Analysis of hydrolysable tannins

This review deals with the main issues relating to the analysis of hydrolysable tannins (HTs). It covers briefly their distribution in the plant kingdom and describes their main structural features. HTs have often been ignored because they are apparently more difficult to analyse than condensed tannins. Meaningful analytical data are critically dependent on appropriate sample preparation, storage and extraction techniques. This requires some understanding of the reactivities of hydrolysable tannins. Mixtures of HTs have been measured by general tannin assays, such as precipitation with metals or proteins, and by colorimetric assays for total phenols. Some HTs can also be measured by more specific colorimetric tests. Although colorimetric assays are widely used for tannin analyses, they generally do not provide accurate quantitative data. At best, they provide data for comparative purposes. Thin layer chromatography (TLC) and high performance liquid chromatography (HPLC) are useful tools to screen samples for the different types of tannins, hydrolysable or condensed tannins. In addition, there are physicochemical techniques, such as nuclear magnetic resonance (NMR) and mass spectrometry (MS), which are used to identify pure tannin compounds. More recently, other MS techniques have been developed that are capable of yielding molecular weights of crude tannin mixtures.