Proteins tannins and

Figure 16.8 shows a typical flow nephelometric graphic involving the interactions between three substrates protein, tannin, and carbohydrate. In an initial phase there is an increase in turbidity corresponding to the stabilization of protein and tannin concentrations. The maximum value of turbidity corresponds to the TSA of the tannin solution in relation to the protein (BSA). In a second phase a gradient of increasing carbohydrate concentration is started, leading to a decrease... 

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). 

Minerals, particularly Bentonite, ate used to remove proteins that tend to cause haze in white wines. The natural tannin of ted wines usually removes unstable proteins from them. Excess tannin and related phenols can be removed and haze from them prevented by addition of proteins or adsorbents such as polyvinylpyttohdone. Addition of protein such as gelatin along with tannic acid can even be used to remove other proteins from white wines. Egg whites or albumen ate often used to fine ted wines. Casein can be used for either process, because it becomes insoluble in acidic solutions like wines. 

Investigations have focused on the content of polyphenoHcs, tannins, and related compounds in various foods and the influence on nutrient availabiHty and protein digestibiHty. It has been estabHshed that naturally occurring concentrations of polyphenoloxidase and polyphenols in products such as mushrooms can result in reduced iron bio availabiHty (75). Likewise, several studies have focused on decreased protein digestibiHty caused by the tannins of common beans and rapeseed (canola) (76—78). 

The mechanism of the tarmage is accepted to be largely one of replacement of the bound water molecules by the phenoHc groups of the tannin and subsequent formation of hydrogen bonds with the peptide bonds of the protein. The effect of this bonding is to make the leather almost completely biorefractive. 

Filtering. Conditioning or 1 agering gives the beer its desired organoleptic properties, but it stiU contains yeast, protein-tannin complexes, etc, ie, it has a hazy appearance. A high quaHty beer must be clear and totaUy sterile, have coUoidal stabiHty, and yeast must be removed to aUow the beer to have biological stabiHty. The protein-tannin complexes must also be removed so as not to upset the coUoidal stabiHty. 

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). 

Like alfalfa, the clover family (Trifolium spp.) consists of perennial legume plants that fix their own nitrogen. However, they suffer several limitations as general production crops for molecular farming, such as restricted perenniality, low protein content and the presence of high levels of condensed tannins which interfere with protein extraction. Despite these disadvantages, clovers are forage crops and are there-... 

Some animals process plants to reduce defense compounds even before eating. The meadow vole M. pennsylvanicus cuts winter branches of white spruce, Norway spruce, white pine, and Norway pine and leaves them on the snow for 2-3 days before eating them. This reduces the levels of condensed tannins and other phenolics by one half, to their summer levels. A high level of protein 12%) and reduced phenolics (1.5% of dry matter) now render the food acceptable. It is not clear how the phenolics are being lost, possibly by polymerization or oxidation (Roy and Bergeron, 1990b). 

Dietz, B. A., Hagerman, A. E., and Barrett, G. W. (1994). Role of condensed tannin on salivary tannin-binding proteins, bioenergetics and nitrogen digestibility in Micro-tuspennsylvanicus. Journal of Mammalogy 75,880-889. 

Mehansho, H., Butler, L. G., and Carlson, D. M. (1987). Dietary tannins and salivary proline-rich proteins interactions, induction, and defense mechanism. Ann Review of Nutrition 7,423-440. 

Very little information exists on the phenolic protein-conplexing resins, except for that on creosote bush and sane arctic plants. In contrast to condensed tannins and hydrolyzable tannins, these are typically ether-soluble compounds this may allow their entry into the body across cell membranes, and thios give them the potential for action outside the gut lumen. Reports for animals consuming seaweed indicate that astringent protein-precipitating substances (presumably phlorotannins), are potentially important in marine plant-herbivore 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. 

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... 

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