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

This work is part of a wider project (OLinWASTE) which aim is the development of an integrated and innovative zero-emission biorefinery transforming the olive mill waste (OMW), including pomace, stones, and leaves into valuable resources. As a first step in the design of the biorefinery, the extraction of phenolic compounds from leaves and twigs (OL) generated in the Italian olive mill, Agrolio (Italy) was studied by subcritical water (SW) treatment. Experiments were carried out in a batch reactor of 0.5 L capacity. OL, with a humidity content of 44 ± 2 % (wt), were grinded without drying by using an IKA® A10basic and then subjected at 120 ºC, 130ºC and 140 ºC for 30 min. The percentage of total amount of soluble solids increased with temperature with values of 29.8 ± 0.5, 34.2 ± 0.7 and 37.2 ± 0.9, respectively. Extracts presented an interesting phenolic profile and antioxidant capacity, observing that luteolin-7-O-glucoside, oleuropein and hydroxytirosol were the major identified phenolic compounds, accounting for nearly 90 % of the identified phenolic compounds. Luteolin-7-glucoside and oleuropein content decreased with temperature, while hydroxytirosol increased due to thermal degradation of oleuropein releasing hydroxytyrosol, being 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 identified in the total solid extract at 140 ºC. These results shows that valorization of OMW by SWE offers a great potential in the production of valuable bioactive compounds.