Protein tannin complexes

Process Va.ria.tlons. The conventional techniques for tea manufacture have been replaced in part by newer processing methods adopted for a greater degree of automation and control. These newer methods include withering modification (78), different types of maceration equipment (79), closed systems for fermentation (80), and fluid-bed dryers (81). A thermal process has been described which utilizes decreased time periods for enzymatic reactions but depends on heat treatment at 50—65°C to develop black tea character (82). It is claimed that tannin—protein complex formation is decreased and, therefore, greater tannin extractabiUty is achieved. Tea value is beheved to be increased through use of this process. 

Riedl, K.M. and Hagerman, A.E., Tannin-protein complexes as radical scavengers and radical sinks, J. Agric. Food Chem., 49, 4917, 2001. 

More complex, second order interactions may be imagined, involving more than one natural enemy. For example, consider insects to which tannins are important deterrents and digestion inhibitors. As mentioned above, elevated gut pH appears to be a way of dealing with tannins, since tannin-protein complexes are dissociated or inhibited at alkaline pH (16,32). Indeed, using a model in vitro system in which hemoglobin is employed as a protein substrate, we found that several natural tannins and phenolic extracts do not precipitate this protein when the pH exceeds about... 

Hagerman, A. E. (1989). Chemistry of tannin-protein complexation in Hemingway, R. W., Karchesy, J. J. (Eds), Chemistry and significance of condensed tannins. Plenum Press, pp. 323-331. 

Perez-Maldonado, R. A., Norton, B. W., Kerven, G. L. (1995). Factors affecting in vitro formation of tannin -protein complexes. J. Sci. Food Agile., 69, 291-298. 

The acetate buffer is set at pH 4.9 because 4.9 is the pi of BSA and thus affords maximum precipitation of the tannin/protein complexes formed during the precipitation reaction. Coincidentally pH 4.9 is particularly good for measuring absorbance due to polymeric pigments because the anthocyanins have their minimum absorbance at that pH (d). Since any remaining monomeric anthocyanins are bleached with bisulfite, this procedure assures that all of the remaining 520 nm absorbance is due to polymeric pigments (LPP + SPP). 

The addition of enological tannins only eliminates part of the proteins, and the new tannin-protein complexes in the wine are generally more heat sensitive than the original proteins. 

The model of interactions between tannins and proteins (Figure 6.22) described by Haslam in 1981 is still in use today. In the case of small quantities of proteins, the polyphenols spread over the surface in a single layer, thus decreasing their hydrophilic character. The proteins clump together and, eventually, precipitate. When the protein concentration increases, phenolic compounds spread over their surface act as ligands or cross-linking agents between the various molecules. The superficial hydrophobic layer then recombines and causes the proteins to precipitate. Therefore, the relative concentrations of tannins and proteins affect the formation and precipitation of tannin-protein complexes. 

Fig. 6.35. Simplified method for fractionating various classes of phenolic compounds in wine Al, free anthocyanins TA, tannin-anthocyanin combinations C,P, catechins and little-polymerized procyanidins CT, condensed tannins TP, tannin-polysaccharide and tannin-protein complexes (Glories, 1978)...

TP fraction. The precipitate obtained by adding 9 volumes of ethanol to 1 volume of wine is known as the TP fraction. It consists of salts and polysaccharides, as well as tannin-polysaccharide and tannin-protein complexes with molecular weights above 5000. 

The various parts of grape bunches (stalks, seeds, skins) contain phenolic compounds that may be fractionated (Section 6.4.6) into four groups. The skins have a particularly high concentration of tannin-polysaccharide and tannin-protein complexes that give a nicely rounded impression. On... 

Bentonite has a negative charge that fixes the positive unstable colloids and pulls them down. It is more efficient than cold flocculation of the colloids. The problem is different in the case of protein-based fining agents. Some of the colloids are pnlled down by the flakes of tannin-protein complexes, while the rest are stabilized by residnal proteins that are also part of the wine s colloidal strnctnre. 

Tannin-protein complexation is reversible, provided that covalent bonds are not involved and that both condensation and aggregation are limited. If this is not the case, quinoid intermediaries are formed. These are highly reactive with proteins and the combinations formed are insoluble and irreversible (Heart et al., 1985 Gal and Car-bonell, 1992 Metche, 1993). 

Hagerman, A. E. and C. T. Robbins, Implications of soluble tannin-protein complexes for tannin analysis and plant defense mechanisms, J. Chem. Ecol., 13, 1243-1259 (1987). 

As mentioned before, the amount of soluble tannin that causes astringency in persimmon fruits is usually estimated visually by the tannin print method and can be measured quantitatively by the Folin-Denis method. There is also a protein precipitation method for the measurement of soluble tannins (Hagerman and Butler 1978). In that method, the soluble tannin content is assayed by the addition of the sample to a standard solution of protein and the isolation of insoluble tannin-protein complexes. The complexes are dissolved in alkaline solution, to which ferric chloride is added. The absorbance of the solution at 510 nm is measured. 

Jones et al. (1973) also reported a complete absence of soluble proteins in the rumen of cattle grazing on species that contained condensed tannins. In an another study, Jones and Mangan (1977) observed that the site of nitrogen metabolism is transferred from the rumen to the intestine of sheep when they are fed sainfoin with condensed tannins, which include the formation of tannin-protein complexes that are stable in the rumen (pH 6.5) but not in the abomasum-duodenum (pH 2.5). 

Although tannins appear to protect protein from microbial attack in the rumen, their ultimate effects on ruminant nutrition may depend upon how available this bypass protein is to the animal. According to Price and Butler (1980), the same factors that cause tannins to have a deleterious effect on monogastric nutrition, will presumably be important in the post-rumen digestive tract. If the tannin-protein complex does not dissociate in the abomasum or intestine, there will be no benefit to the animal from the protein having been protected in the rumen. If it does dissociate, the liberated tannin may damage the intestinal tract or form new complexes at some point with endogenous proteins. 

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