Precipitating tannin

Despite their very short sequence (7 to 38 amino acid residues for the 12 histatins identified so far), the histidine-rich salivary protein histatins have also been reported to precipitate tannins, eventually more efficiently than proline-rich proteins, especially at neutral pH and high tannin concentration. A detailed NMR analysis of the binding between EGCG and histatin 5, a 24-mer that is very rich in basic His, Lys, and Arg residues ( 60%) and devoid of secondary structure, has revealed noncooperative binding of six to seven flavanol molecules with a dissociation constant of 1 mM (pH 3.0, 25°C). ... 

Salivary histatins have a high ability to precipitate tannins [18, 40, 71]. Histatins are histidine-rich proteins that can be found in both sublingal/submandibular saliva and parotid saliva and represent about 2.6% of the total salivary proteins. It has been proposed that histatins could be important in neutralizing tannins in situations where they come not from the diet but from other situations when the saliva is not stimulated, such as when cotton workers breathe tannin-rich dust [181-... 

All salivary PRPs have shown affinity for dietary tannins [43] and are easily precipitated [15, 40]. Basic PRPs seem to be more effective in precipitating tannins when compared to acidic and glycosylated PRPs [96]. 

Testa for Albumen. A solution of bichloride id mercury dropped into u fluid containin< albumen, occasions a white precipitate. Tannin or tincture of galls gives a yellow, pitchy precipitate. 

Niobic Acid. Niobic acid, Nb20 XH2O, includes all hydrated forms of niobium pentoxide, where the degree of hydration depends on the method of preparation, age, etc. It is a white insoluble precipitate formed by acid hydrolysis of niobates that are prepared by alkaH pyrosulfate, carbonate, or hydroxide fusion base hydrolysis of niobium fluoride solutions or aqueous hydrolysis of chlorides or bromides. When it is formed in the presence of tannin, a volurninous red complex forms. Freshly precipitated niobic acid usually is coUoidal and is peptized by water washing, thus it is difficult to free from traces of electrolyte. Its properties vary with age and reactivity is noticeably diminished on standing for even a few days. It is soluble in concentrated hydrochloric and sulfuric acids but is reprecipitated on dilution and boiling and can be complexed when it is freshly made with oxaHc or tartaric acid. It is soluble in hydrofluoric acid of any concentration. 

Treatment of the Wort. The hot wort produced in the brewhouse cannot be transferred directly to the fermenting room. If natural hops are used they must be separated by a hop strainer as shown in Figure 8. During boiling, protein—tannin complexes are precipitated in the form of warm sludge. 

These condensed tannins and their derivatives, all of high molecular weight, function as anionic polyelectrolyte sludge conditioners, tending to sequester hardness salts and hinder their precipitation as crystalline scales. In addition, when precipitation does occur, the condensed tannins coagulate the particles, resulting in a mobile sludge that can be easily blown down. 

Where sodium sulfite is added as a component of multifunctional or one-drum products designed for smaller boilers, no cobalt catalyst is added because of the cobalt alkaline precipitation problem. Consequently, if the FW temperature is low this type of formulation is unsuitable because the sulfite requirement will be too high and the available reaction time too short. Probably a tannin-based, one-drum product would be more suitable (although here again there may be a problem because tannin-based products, unlike sulfite cannot be mixed with amines). 

Sami-Manchado, P. et al.. Analysis and characterization of wine condensed tannins precipitated by proteins used as fining agent in enology. Am. J. Enol. Viticult., 50, 81, 1999. 

Tannins are one of several antinutritional factors present in dry beans. Any polyphenolic compound that precipitates proteins from an aqueous solution can be regarded as a tannin (11). Tannins precipitate proteins due to functional groups that complex strongly with two or more protein molecules, building up a large cross-linked protein-tannin complex (12). 

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