Tannin (Polyphenol) Stability in Aqueous Solutions.

Understanding the chemical stability of tannins (polyphenolics) in soils is critical to understanding their biological activities and fate.  We examined the stability of chemically defined tannins in aqueous solutions under conditions simulating natural and laboratory conditions.  We evaluated tannin stability by HPLC, looking for both losses and conversion to other simpler phenolics.  In our initial studies we used a hydrolyzable tannin, pentagalloyl glucose, (PGG), and a simple flavonoid ester tannin, epigallocatechin gallate, (EGCG).  Both compounds were stable for over 80 hours at room temperature with ambient atmosphere.  Both compounds degraded, with half lives of 100 h, at elevated temperatures typically used in the lab to extract sorbed compounds from soils (80^oC).  Pentagalloyl glucose had a half life of only 33 hours when incubated at 80^oC in aqueous soil extract containing Fe (III).   The stability of EGCG was not affected by the iron-enriched soil water, suggesting that different tannins decompose by distinct pathways that are, in some cases, metal-catalyzed.  Further studies used solutions that simulate biological conditions, to try to determine the chemical stability of tannins under natural (biotic) soil conditions.  At biological pH (7.4) the tannins rapidly decomposed unless oxygen was excluded from the system.  Like other phenolics, tannins are generally more susceptible to oxidation at higher pH where the easily oxidized phenolate ion predominates.  Our study suggests tannins are relatively stable at room temperature in slightly acidic soils but both higher temperature and neutral to basic pH may facilitate chemical degradation.  We have also demonstrated that stability varies as a function of polyphenol structure, emphasizing the need for studies of tannins of defined structures.