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Hydrolysis of Procyanidins 

Historically, winemakers thought the distribution of various sized, condensed tannins extracted from seeds and skins depended to a high degree on maturity of the grapes. However, Haslam first suggested in 1980 and later wrote in a 1988 review paper, that condensed tannins (polyphenolics) are not stable to hydrolysis. The conditions found in wine (low pH, high acidic conditions) favor hydrolysis at the 4–>8 positions or the 4–>6 positions.   Why is the phenomena important?

Hydrolosis of Procyanidins

Proposed mechanisms leading to T-A and T-T adducts 
under non-oxidative conditions. 
Hydrolysis of tannin oligomers or polymers is the central point a mechanism proposed by Véronique Cheynier under reductive conditions. This mechanism helps to explain compounds isolated from red wines and how these compounds may be formed. As Haslam has suggested, a tannin molecule, e.g. dp 8 (8 catechin or epicatechin units) in length, may be hydrolyzed under acidic conditions in wine. For example, as shown in the figure below, this hydrolysis may produce two shorter oligomers, dp 4 in length. However, the hydrolysis produces one neutral oligomer and one positively charge oligomer (carbocation).
  Depending on the concentrations of available tannins or anthocyanins, the carbocation formed will react with one or the other., If the reaction is with another tannin a longer oligomer or polymer will be formed.

tannin molecules

  However, the process differs if an anthocyanin is involved. First, note that the anthocyanin must be in the hydrated or colorless form. This form provides an electron-rich molecule which more readily reacts with the formed carbocation.
  The reaction occurs between the two molecules at the 4 and 8 positions and a covalent bond is formed. Once formed, the tannin part acts as an electron sink and favors the loss of water (the hydration of the anthocyanin) and a stabilized color or anthocyanin-tannin adduct is formed.
  Losing the water of hydration has another unique aspect: the terminal molecule (the anthocyanin) no longer has an excess of available electrons. Thus, the anthocyanin acts as a terminus or terminal quencher for any further reaction at this end of the oligomers or polymers being formed.

Proposed condensation reactions under non-oxidative conditions 

non-oxidative conditions

Here is a more simplistic representation of the previous section. Reactions under reductive conditions occur more slowly, depending on conditions and availability of reactants.
  However, scientists think that tannins formed under these conditions will be longer, with a more uniform structure. The longer and more uniform the tannins, the more likely there will be a strong interaction between the tannin molecules.
  This interaction causes an association and aggregation of similar molecules attempting to protect hydrophobic centers (the aromatic rings). Hydroxyls located on the outer portions of the polymers will hydrogen bond, aiding in the stabilization of the molecule, but at the same time excluding water molecules.
  Once the aggregate of molecules becomes large enough and excludes enough water it will lose its ability to stay soluble and precipitate from solution.
  We see examples of this during extended maceration, especially under highly reductive conditions. At first, it appears that we are extracting more tannin and color. Yet within one to three months, most of this material precipitates from the wine and color is lost.

Summary of reactions under non-oxidative conditions 
1. Under reductive conditions (no crosslinking by acetaldehyde). A distribution of procyanidin oligomers and polymer sizes will tend to redistribute to a median size. For example, range of sizes of dp 2 to 15, will tend to favor formation of a median size such as dp 7.

2. Anthocyanins will interfere with this process by acting as terminal quenchers. Once an anthocyanin has linked to a terminal end of a polymer or oligomer no further reactions with that molecule will occur, other than losing the sugar moiety.

In model wine solutions, with no interfering compounds, a distribution of procyanidin oligomers should, over time, form an average size molecule.
  However, if interfering compounds such as anthocyanins are present, then the distribution of molecule sizes will be much more unpredictable in manner.