Gravimetric determination of soluble phenolics including tannins from leaves by precipitation with trivalent ytterbium

Abstract

A method for the gravimetric determination of soluble phenolics based on precipitation with trivalent ytterbium is described. Solutions of ytterbium‐precipitated phenolics can be prepared in order to study u.v. absorbance, colorimetric assays for phenolics, protein precipitation and cellulase inhibition. The method was applied to mature leaves of 10 East African browse species in the diets of goats and camels. There was a large range in ytterbium‐precipitated phenolics (Yb‐ppt), from 50% dry matter (DM) in Acacia nilotica to less than 15% in Acacia brevispica and Cadaba farinosa. Results from u.v. absorbance and the Folin‐Ciocalteu assay on solutions of ytterbium‐precipitated phenolics indicated that ytterbium precipitates many types of phenolics including proanthocyanidins. The vanillin‐HCl and n‐butanol‐HCl assays for proanthocyanidins separated the samples into two groups with high or low levels. The group with high levels of proanthocyanidins had greater protein precipitation capacity and cellulase inhibition at similar amounts of Yb‐ppt. The results indicate that the method may be useful in nutritional studies involving browse and other forages. Gravimetric analysis avoids problems associated with colorimetric assays because it does not rely on the use of standards. The preparation of solutions of Yb‐precipitated phenolics allows direct comparison between amount and nutritional effects such as protein precipitation and enzyme inhibition.