Antioxidant Properties of Peanut Plant Leaves and Roots and Contribution of Specific Phenolic Compounds to Antioxidant Capacity

Abstract

The peanut seeds represent less than 40 % of the total biomass of the peanut plant. Currently, peanut plants are left in the field after harvest or baled for animal feed. The research presented here was the second step in identifying bioactive compounds from peanut plants that may be the source for value added products for the peanut industry. The objective of this work was to determine the antioxidant capacity and total phenolic content for peanut leaves and roots, and to identify and quantify specific phenolic compounds contributing to the total antioxidant activity. Peanut leaves and roots were collected from a North Carolina (NC) research farm. Additional roots were collected from a Texas (TX) farm. Plant parts were freeze dried and extracted with methanol and aqueous methanol.

Antioxidant activity of the extracts was tested using ORAC and DPPH assays. Total phenolics were measured using the Folin-Ciocalteau method. Chlorophyll was tested with the ORAC assay to dispel concern for potential chlorophyll antioxidant activity. Thin Layer Chromatography (TLC) and High Performance Liquid Chromatography (HPLC) were used for separation and preliminary identification of compounds contributing to the total antioxidant capacity of the peanut leaves and roots. HPLC coupled with Mass Spectrometry (MS) was utilized for identification and quantification of individual phenolic compounds in the peanut plant parts. The identified phenolic compounds were assayed individually and as a quantitative mixture with both antioxidant assays to determine the contribution of the individual compounds to the total antioxidant activity previously determined for peanut plant extracts. NC leaves had the highest antioxidant capacity and total phenolic content, followed by NC roots and TX roots. Aqueous methanol extracted more antioxidant compounds from peanut plants than methanol, which was displayed by significantly higher antioxidant capacities and total phenolic content. The total phenolic content was highly correlated with the antioxidant activity of the ORAC and DPPH suggesting that phenolic compounds may contribute to the antioxidant activity. Compounds identified in the peanut leaves and roots were phydroxybenzoic acid, caffeic acid, chlorogenic acid, ferulic acid, catechin, epicatechin, and gallocatechin gallate. Epigallocatechin gallate was identified in NC leaves and NC roots, while resveratrol was identified in NC roots and TX roots only. Individual assays of the phenoli compounds identified indicated that they contributed approximately 40% of the total antioxidant activity in the ORAC assay, and 15% in the DPPH assay.