Extraction and Composition of Polyphenols from Purple Cowpeas

  Objective  Polyphenol compounds in purple cowpeas were solvent-extracted, purified, and then chemically analyzed by HPLC.

  Method  Single factor and orthogonal experiments were conducted to optimize the polyphenol extraction from purple cowpeas. Using polyphenol yield as the criterion, effects of solvent, solid-liquid ratio, temperature, and pH applied for the process were evaluated for optimization. The crude extract was purified using a Sephadex LH-20 macroporous resin. HPLC with external standards was employed for the chemical determination on the purified product.

  Result  The optimized process applied a peas-to-solvent ratio of 1 ∶ 5 (m ∶ V) with an 80% acetone solution to extract 3 times at 4^oC and pH 6 for 5 h per cycle to reach a maximum total polyphenols content of the gallic acid equivalent of 614 mg·100 g⁻¹. The correlation coefficients on the linear functions of the 6 polyphenols against their respective standards ranged from 0.995 9 to 0.999 9 with a limit of detection (LOD) of 0.1～1.0 mg·100 g⁻¹ and that of quantitation 0.2～2.5 mg·100 g⁻¹ on the HPLC test. The average recoveries on these compounds ranged from 74.3% to 107% with a relative standard deviation (RSDs) of 0.4～8.6%. The 6 polyphenols in the purple cowpeas thus determined averaged between 0.46 mg·100 g⁻¹ and 6.5 mg·100 g⁻¹, with rutin being the most abundant.ing the most abundant.

  Conclusion  The polyphenol extraction from purple cowpeas was optimized to deliver high yields on 6 monomer phenols, which could be accurately determined quantitatively by HPLC.