Abstract. A rapid thiolytic degradation and clean-up procedure was developed for analyzing tannins directly in chlorophyll-containing sainfoin (Onobrychis viciifolia) plants. The technique proved suitable for complex tannins mixtures containing catechin, epicatechin, gallocatechin and epigallocatechin flavan-3-ol units. The reaction time was standardized at 60 min in order to minimize loss of structural information due to epimerization and degradation of terminal flavan-3-ol units. The results were evaluated by separate analysis of extractable and unextractable tannins, which accounted for 63.6 to 113.7% of the in situ plant tannins. It is of note that 70% aqueous acetone extracted tannins with a lower mean degree of polymerization (mDP) than was found for tannins analyzed in situ. Extractable tannins had between 4 to 29 lower mDP-values. The method was validated by comparing results from individual and mixed sample sets.

Tannin composition of different sainfoin accessions covered a range of mean degrees of polymerization (mDP) from 16 to 83, procyanidin/prodelphinidin (PC/PD) ratios from 19.2/80.8 to 45.6/54.4 and cis/trans ratios from 74.1/25.9 to 88.0/12.0. This is the first high throughput screening method, which is suitable for analyzing condensed tannin contents and structural composition directly in green plant tissue.