Utilization of Flavonoid Compounds from Bark and Wood: A Review

Abstract

Flavonoid compounds, which are extracted from bark and wood and used commercially, are flavan 3-ols as monomers and their polymers, which are called “condensed tannins”. Reactions of the condensed tannins with formaldehyde are the basis for wood adhesives. In the late 1940s, tannin research for wood adhesives was begun and the world-first commercial use of wattle tannin from black wattle (Acacia mearnsii) bark as wood adhesives occurred in Australia in the 1960s. In addition, wattle tannin-based adhesives were further developed in South Africa and the uses of these adhesives have been continuing to date. The success of wattle tannin in wood adhesives is demonstrated by the collaboration of the ACIAR with the CAF in the early 1990s. Although radiata pine bark (Pinus radiata) could be a useful resource for the production of wood adhesives, three problems prevented its use in this application: low extractive yields from the bark, variable quality of the tannin extracts and excessive viscosity of the formulated tannin adhesives. In order to overcome these problems, various extraction methods have been proposed. Studies on tannin adhesives from bark of other pine species are also described. Furthermore, the use of the tannin in the bark without extraction is described as “bark adhesives” from radiata pine and black wattle. The use of radiata tannin without formaldehyde for moulded wood products is also described. Owing to the strong antioxidant activity of flavonoid compounds, bark extracts from French maritime pine (Pinus pinaster, synonym P. maritima) and radiata pine have been commercialized as nutritional supplements: Pycnogenol and Enzogenol, respectively. The background and the development of Pycnogenol and the basic difference in the preparation processes between Pycnogenol and Enzogenol are described. On the basis of the discovery that the SOSA value for wattle tannin is approximately 10 times that of extracts from pine bark supplements (Pycnogenol and Enzogenol), chemical, biochemical and clinical studies on wattle tannin were conducted. Results from these studies are outlined. Wattle tannin has been developed as a nutritional supplement and marketed as Acapolia in Japan.