Evaluating the Aqueous Extraction of Phenolic Compounds from Olive Tree Pruning

Olive tree pruning is one of the main waste products generated by the olive oil and table olive industries. Currently, this biomass is either cut and left scattered on the soil for fertilization or burned, with neither practice generating any economic returns. Remarkably, olive biomass is rich in phenolic compounds, including oleuropein derivatives, which have market value in the nutraceutical, cosmetic, and food sectors. Therefore, this study evaluates the aqueous extraction of phenolic compounds from this untapped biomass, which can contribute to its valorization and serve as a first step in its integration into a biorefinery. Process: Olive tree pruning was dried and milled before extraction. Soxhlet extraction (0.1 L extractor, 5% s/l, 24 h) and liquid hot water at 120 °C for 60 min in the laboratory (autoclave, 0.5 L bottles, 15% s/l) and on a larger scale (Büchi-Glass Ulster 20 L reactor, 20% s/l) were applied for comparison. The recovered volume and the phenolic content were evaluated. The results showed that the highest total phenolic content was obtained using Soxhlet (0.038 g/g biomass, dry weight), followed by liquid hot water treatment (up to 0.015 g/g). The oleuropein content also varied depending on the extraction approach. The results suggest that liquid hot water can be used to partially recover phenolic compounds from olive tree pruning, albeit in a shorter time compared to Soxhlet extraction and using higher solid loads to facilitate scaling up. This operation can be integrated into the biorefining of olive tree pruning to produce antioxidants, such as oleuropein and hydroxytyrosol, along with other bioproducts.