Tannin condensed, identification

Interest in use of condensed tannins as components of adhesive formulations began about three decades ago. While research studies have been carried out in widely scattered laboratories around the world, three major areas of activity can be distinguished. These are 1) development of bark extracts and commercial production facilities on the west coast of North America, 1953 to 1975 2) application of tannins in adhesive formulations in South Africa based on indigenously produced mimosa (wattle) tannin, early 1970 s to the present and 3) a resurgence of interest in pine bark as raw material for tannin-based adhesives, beginning in the middle 1970 s. Each of these activities has been characterized by parallel efforts on structural identification of the tannins and development of unique methods for incorporating the isolated tannins into adhesives. 

Characterization of hydrolyzable tannins can be accomplished by hydrolysis and identification of the fragments by standard techniques such as cochromatography on paper, TLC or HPLC and NMR, UV, and MS (Karchesy et al., 1989 Okuda et al., 1989a Porter, 1989). Condensed tannins can be cleaved in mild acidic solution to form a flavan-3-ol and a quinone methide. The quinone methide can be captured with a suitable nucleophile (e.g., -SCH2Ph) to form 4-thio-benzyl derivatives and permit determination of the constituent flavonoid units (Porter, 1989). The 4-thiobenzyl derivatives may be reduced with Raney-nickel to yield flavan-3-ols (Porter, 1989). 

The major phenolic compounds are the anthocyanins and tannins (proantho-cyanidins or condensed tannins) [9-12]. The identification of these compounds and their distribution in the grape berry have been investigated. 

Since Somers published his first observations on anthocyanin-tannin condensations, many advances have been made in the identification of new... 

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