Ro Y Or Or

Figure 5. Camelliin B.

the protein and the hydroxyl groups of the polyphenol. The polyfunctional tannin molecule is able to bind at many sites on the surface of the protein molecule, thus increasing its hydrophobic nature and promoting precipitation. Inhibition of enzymic activity is often a result of tannin-protein complexation. At high protein concentrations the interaction results in cross-linking. There is also evidence for some hydrophobic bonding.

The formation of the tannin-protein complex is pH dependent and reaches a maximum at the isoelectric point of the protein. Most complex formation is reversed at pH >9. Solvents, such as acetone, that tend to form hydrogen bonds competitively inhibit the interaction. Polyphenol molecules with a high degree of flexibility are most reactive. Proanthocyanidins are more rigid than the gallotan-nins and are less active complexing agents on a molar basis.

Tannin-protein complexation is favored as the polyphenol solubility is decreased. Proteins with a high proline or hydroxyproline content, such as gelatine, have a greater affinity for polyphenols as these amino acids impart flexibility to the protein molecule and form strong hydrogen bonds. The tendency of a protein to bind to a tannin may be determined by the degree to which the protein competitively prevents inhibition of a specific enzymic activity by the tannin.

Complexation and precipitation of caffeine by tannins has been recognized as the most important cause of "cream" formation when infusions of black tea are allowed to cool. The enzyme tannase catalyzes the cleavage of the gallate linkage in tannin molecules and has been used to solubilize caffeine-polyphenol adducts during preparation of instant tea (see the article Tea). It may be induced in large quantities in some strains of Aspergillus molds. Caffeine-polyphenol systems have been used as models for the investigation of tannin—protein interactions because there is a resemblance between the molecular structures of caffeine and peptides.

The tanning of animal hides with vegetable extracts accounts for only a small proportion of the leather now produced in the United States. The use of chromium salts has supplanted the traditional process for most applications. Vegetable tannage depends on packing the amorphous regions of collagen fibers with large quantities of tannins so as to bring about extensive cross-linking. The collagen may adsorb up to 50% of its weight of tannins. Tanning renders the mass impermeable to bacterial attack, decreases water penetration, and increases tensile strength. Vegetable-tanned leather has superior qualities of resilience. The extracts used are derived from oak galls, chestnuts, and the bark and heartwood of trees such as quebracho, gambier, and mimosa. Wastewater from tanneries is highly contaminated. Its purification before discharge is made difficult by the inhibitory action of residual tannins on the microbiological organisms normally utilized for anaerobic fermentation (21).

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