Proteins reaction with tannins

Drawing on this and the reported specificity of tannin-protein interactions ( ) leads to the conclusion that any useful in vitro modelling of the impact of tannins on digestion must consider more than pH and the concentrations of the buffer, enzyme, substrate, and tannin. The actual enzyme-substrate system must be nutritionally realistic to control for specificities of the reaction of tannins with proteins (including enzymes) gastrointestinal mucoproteins should perhaps also be included on the same grounds. Besides all this, misleading results nay still be obtained if bile surfactants are omitted from the equation. 

Proteins react with polyphenols such as phenolic acids, flavonoids, and tannins, which occur widely in plant products. These reactions may result in the lowering of available lysine, protein digestibility, and biological value (Hurrell 1984). 

Reactions Involving Tannins with Protein and Polysaccharides... 

According to Lea (1992) the reaction between tannins and proteins depends on the degree of polymerization of the procyanidins. Astringency increases up to heptamer level and then decreases, as the molecules are too bulky. Maximum bitterness occurs with tetrameric procyanidins. These findings were confirmed by Mirabel (2000), showing that the difference between bitterness and astringency varied widely from one taster to another and that the distinction was not clear. 

Furthermore, at low protein concentrations, polyphenols bond to the surface of the protein at one or more sites, forming a monolayer that is less hydrophilic than the protein alone. This is followed by aggregation and precipitation. When the protein concentration is high, an identical phenomenon occurs, with the superposed formation of cross-bonds between various protein molecules. This explains the non-stoichiometry of the tannin-protein reaction observed by many authors (Ribereau-Gayon et al., 1977). 

Compared to previous gluten based adhesives the present resins are applicable in liquid form, thus without any need for modifications of the application systems in particleboard factories. Relative to other protein adhesives such as soy-based adhesives based on reaction with formaldehyde some of the resins presented here have several advantages (i) they cannot and do not produce any aldehyde emission as neither formaldehyde nor any other volatile aldehyde was used in some of the formulations (ii) the percentage of natural materials was increased up to 70% for one type of formulation and up to 95% for others. Furthermore, in relation to resin formulations based on different cross-linking reactions other than those with formaldehyde the resins presented here have other advantages they are competitive with alternate natural resin systems such as those based exclusively on tannins and/or lignins. 

Proanthocyanidins have different physiological effects. Recently acknowledged are their anti-inflammatory and anti-aUergic effects, and beneficial effects in the development of atherosclerosis (the effect is related to their reaction with free radicals). Condensed tannins have also found use as food additives. A diet that is high in tannins may, however, also show negative effects, such as lower utihsation of proteins. 

The natural moisture of the cocoa bean combined with the heat of roasting cause many chemical reactions other than flavor changes. Some of these reactions remove unpleasant volatile acids and astringent compounds, partially break down sugars, modify tannins and other nonvolatile compounds with a reduction in bitterness, and convert proteins to amino acids that react with sugars to form flavor compounds, particularly pyrazines (4). To date, over 300 different compounds, many of them formed during roasting, have been identified in the chocolate flavor (5). 

The conversion of animal hides into leather by treatment with water-soluble plant extractives has been practiced since antiquity. This process became known as tanning and obviously involved the reaction of a naturally occurring extractive, tannin, with the protein in the hide. We now know, of course, that tannins comprise a whole spectrum of chemical compounds, but generally they are polyphenolic and polymeric. Tannins have been isolated from a wide variety of raw materials, including insect galls, fruit skins, seed hulls, leaves, bark, and heartwood. Indeed, tannins are of nearly ubiquitous occurrence in higher orders... 

Tannins are widely distributed in plant flora. They are phenolic compounds of high molecular weight. Tannins are soluble in water and alcohol and are found in root, bark, stem and outer layers of plant tissue. Tannins have a characteristic feature to tan, i.e. to convert the things into leather. The tannins are acidic in reaction and it is attributed to the presence of phenolic or carboxylic group. Tannins form complex with proteins, carbohydrates, gelatin and alkaloids. 

Tannins are a phytochemically heterogenous group of natural products, the most important common property of which is their traditional ability to form insoluble compounds with proteins. This reaction is the basis for their extensive use in leather production. The same property is the reason for their traditional therapeutic use, which includes the treatment of diarrhoea, bleeding gums, and skin injuries through an astringent effect. 

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 procedure in the polygon in Figure 1 is the Hagerman Butler assay for tannin. The only modification is that we retain the supernatant after centrifugation of the precipitation reaction mixture and bleach the monomeric anthocyanins with bisulfite. The residual absorbance represents pigments that do not precipitate with protein and do not bleach in the presence of bisulfite. We have designated this material small polymeric pigment solely to denote these two characteristics. The LPP removed from the sample by protein precipitation... 

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