Characterisation of tannins and in vitro protein digestibility of several Lotus corniculatus varieties

Seven birdsfoot trefoil (BFT) varieties (Lotus corniculatus) grown in Sweden, were harvested at the 50% flowering stage and analysed for tannins by the radial diffusion and HCl-butanol methods. The flavan-3-ol composition of different BFT tannins was determined by HPLC. Tannins were isolated and examined for their molecular weight distributions by HPLC gel permeation chromatography (GPC) and MALDI-TOF mass spectrometry. Ruminal protein degradability was determined in vitro and related to tannin chemistry. Tannin concentrations of the BFT varieties were generally low and ranged between 0.3-1.0% (radial diffusion assay) and 0.2-1.7% (HCl-butanol assay) on a DM basis. The delphinidin:cyanidin ratios showed considerable variation ranging from 16:84 to 33:67 amongst the seven varieties. GPC analysis revealed small differences between the varieties with most of the variation occurring in the relative proportions of the higher molecular weight tannins. MALDI-TOF mass spectrometry of tannins from two varieties gave well-resolved spectra of tetramers, pentamers and hexamers. Oligomers up to the decamers were also detectable. Each of these oligomers had a subset of structures incorporating catechin/epicatechin (CE) and gallocatechin/epigallocatechin (GE) units. Some homopolymers containing CE units only (i.e. procyanidins), but none with GE units only (i.e, prodelphinidins), were detected. Most mixed CE/GE oligomers of all sizes contained one or two GE units. There were significant differences (P less than or equal to 0.05) in vitro N-degradability between four varieties. The data suggest that degradability of the soluble proteins in birdsfoot trefoil were negatively correlated to tannin concentrations (R-2 = 0.93) despite the fact that their overall concentrations were very low. (C) 2000 Elsevier Science B.V. All rights reserved.