Biorefinery concept to valorize olive leaves and twigs generated in the olive mill by mild subcritical water

Abstract

This work is part of a wider project (OLinWASTE) aimed at developing an innovative zeroemission biorefinery transforming the olive mill waste (OMW), including pomace, stones, and leaves into valuable resources. The olive leaves at twigs (OL) were provided by the Italian olive mill Agrolio (Coratina cultivar). OL contain 44 ± 2 % (wt.) moisture and has the following monosaccharide composition: xylose (43 ± 4), followed by arabinose (20 ± 5) and galacturonic acid (11 ± 3), with lower amounts of other monosaccharides and bioactive compounds. The extraction of phenolic compounds from wet, ground OL was studied using subcritical water (SW) treatment under mild conditions: 120, 130 and 140 ºC for 30 min, in a batch reactor of 0.5 L. The percentage of total soluble solids increased with temperature with values of 29.8 ± 0.5, 34.2 ± 0.7 and 37.2 ± 0.9, respectively. The extracts presented an interesting phenolic profile and antioxidant capacity, with oleuropein, hydroxytirosol, and luteolin-7-O-glucoside being the major identified phenolic compounds, accounting for nearly 90 % of the phenolic content. Luteolin-7-O-glucoside and oleuropein content decreased with temperature, while hydroxytirosol increased due to thermal degradation of oleuropein, releasing hydroxytyrosol, a potent antioxidant, reaching values of 469 ± 28 mg per 100 g of total solids in the extract, from a total of 636 ± 34 phenolic compounds in the total solid extract at 140 ºC. These results show that valorization of OMW by SW offers significant potential for producing valuable bioactive compounds.