Different approaches to evaluate tannin content and structure of selected plant extracts – review and new aspects

Tannins occur in many field herbs and legumes, providing an immense variability in structure and molecular weight. This leads to complications when measuring tannin content; comparability of different methods is problematic. The present investigations aimed at characterizing four different tannin extracts: quebracho (Schinopsis lorentzii), mimosa (Acacia mearnsii), tara (Caesalpinia spinosa), and gambier (Uncaria gambir) and impact of storage conditions. Using photometrical methods as well as HPLC-ESI-MS, fundamental differences could be determined. Quebracho, mimosa, and gambier contained 164.3, 108.2, and 169.3 g kg-1 of tannin (calculated as procyanidin C1); tara reached 647.5 g kg-1 (calculated as epigallocatechin gallate). Alongside with compounds already described in the literature, several tannin molecules were found that have not been observed before in the analyzed sources. Extraction with hot water provided clear advantage over treatment with acetone or methanol; the organic solvents resulted in 9.2 to 15.3 % less tannin isolation. Tannin content decreased by a maximum of 1 % per year stored at room temperature compared to 4 °C, but proportions of some compounds slightly shifted. Oven drying of material should be avoided. In general, the tannin extracts proved to have very diverse structures, making application of an overall standard method difficult.