are probably also involved. These reactions modify and stabilize wine color. 

Condensed tannins of the prodelphinidin, procyanidin and propelargo-nidin series should, however, be amenable to the same direct synthetic approach as outlined in the foregoing [cf. biflavanoid procyanidin formation (72)] although 5-hydroxylation probably introduces an additional steric factor into condensation reactions, thereby reducing yields. 

Botha, J. J., P. M. Viviers, D. Ferreira, and D. G. Roux Condensed tannins Competing nucleophilic centres in biomimetic condensation reactions. Phytochemistry 1982 (in press). 

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. 

Flavan-3,4-diols FIavan-3,4-diols, also known as leucoanthocyanidins, are not particularly prevalent in the plant kingdom, instead being themselves precursors of flavan-3-ols (catechins), anthocyanidins, and condensed tannins (proanthocyanidins) (see Fig. 5.4). Flavan-3,4-diols are synthesized from dihydroflavonol precursors by the enzyme dihydroflavonol 4-reductase (DFR), through an NADPH-dependent reaction (Anderson and Markham 2006). The substrate binding affinity of DFR is paramount in determining which types of downstream anthocyanins are synthesized, with many fruits and flowers unable to synthesize pelargonidin type anthocyanins, because their particular DFR enzymes cannot accept dihydrokaempferol as a substrate (Anderson and Markham 2006). 

Edwards and Jones (31) identified the condensed tannin extracted from cotton plant bracts as a tannin-like polymer of 5, 7, 3, 4 tetrahydroxyflaven 3-4 diol (THF). They demonstrated nonspecific precipitation with IgG, IgM, IgA, five myeloma IgG s and positive gel diffusion reactions with heavy and light chains. Fab and Fc pieces of IgG. Nevertheless, they refuted this reaction as a true antigen-antibody reaction, and subsequently suggested that byssinosis was not an immune complex mediated pulmonary disease. 

Hermingway RW (1989) Reactions at the interflavanoid bond of proanthocyanidins. In Hermingway RW, Karchesy JJ (eds) Chemistry and significance of condensed tannins. Plenum Press, New York, NY... 

Thiolysis also proved useful for the analysis of derived tannins. Methylmethine-linked tannin-like compounds resulting from acetaldehyde-mediated condensation of flavanols (see Section 5.5.S.2) yielded several adducts when submitted to acid-catalyzed cleavage in the presence of ethanethiol, providing information on the position of linkages in the original ethyl-linked compounds. " Thiolysis of red wine extracts released benzylthioether derivatives of several anthocyanin-flavanol adducts, indicating that such structures were initially linked to proanthocyanidins. However, some of the flavonoid derivatives present in wine (e.g., flavanol-anthocyanins ) are resistant to thiolysis, while others (e.g., flavanol-ethyl anthocyanins) were only partly cleaved. Thiolysis, thus, appears as a rather simple, sensitive, and powerful tool for quantification and characterization of proanthocyanidins, but provides mostly qualitative data for their reaction products. 

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