Effect of gamma radiation on bioactive compounds of olive wastes

The olive pomace is an environmentally detrimental residue from olive oil industry. This residue contains large amounts of bioactive compounds, such as hydroxytyrosol and tyrosol, secoiridoid derivatives, phenolic acids and flavonoids1 that might be used by the food industry as preservatives. The aim of this work was to study the gamma radiation potential to improve the extractability of the bioactive compounds present in olive wastes. Gamma radiation is an eco-friendly technology that can be used to enhance the benefic properties of different agro-industrial products. Olive pomace samples (crude olive pomace - COP - and extracted olive pomace - EOP) were collected from UCASUL - União de Cooperativas Agrícolas do Sul, located in Alentejo region, in Portugal. The irradiation experiments were carried out at room temperature in a Co-60 semi-industrial facility (absorbed doses: 5-22 kGy; dose rate: 16 kGy/h). The characterization of the phenolic profile in the extracts of olive pomace and the identification of the radiolytic products were carried out by HPLC-DAD-ESI/MS2. The major phenolic compounds present in olive pomace extracts were hydroxytyrosol, hydroxytyrosol-1-β-glucoside, tyrosol, syringic acid and luteolin-7-O-rutinoside. Caffeic acid, vanillin, verbascoside and its derivatives and oleuropein aglycons were also found in the extracts although in lower concentrations. The obtained results demonstrated that gamma radiation significantly improved the extraction of phenolic compounds from both olive pomace extracts, obtaining the highest yield at 10 kGy for EOP and at 22 kGy for COP. At these doses, the total concentration of phenolic compounds in the extracts was 159±7 mg/g in the EOP and 161±2 mg/g in the COP ones. Comparing with non-irradiated samples, these values represent an increase in extractable phenolic compounds of 2.5 and 2.4 fold, respectively. Nevertheless, for EOP it was found that an absorbed dose of 5 kGy was capable to increase the phenolic content with no significant difference from the higher applied doses. These results demonstrated that gamma radiation could be a suitable technology for the valorization of olive oil by-products, contributing to enhance extraction of phenolic compounds. This outcome can help the olive oil industry to adopt clean processes and promote the sustainable development.cknowledgements: We would like to thank. J. Madureira received a Ph.D. scholarship from Fundação para a Ciência e Tecnologia (FCT)— SFRH/BD/136506/2018; L. Barros and M.I. Dias thank FCT, P.I., for their institutional scientific contract; to UCASUL - União de Cooperativas Agrícolas do Sul” agro industrial cooperative for providing the samples; to Technological Unit of Radiosterilization (University of Lisbon) for the samples irradiation. Funding: The authors are grateful to FCT, Portugal and FEDER under Programme PT2020 for financial support to CIMO (UID/AGR/00690/2019) and C2TN (UID/Multi/04349/2019), FEDER-Interreg España-Portugal programme through the project 0377_Iberphenol_6_E, and International Atomic Energy Agency (IAEA).