Protein-tannin aggregates

Phenolic compounds also contribute directly to the flavor due to their astrin-gency and bitter taste characteristics [11]. In fact, astringency is believed to be due to the interaction between tannins and salivary proteins, resulting in the formation of protein-tannin aggregates in the mouth, as discussed in more detail below [12-15],... 

Polysaccharides, being polyelectrolytes, could form ternary complexes with the protein-tannin aggregate, enhancing its solubility in aqueous medium. 

Some MP fractions obtained from wine also had the ability to inhibit protein-tannin aggregation (unpublished results). Polysaccharides showed effects at concentrations at which they are present in wine, which means that they could have an influence on wine astringency. 

Fig. 9D.11 Schematic interaction between tannins and proteins a main driving forces between phenolic rings (cross-linkers) of tannins and the amide groups and apolar side chains of antino acids such as proUne b protein-tannin aggregates the grey Ss represent proteins with a number of tannin binding places, and the black arrows represent tannins with protein binding sites...

In general, the effectiveness of carbohydrates to prevent protein-tannin aggregation increase with their ionic character (Luck et al. 1994 Carvalho et al. 2006b de Freitas et al. 2003) neutral carbohydrates practically do not affect aggregation. 

Mateus, N., Carvalho, E., Luis, C., de Freitas, V. A. P. (2004a). Influence of the tannin structure towards the disruption effect of carbohydrates on protein-tannin aggregates. Anal Chim. Acta, 513, 135-140. 

De Freitas, V. Carvalho, E. Mateus, N. Study of carbohydrate influence on protein-tannin aggregation by nephelometry. Food Chem. 2003, 81, 503-509. 

Freitas VD, Mateus N (2002) Nephelometric study of salivary protein-tannin aggregates. J 8ci Food Agric 82 113-119... 

However, the most intuitive way of measuring aggregate formation in solution is the measurement of the light scattered by protein-tannin particles. In fact nephelometry and other light-scattering measurement techniques have been widely used to study protein-tannin interactions [33, 34, 44—49]. 

This does not seem to occur in all cases. Some authors have observed that with an increase in tannin/protein ratio there is the formation of large particles that eventually precipitate. A three-phase model has been proposed to explain this phenomenon [33, 51]. The simultaneous binding of the multidentate tannin to several places in the protein leads to the protein wrapping around the tannins. As the concentration of tannin increases, several tannin molecules bind to the protein surface and crosslink with other proteins, leading to protein association. As more tannin is added, more protein-tannin complexes aggregate, forming larger particles that precipitate. 

This decrease in aggregation could be due to ion adsorption on the tannin or protein surface restraining the interaction, or to adsorption of ions onto the surface of the protein or protein-tannin complex leading to higher solvation. 

Studies performed over the years have contributed to better understanding of the interactions between proteins and tannins, which are important not only due to their astringency but also because of their impact on food nutritional characteristics, on human health, and on plant metabolism. It is clear that protein-tannin interactions are influenced by several factors, among which polysaccharides could be important because they are also present in tannin-rich vegetables. Much remains to be studied in this field, particularly the specific phenomenon that occurs between proteins, tannins, and polysaccharides that leads to a decrease in aggregation, and further studies are needed involving other salivary proteins and digestive enzymes. 

The affinity of tannins to bind proteins is favored by their ability to work as multidentate ligands (cross-linking) in which one tannin is able to bind to more than one protein at one time (Fig. 9D.llb) or to bind to more than one point in the same protein (Charlton et al. 2002a Siebert et al. 1996). These associations between tannin and protein could result in aggregates that precipitate depending on the ratio of protein/tannin and also the concentration of protein (Frazier et al. 2003 Poncet-Legrand et al. 2006). 

Recently, Carvalho et al. (2006b) studied the influence of wine polysaccharides (AGP, RGll and MP) on salivary protein-tannin interactions. The results showed that the most acidic fractions of AGPs and MPs have the ability to inhibit the formation of aggregates between condensed tannins and two different salivary proteins (a-amylase and lB8c). The concentrations tested are below to those present in wine which means that they could have an influence in wine astringency. 

Figure 16.6 Possible mechanisms (i and ii) involved in the inhibition of the aggregation of tannins (T) and proteins (P) by carbohydrates (C) [82],...

Previous works have shown that a basic PRP (IB8c) binds to condensed tannins much more effectively than a-amylase (de Freitas and Mateus 2002). This can be explained by the 3D structure of proteins a-amylase is a globular protein, and IB8c is likely to adopt an extended random coil conformation, which would allow the protein to offer more contact sites to interact with tannins. However, a-amylase seems to be more specific and selective than PRPs in the aggregation with samples containing different amounts of procyanidins (Mateus et al. 2004c). 

The formation of aggregates of tannin particles, or tannins and proteins, may be inhibited by the presence of polysaccharides (macromolecular... 

---