Comparison of tannin analysis by protein precipitation and normal-phase HPLC

Abstract

Tannins are a heterogenous class of polymeric phenolics found in grapes, oak barrels and wine. In red wine tannins are primarily responsible for astringency, though they also have an important role in reacting with and stabilizing pigments. There are numerous sub-classes of tannins found in wine but they all share structural heterogeneity within each sub-class, with varied polymer composition, configuration and length. 

Numerous methodologies exist for the quantification of tannins, however, protein precipitation using bovine serum albumin has proved itself useful due to its strong correlation to the sensory perception of astringency and the basic instruments required for the method. Though the method can yield valuable insights into tannin composition, it cannot be automated easily and necessitates well-trained personnel. 

RP-HPLC analysis has been used for the quantification of low molecular phenolic compounds for a long time, but it is not suitable for the quantification of tannins. A normal-phase (NP)-HPLC method using a ternary solvent system is suggested, which is able to separate the phenolic compounds from red wine into three major fractions. Comparison with standard phenolic compounds allowed the characterization and quantification of these fractions and the results were compared to those obtained by protein precipitation.