You are here TA/TT mechanisms - non-oxidative Creativity and innovation leads to discovery Proposed role of Co-pigmentation Anthocyanins and Procyanidins Tannins and the Structure of the main procyanidin Oak and Oxygen
Aldehyde compounds, extracted from oak,
capable of crosslinking tannins and anthocyanins

Aldehyde compounds

Sugar dehydration products, formed during toasting of oak, e.g. furfural and HMF (shown above), are able to crosslink tannins and tannins with anthocyanins. However, Cheynier's group has shown the aromatic aldehydes, e.g. vanillan and coniferaldehyde (shown above), have not been found to crosslink tannins. Evidently, there is a steric hindrance by the aromatic group. However, the same aromatic group may allow hydrophobic interaction with tannins and anthocyanins, allowing the aromatic aldehyde to act as a co-factor in co-pigmentation.

Crosslinking procyanidins with aldehydes
There are clear structural differences between crosslinked procyanidins and natively formed (or reductive linked) procyanidins. The important point here is that different procyanidin or tannin structures can be formed when acetaldehyde or oak extractives are present.

procyanidins with aldehydes

Tannin-anthocyanin (T-A) /tannin-tannin (T-T)
crosslinking under oxidative conditions


The proposed mechanism of crosslinking between procyanidins and anthocyanidins follows:

  Acetaldehyde has been shown to react first and preferentially with the procyanidin or tannin, not with anthocyanins. However, as Cheynier's group established, the crosslinker could be furfural, HMF and other oak derived components. Secondly, acetaldehyde tannin adduct with dehydrate to form a carbocation. From this point, the reaction is much the same as described previously for reductively formed adducts. Depending on concentrations and proximity for reaction, the carbocation had the choice of reacting with either tannin or anthocyanin. As before, the anthocyanin reacts in the hydrated or colorless form and the tannin acts as the electro-sink to favor elimination of water, thereby forming the flavylium cation and stabilizing the color.

Proposed condensation reactions under oxidative conditions

oxidative conditions

Above is a simplified pictorial of the crosslinking reaction. The main point of interest is the difference in the structure comparing crosslinked vs. reductively formed or native tannin oligomers. The many kinks added into the oligomer or polymer prevent tight association and hence aggregation minimizing precipitation.
  The crosslink formed between the procyanidins is no more stable than those existing in the native procyanidin. Therefore, they are labile and prone to hydrolysis, just as negative oligomers and polymers are. However, Cheynier's group has found that crosslinks formed with furfural, HMF and 5-methyl furfural are more stable (less prone to hydrolysis) than those made with acetaldehyde. Remember that linking an anthocyanin to a tannin prevents further polymerization at that end. We should see a higher percentage of smaller oligomers formed when acetaldehyde or oak derived crosslinkers are present. This should be especially prevalent during fermentation, when anthocyanins are in high concentration and available for reaction.

Acetaldehyde-induced condensation of epicatechin and malvidin-3-glucoside


In model solutions, we should see more smaller polymers formed when enough reactants (procyanidins and anthocyanins) are available. The trimer adduct should be the most stable that is formed.