Preparation and antimicrobial activity of olive oil nanoemulsions

Due to their health benefits including antioxidant, anti-inflammatory and also antimicrobial activities, extra virgin olive oil (EVOO) has received much attention in food, cosmetics, and pharmaceutical industries. Nanoemulsions (NEs) solubilize and increase absorption and bioavailability of lipophilic substances (i.e. olive oil). However, there is insufficient data in the formation of olive oil NEs. This study aimed to investigate the formation of oil-in-water NEs containing EVOO using low-energy approach and high-energy approach. NEs were characterized for size, polydispersity and zeta potential after storage at different conditions for up to 180 days. They were determined for antimicrobial susceptibility. From the result, NEs prepared by low energy emulsification using Tween® 80 (surfactant) indicated the requirement of co-surfactant (Imwitor® 308) to form NEs with smaller droplet sizes (< 200 nm). An increase in oil amount resulted in larger NEs droplets and more surfactant needed to form an interfacial film of droplet. For NEs prepared by high energy ultrasonication, parameters to generate the optimal NEs were ultrasound amplitude 25%, treatment time 5 min and input energy 2250 J. NEs containing 5% EVOO, 30% surfactant and 15% co-surfactant (T30 O5) was the most stable after storage at 25 °C ± 2 °C / 60% RH ± 5% RH, 30 °C ± 2 °C / 65% RH  5% RH and 40 °C ± 2 °C / 75% RH ± 5% for 180 days. The antimicrobial activity of NEs was measured as a zone of inhibition by disc diffusion, minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC) and minimum fungicidal concentration (MFC) by broth microdilution method. The results showed that encapsulation of EVOO into NEs increased antimicrobial activity about 42-105 times against bacteria and 28-42 times against fungi compared to corresponding sole EVOO. Furthermore, adding more olive oil in preparation resulted in more antimicrobial activity. T30 O10 was the most effective formulation in antimicrobials against tested organisms. Conclusively, EVOO NEs showed antimicrobial activity which depends on the compositions of EVOO NEs. Further investigation should be the mechanism of NEs as antimicrobials in order to apply the NEs employed in food and pharmaceutical aspects.