Effect of hydrothermal treatment of wet olive pomace to obtain phenolic compounds with antioxidant activity

Abstract

Wet olive pomace (WP) is the by-product from oil mills that use a two-phase extraction system to obtain extra virgin olive oil. After the extraction process, approximately 98-99% of the phenolic compounds (PC) present in the fruit remain in the WP, so this by-product is a rich source of this type of compounds, which have strong antioxidant activity (AA) [1]. The most widely used method for separating PC from natural products is conventional solid-liquid extraction with organic solvents (OS). The use of OS has numerous disadvantages. Emphasis has been put in the study of alternatives to the use of conventional OS. The hydrothermal extraction with water as solvent is one of these alternatives. During this process, the lignin structure can be broken into smaller molecules, increasing the solubilization of PC with AA [2]. The aim of this work is to study different hydrothermal extraction conditions as a previous step to define the levels of three independent variables (temperature (T), time (t) and liquid-solid ratio (LS)) for its use in future optimization studies in the extraction of PC from WP. WP composed of a 50% mixture of Arbequina-Picual varieties with 65.5% moisture was used. Different combinations of T (120, 145, 170 and 180°C), t (15, 30, 60 and 90 min) and LS (2.58, 13.5 and 20 mL/gWP in dry basis (db)) were tested in a 2 L electrically heated reactor with agitation (200 rpm), using water as extraction solvent. After separation by centrifugation, the extracts were characterized by its total phenols content (TP) by Folin-Ciocalteu method, AA by FRAP and DPPH assays and hydroxytyrosol (HT) and hydroxymethylfurfural (HF) contents by HPLC-DAD. In general, the lowest T and LS extraction conditions gave the lowest levels of TP (14.5 mgGAE/gWP db), FRAP (154.1 μmol Fe2+/gWP db), DPPH (57.5 μmol Trolox/gWP db), HT (2.53 mg/gWP db) and HF (non-detected) contents. Extractions at 120°C were not satisfactory due to poor solid-liquid separation, evaluated as visual turbidity in the extracts post-centrifugation. Extractions at 170°C, 90 min and LS of 20 mL/gWP db gave the highest levels of TP (31.8 mgGAE/gWP db), FRAP (365.9 μmol Fe2+/gWP db) and DPPH (121.1 μmol Trolox/gWP db), with HT content of 2.67 mg/gWP db. Extractions at 180°C, 30 min and 170°C, 90 min, both with LS of 20 mL/gWP db, gave the highest HF content (2.79 mg/gWP db and 3.46 mg/gWP db, respectively). As a degradation compound, HF is not a desirable target to extract. Extractions at 170°C, 30 min and LS of 20 mL/gWP db gave the highest HT content (3.50 mg/gWP db). In conclusion, extraction conditions with T between 145-170°C, t between 15-60 min and LS between 5-20 mL/gWP db will be used to study the best conditions to extract PC from WP applying hydrothermal extraction with the equipment presented in this work.