Chemical characterization and efficient extraction of condensed tannin from the bark of juvenile Salix species

Abstract

Short-rotation harvesting of Salix species has been studied for potential biomass resources because of their great bud flush. The chemical characteristics and extractability of bark extractives, especially condensed tannin, from 1-2- and 3-5-year-old Salix pet-susu and Salix sachalinensis trees were investigated. Among the results, the yields of 70% acetone aqueous extractives from the bark of S. pet-susu and S. sachalinensis were 29.0% and 25.8% (based on oven dried bark), respectively; and the tannin polymer contents of the extractives of S. pet-susu and S. sachalinensis were 43.4% and 39.4% (based on 70% acetone aqueous extractives), respectively. The chemical characteristics of the tannin polymers were determined by ¹³C-NMR, Py-GC/MS, and GPC analyses, and the tannins of S. pet-susu and S. sachalinensis consisted mainly of procyanidin and prodelphinidin. The approximate degree of polymerization was calculated as 6 (S. pet-susu) and 5 (S. sachalinensis). From the results of total extractives with water extraction at various temperatures, and with organic solvent extraction, the highest yields of total extractives and total polyphenols were obtained with 100 °C water extraction. On the other hand, those of total flavanols were observed with 70% acetone extraction. In order to consider an alternative treatment for debarking and grinding treatment, crush treatment was conducted, and the results indicated that such treatment raised the yields of total extractives, total polyphenols, and total flavanols to those from powder form. Additionally, it is evident that a 2-hour extraction time was the most effective to obtain condensed tannin from the crushed sample of S. sachalinensis and a 1-hour extraction time was adequate in the case of S. pet-susu. Based on the results, we conclude that short-rotation harvested Salix trees are potentially valuable natural resources for useful chemicals, especially condensed tannin.