Physical and antimicrobial properties of neutral nanoemulsions self-assembled from alkaline thyme oil and sodium caseinate mixtures.

The objective of this study was to utilize self-assembly properties of sodium caseinate to prepare nanoemulsions after neutralization of alkaline mixtures with 2% w/v caseinate and up to 3% v/v thyme oil. The encapsulation efficiency was >90% at a surfactant-to-oil ratio of up to 1:1.25. The nanoemulsions had structures smaller than 75 nm according to atomic force microscopy and stable hydrodynamic diameters of ~90-200 nm during 20-day storage at room temperature. When tested against Escherichia coli O157:H7 ATCC 43895 and Staphylococcus aureus ATCC 25923 in tryptic soy broth and milk, the encapsulated thyme oil was more active than free thyme oil. Additionally, S. aureus was more resistant than E. coli O157:H7, requiring 11.7 and 5 g/L of encapsulated thyme oil, respectively, to completely deactivate bacteria in milk in 4 h. The findings suggest that thyme oil nanoemulsions can be potentially used as natural preservatives to improve food safety.