Characterisation and quantification of condensed tannins in rapeseed hulls and meals by depolymerization methods

Abstract

Condensed tannins (i.e. polymerized procyanidins) were quantified in crude samples of rapeseed meal and hulls using two acidolysis methods. When it was applied directly on crude material, the colorimetric butanol-HCl assay allowed an estimation of two grams of tannins per kilogram of dried hulls. Interestingly, the estimation was more than double after solvent extraction using sodium dodecyl sulphate (SDS) when the colorimetric method was applied on both the extract and the insoluble residue. By altering the plant tissues organization, SDS treatment may allow a better accessibility of the depolymerizing reagents. However, this colorimetric assay was judged approximate because of browning side-reaction likely involving other components. Phloroglucinolysis combined with HPLC-UV-MS analysis was more rigorous and informative confirming that rapeseed hulls condensed tannins were essentially constituted of epicatechin units with an average degree of polymerization of close to 11. According to this method, the concentration of condensed tannins in hulls was estimated to 5.19 g/kg in the optimized conditions established through a factorial experimental design allowing optimization of hydrochloric acid concentration, reaction duration and temperature.