Tannins gallic acid based

Scalbert et al. (1989) described several methods to determine the concentration of hydrolysable and condensed tannins (proanthocyanidins see Chapter 1, section 3.13.1) in wood extracts. Tannins are complex and heterogeneous In addition to the distinction between the flavonoid-based condensed tannins and the gallic acid-based hydrolysable tannins, each group can display a large degree of chemical variability that can affect the efficacy of the different assays. Interference of chemically related nontannin compounds, such as flavonoids, can in certain cases bias the results. 

Singleton and Esau (I) reviewed the methods for phenol analysis of wine. They pointed out that study would be greatly advanced if one could determine the total content of phenolic substances and express it in such a way that analysis of subclasses of phenols could be related to the original total and a balance sheet could be obtained. One could then say, for example, this wine has a total phenolic content of 1200 mg/liter calculated as gallic acid, and of that total, cinnamic acid derivatives account for 200 mg/liter, anthocyanins for 300 mg/liter, other small flavonoids for 200 mg/liter, and condensed tannins complete the total with 500 mg/liter of gallic acid equivalent. To accomplish this, the total phenol analysis not only must meet ordinary criteria of reproducibility and precision, but it also must be based on chemical relationships such that fractions determined separately can be converted to units of the total. Of course when clearcut fractionation can be accomplished by... 

C.I. Natural Brown 6 (constitution not available), basic structure gallic acid (gallotannin 1), is based on a large variety of vegetable products having a high content of hydrolyzable tannin, which mostly form gallic acid when hydrolyzed. 

In the adsorption of some metal ion by tannin adsorbents [16], tannins are widely distributed in nature and have multiple adjacent phenolic hydroxyl groups and exhibit specific chelation ability toward metal ions [17]. According to the chemical structures of tannins, they can usually be classified into hydrolyzable tannins, condensed tannins and complex tannins. Hydrolyzed tannins yield gallic acid or eUagic acid when hydrolyzed by acid, base or some enzymes [18]. Turkish sumac tannin (hydrolyzable tannin) is illustrated in Fig. 28.1 whose basic structure is of flavan-3-ols. 

When tannins are extracted into hot water, some of these compounds are partially hydrolyzed to form free gallic acid. The taimins, because of their phenolic groups, and gallic acid, because of its carboxyl groups, are both acidic. If sodium carbonate, a base, is added to tea water, these acids are converted to their sodium salts that are highly soluble in water. 

Most hydrolyzable tannins contain a central core of glucose or other polyhydric alcohol esterified with gallic acid (gallotannins) or hexahydroxydiphenic acid (ellagitannins) (Fig. 2). The latter is isolated as its stable dilactone, ellagic acid. These Types of tannins are readily hydrolyzed by acids, bases or certain enzymes. 

Vegetable tans, however, are based on wood, bark, leaves and roots of mimosa, quebracho, chestnut and oak. - Tannin is the active ingredient, a polyphenol (sugar derivative of gallic acid), which binds to the collagen (- proteins) by hydrogen bonds. Due to a m.w. of 500-5000, the penetration into the fibrils is slower than with chromium tans and results in heavier and fuller leathers. 

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