Measurement of low molecular weight tannins: Indicators of methanogenic toxic tannins

The effectiveness of several low molecular weight (MW) tannin measurement methods for indicating the tannin toxicity to methan bacteria was evaluated. The methanogenic toxicity of the low and high MW tannins from autoxidized bark extracts was studied by selective removal of MW fractions from the extract with active carbon adsorption and calcium precipitation treatments. The toxicity of the low MW tannin fraction and the nontoxicity of the high MW tannin fraction were demonstrated. The low MW tannin concentration, measured by HPLC and a method based on the loss of tannins by treatment with granular active carbon (AC), had a very close relationship with the methanogenic toxicity, but a poor relationship was found with the total tannin concentration. The low MW tannins detected by the HPLC and AC methods had similar peak area positions in HPLC chromatograms as the tannins that were adsorbed by polyamide (trisacryl GF05) gel beads. These gel beads have an exclusion limit of 3000 g·mol^-1, indicating that this is the approximate MW boundary between toxic and non-toxic tannins.