Characterization of Condensed Tannins from Freeze-Dried, Silage or Hay Purple Prairie Clover (Dalea purpurea Vent.): Structure Composition, Protein Precipitation and Anti-Escherichia coli Properties

Conservation methods have been shown to affect forage nutrient composition and value, but little information is available about the effect of forage conservation on plant condensed tannins (CT). The objective of this study was to assess the effects of conservation method on the concentration, chemical composition and biological activity of CT. Whole plant purple prairie clover (PPC, Dalea purpurea Vent.) was harvested at the full-flower stage and conserved as freeze-dried forage (FD), silage (SIL) or hay (HAY). Concentration of CT in conserved PPC was determined by the butanol-HCl-acetone method. Structural composition, protein-precipitation capacity and anti-bacterial activity of CT isolated from conserved forage were determined by in situ thiolytic degradation followed by LC-MS analysis, a protein precipitation assay using bovine serum albumin (BSA) and ribulose 1,5-disphosphate carboxylase (Rubisco) as model proteins and an Escherichia coli growth test, respectively. Conservation method had no effect on concentration of total CT, but ensiling decreased (P < 0.001) extractable CT and increased (P < 0.001) fiber-bound and protein-bound CT. In contrast, hay only increased (P < 0.01) protein-bound CT. Regardless of conservation method, epigallocatechin (EGC), catechin (C) and epicatechin (EC) were the major flavan-3-ol subunits, and gallocatechin (GC) was absent from both terminal and extension units of PPC CT. The SIL CT had the lowest (P < 0.001) EGC but the highest (P <0.01) EC in the extension units. Similarly, SIL CT exhibited a lower (P < 0.001) mean degree of polymerization (mPD), but a higher (P < 0.001) procyanidin/prodelphinidin ratio (PC/PD) than FD or HAY CT. The protein-precipitating capacity of CT in conserved PPC ranked (P < 0.001) as FD > HAY > SIL. Growth of E. coli in M9 medium was inhibited by 25-100 µg/ml of CT isolated from FD, SIL and HAY (P < 0.05), but preservation method had no effect on the ability of CT to inhibit bacterial growth. The results demonstrated that ensiling decreased the extractability and protein-precipitating capacity of CT by increasing the PC/PD ratio. Purple prairie clover conserved as hay retained more biologically active CT than if it was conserved as silage.