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.
