Tannins colloidal nature

In recent years the importance of the marked colloidal nature of tannin extract solutions has come to the fore [27,36-45]. It is the presence of both polymeric carbohydrates in the extract as well as of the higher molecular fraction of the polyphenolic tannins which determines the colloidal state of tannin extract solutions in water [26,36]. The realization of the existence of the tannin in this particular state affects many of the reactions that lead to the formation and curing of tannin adhesives, to the point that reactions not thought possible in solution become instead not only possible but the favored ones [26,36], while reactions mooted to be of determinant importance when foimd on models not in the colloidal state have in reality been shown to be inconsequential to tannin adhesives and their tannin applications [43,44]. 

One of the prime environmental objectives is the removal of suspended contaminants from water and waste streams. Water turbidity in nature is the result of colloidal clay dispersion and the color is from decayed wood and leaves (tannins and lignins) and organic soil matter. In addition to these contaminants, there are viruses, algae, bacteria, metal oxides, oils and other pollutants. In recent years, synthetic organic polyelectrolyes, in particular the cationic polymers, have been used very effectively in water treatment (3). These polyelectrolytes are high... 

It is now more readily accepted that this denatu-ration results from the adsorption of tannins without dehydration (Kawamoto and Nakatsubo, 1997) (Figure 9.5). The new complex is an electronegative hydrophobic colloid. It remains stable in a clear solution if no salts are present, otherwise it flocculates. In the same way, natural wine proteins are denatured by heating and flocculate as the liquid cools. 

Most wines certainly contain mucilaginous substances that act as protective colloids mannoproteins. Their existence is demonstrated by the elimination of the protective effect after fine ultrafiltration or dialysis. This phenomenon is well known in red wines, where colloidal coloring matter and tannins inhibit tartrate precipitation. It also exists in white wines and may be attributed to neutral polysaccharides (gum). According to the desired result, these substances may either be eliminated by fine filtration (e.g. to facilitate tartrate stabilization) or, on the contrary, protective colloids such as gum arabic may be added to a clear wine just before bottling to compensate for insufficient natural protection. 

The anulability of silica may be relatively high in soils high in humus. There is reported to be an area in Germany where soil contains silidc add in combination with the tannin-like components of humus which is stable at pH 9-10 (144). Rice plants take up. more silica from the growth medium when poly (2-vinylpyridine oxide) is present (145). This is a material which reportedly reduces the toxidty of colloidal or polysilidc add. The nature of the complex formed, if any, is not understood (Chapter 1) unless it is only a pH effect. 

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