Ultrasound-Assisted Preparation of Chitosan Oligosaccharide-Stabilized Thyme Oil-in-Water Nanoemulsions: Enhanced Storage Stability and Antimicrobial Properties

Thyme oil (TO), an aromatic compound derived from Thymus species, exhibits potent antioxidant and antibacterial properties. To address its defects of high volatility and susceptibility to oxidation, TO was encapsulated in chitosan oligosaccharide (COS)-stabilized oil-in-water emulsions using a two-step emulsification method with ultrasound assistance. The droplet size of TO-in-water emulsions decreased significantly with increasing ultrasound power and treatment time, achieving sizes below 240 nm with an encapsulation efficiency exceeding 90%. The COS interface layer, combined with polyvinyl alcohol (PVA), effectively enhanced emulsion stability by preventing phase separation and maintaining droplet size and zeta potential during storage. Compared to its free form, the encapsulation of TO in the emulsion significantly improved the antioxidant activities, as evidenced by the enhanced ABTS (1.25-fold) and DPPH (1.33-fold) radical scavenging activities, at equivalent concentrations. Additionally, the TO emulsions exhibited superior antibacterial and antifungal properties, with minimum inhibitory concentration (MIC) values reduced by half and effective inhibition of Escherichia coli, Staphylococcus aureus, and Penicillium italicum growth. These findings highlight the potential of TO emulsions as an effective delivery system for improving the functionality and stability of TO in fresh food preservation applications.