Isolation and identification of olive tree leaf phenols through a resin adsorption/desorption process

Abstract

Agricultural by-products contain substantial amounts of phenolics with significant antioxidant properties. In this work, a method to recover high concentrations of phenolic substances from a typical agricultural by-product, such as olive tree leaves, was investigated. Phenolic compounds previously separated through extraction and fractionated on a membrane array: Ultrafiltration (UF)-Nanofiltration (NF)-Reverse Osmosis (RO), were examined, alongside standard compounds. More specifically, the NF concentrate fraction, which contained the low molecular weight organics (simple phenols, carbohydrates) was treated by sorption/desorption processes in a cylindrical column packed with XAD16 N resin. The aim of this study was to optimize the isolation and the enrichment of phenolic compounds through their selective adsorption on resin grains. Standard compounds such as gallic acid and glucose were tested to study the sorption and desorption kinetics of phenol and carbohydrate families and the sorption parameters were estimated using the Thomas model. The results showed that XAD16 N resin selectively adsorbed phenolic compounds over carbohydrates. This was evident by the adsorption capacity exhibited by the resin when treating the glucose and gallic acid solutions. Regarding the NF concentrate fraction of olive leaf phenols, after the adsorption step, the adsorbed hydrophilic carbohydrates were removed by desorption using pure water. Immediately afterwards, an alcohol/water mixture was introduced to recover and extract the phenolic compounds. To identify the distinct phenols, a thorough liquid chromatography study was performed on the sorption and desorption samples collected at the outlet of the packed column, identifying the different time frame each phenol left the system.