Effect of Drying Temperature Regime on Antioxidant Activity of Lespedeza bicolor Turcz. Extracts

Lespedeza bicolor Turcz. (bush clover) is a well-known medicinal, forage, and honey plant, the plant extracts of which exhibit numerous biological activities such as antioxidant, antimicrobial, anti-inflammatory, etc. This study aimed to determine the antioxidant capacity of L. bicolor ethanol extracts from plant raw material dried at 45 and 65 °C. Plants were collected from the experimental sets of the M.M. Gryshko National Botanical Garden of the National Academy of Sciences of Ukraine. It was determined the total polyphenol content (TPC) was determined by the Folin-Ciocalteu method, antioxidant activity by the DPPH method (with 2,2-diphenyl-1-picrylhydrazyl), FRAP method (with 2,4,6-tris(2-pyridyl)-S-triazine), and ABTS ( with 2,2ʹ-azinobis-3-ethylbenzothiazoline-6-sulfonic acid) assay of ethanol extracts. The TPC, antioxidant activity by DPPH, FRAP, and ABTS methods of L. bicolor extracts obtained from plant raw dried at 45 °C were 15.16–53.18 mg GAE.g-1, 46.84–49.19 μmol TE.g-1, 133.16–335.62 μmol TE.g-1, and 2.11–2.39 μmol TE.g-1, respectively, depending on plant part. The TPC, antioxidant activity by DPPH, FRAP, and ABTS methods of extracts obtained from plant raw dried at 65 °C were 22.9–106.68 mg GAE.g-1, 48.86–51.68 μmol TE.g-1, 130.17–345.64 μmol TE.g-1, and 2.19–2.39 μmol TE.g-1, respectively, depending on plant part. The Pearson’s coefficients were higher between the investigated parameters of plants dried at 65 °C and found a strong correlation: r = 0.858 (TPC vs. DPPH), r = 0.952 (TPC vs. FRAP), and r = 0.858 (TPC vs. ABTS). Thus, L. bicolor plant extracts are a valuable source of bioactive compounds with antioxidant activity. The study of TPC and antioxidant activity by different methods showed that the temperature regime of plant raw material drying affected the polyphenols' content 1.5–2.0 times. The drying temperature did not significantly affect the antioxidant activity results: a slight increase was observed in flower and leaf extracts by the DPPH and FRAP methods, stem and leaf extracts by the ABTS assay. The obtained data can be used in further biochemical and pharmacological investigations.