Tannin interactions

Frazier, R.A., Papadopoulou, A., Mueller-Harvey, I., Kissoon, D., and Green, R.J., Probing protein-tannin interactions by isothermal titration microcalorimetry, J. Agric. Food Chem., 51, 5189, 2003. 

Charlton, A.J. et al.. Tannin interactions with a full-length human salivary protein display a stronger affinity than with single proline-rich repeats, FEBS Lett., 382, 289, 1996. 

It is believed that proteins and tannins interact via hydrogen bonding and hydro-phobic effects. Hydrogen bonds can be formed between the hydroxyl groups of phenolic compounds and carbonyl and amide groups of proteins. Hydrophobic interaction can occur between the benzenic nuclei of phenolic compounds and the apolar side-chains of amino acids such as leucine, lysine, or proline in proteins. Several authors have observed the occurrence of hydrophobic interactions between proteins and tannins [16-20]. 

Protein-tannin interactions have been studied using several techniques, such as NMR [18, 22, 23, 25], microcalorimetry [28, 29], enzyme inhibition [30-32],... 

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]. 

Some studies have been made relating specifically to astringency. Some of these studies have focused directly on interactions between tannins and salivary proteins [21, 23, 25, 46, 50-53], and on the changes in saliva protein composition after interaction with tannins [15, 38, 54, 55]. Other studies have correlated the sensorial astringency with protein-tannin interactions [45, 56, 57]. In fact, the astringency felt when sampling different tannin solutions can be correlated with the ability of these tannins to precipitate proteins. This effect has been observed with several proteins such as mucin [45], ovalbumin and gelatin [57], bovine semm albumin (BSA), and salivary proteins [58]. 

An electronic tongue based on protein-tannin interactions has been developed to measure astringency [42]. 

Apart from the relative concentrations of protein and tannin, protein-tannin interactions are influenced by several other factors. 

Another factor that influences protein-tannin interactions is pH. Apparently there is higher protein precipitation at pH values close to the protein isoelectric... 

Protein-tannin interactions also seem to be affected by the presence of ethanol [2, 62] and other compounds in solution [65] and by temperature [2, 77]. 

Protein-tannin interactions appear to be inhibited by the presence of some carbohydrates in solution [32, 65, 78, 81, 82]. This is particularly evident in the graph of Figure 16.5. 

Two mechanisms have been proposed to explain the influence of polysaccharides on protein-tannin interactions [2, 65] (Figure 16.6) ... 

Flow Nephelometric Analysis of Protein-Tannin Interactions 385... 

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. 

In the last part of the book some examples are reported on the importance of the contribution of chemical studies to fields that are of increasing concern for the public opinion such as health, food, and the environment. One of these describes investigation of the protein-tannin interaction in order to better understand organoleptic properties of foodstuffs, and in particular those of red wine. 

Helm, R.F. Ranatunga, T.D. Chandra, M. 1997. Lignin-hydrolyzable tannin interactions in wood. J. Agric. Food Chem. 45 3100-3106. 

Research work performed in the end of the last century by Haslam and co-workers showed that carbohydrates inhibit the protein-tannin interactions in solution (Luck et al. 1994 Ozawa et al. 1987). More recently, similar findings have been reported by de Freitas and co-workers using nephelometric techniques (Fig. 9D.12) for different carbohydrates and protein-tannin systems (Carvalho et al. 2006a, b de Freitas et al. 2003 Mateus et al 2004a). 

Fig. 9D.13 Mechanism of carbohydrate inhibition of protein tannin interactions (Mateus et al. 2004) (adapted from Mateus et al. 2004a)...

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. 

Apart from salivary proteins, other proteins have been used in the tannin-protein interaction studies due to some characteristics that make them similar to PRPs, like casein, gelatin, polyproline (Jobstl et al. 2004 Calderon et al. 1968 Luck et al. 1994 Poncet-Legrand et al. 2006 Siebert et al. 1996). Although it is not a protein, the polymer polyvinylpolypyrrolidone as also been used in these studies (Hagerman and Butler 1981 Laborde et al. 2006). Recently, an electronic tongue based on protein-tannin interactions has been developed to measure astringency (Edelmann and Lendl 2002). Despite the unquestionable importance of all these works to understand the interaction between tannins and proteins, extrapolation to the real context of wine sensory should be done with care. 

Carvalho, E., Povoas, M., Mateus, N., de Freitas, V. A. P. (2006a). Application of flow neph-elometry to the analysis of carbohydrate influence on protein-tannin interactions. J. Sci. Food Agric., 86, 891-896. 

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