Cinnamon oil nanoemulsions for inhibition of contaminated pathogens on Asian seabass fillet

Abstract

Cinnamon oil is an effective natural antimicrobial agent, but the utilization is limited by its low water-solubility. In this study, cinnamon oil nanoemulsions were prepared using a phase inversion temperature (PIT) method, which simply involves heating a mixture of surfactant, oil, and water about the PIT and then quench cooling with stirring. The impact of oil phase composition (cinnamon to ripening inhibitor ratio) and surfactant concentration on the formation, stability, antimicrobial activity and chemical stability of the nanoemulsions were determined. Optimal mean droplet diameters with narrow size distribution were obtained at intermediate cinnamon oil levels (30-40 wt%) in the oil phase. Conversely, relatively larger droplets were generated at lower (0 to 20 wt%) and higher (60 to 100 wt%) cinnamon oil levels. In the present study, the system containing 40 wt% cinnamon oil and 60 wt% medium chain triglyceride (MCT) oil in the oil phase was selected for further studies since it gave the highest stability of antimicrobial cinnamon oil nanoemulsion with the smallest droplet size and lowest PIT. The impact of oil phase composition on the minimum inhibitory concentration (MIC) of cinnamon oil nanoemulsions against Escherichia coli, Salmonella Typhimurium, Staphylococcus aureus and Vibrio parahaemolyticus was largely due to cinnamaldehyde, which is highly susceptible to chemical degradation. For this reason, a decrease in cinnamaldehyde content and an increase in a major reaction product (benzaldehyde) were observed in the cinnamon oil nanoemulsions during storage. The antimicrobial activity of cinnamon oil nanoemulsions increased (lower MICs) as the droplet size decreased, even though the cinnamaldehyde content was lower. Cinnamaldehyde degraded during emulsification and throughout storage. The impact of surfactant concentration (10-20 wt%) on PIT, droplet size, stability, particle morphology, the MIC, dynamic time kill, and changes in bacteria morphology were determined. Increasing non-ionic surfactant (Tween®80) concentration from 10 to 20 wt% significantly decreased droplet size of the nanoemulsions but had no effect on the PIT, stability (at 4 and 25 °C), particle morphology and MIC values of the nanoemulsions. However, dynamic time kill plots revealed that nanoemulsions with higher surfactant concentrations (15 and 20 wt%) led to faster killing or better prolonged inhibition of bacteria compared to those with lower concentration (10 wt%) or with bulk cinnamon oil. Morphological changes of the bacteria were more promoted for nanoemulsions containing higher surfactant concentrations as shown by field emission scanning electron microscopy (FE-SEM). The antimicrobial activity of the cinnamon oil nanoemulsions was attributed to their ability to disrupt bacterial cell wall structures and promote expulsion of internal cellular material. For further study, cinnamon oil nanoemulsion fabricated with 15 wt% surfactant was selected to study its antimicrobial activity on the shelf-life of Asian seabass flesh during chilled storage (4 °C). Decreasing total viable count (TVC) was observed in the samples treated with cinnamon oil nanoemulsion. Thus, the shelf-life of the cinnamon oil nanoemulsion treated samples can be extended up to 4 days, longer than untreated (control) sample. These results indicated that cinnamon oil nanoemulsion have high potential to be used as antimicrobial agent for preservation of fish fleshes.