If freeze drying is too expensive or the equipment is not available, freezing without thawing of the sample before extraction is suggested. Acetone inhibits tannin-protein interaction. This is a limitation in protein precipitation assays. This unextractable fraction cannot be ignored because of its nutritional effects. Sample purification and isolation Tannins need to be purified from low molecular weight phenolics and pigments that are present in crude plants extracts.
Purification is essential for the preparation of suitable standards.
Purification of large quantities of tannins can be done by taking advantage of their absorption by Sephadex LH-20. The reaction is based on the reduction of phosphomolybdic acid by phenols in aqueous alkali. Interfering compounds such as ascorbic acid, tyrosine and possibly glucose are also measured. Vanillin-HCl assay Specific for PAs or condensed tannins . This monomer gives the maximum optical density leading to underestimation of large polymers.