Evaluation of the stability and antimicrobial activity of emulsions loaded with thymol for the post-harvest sanitization of lettuce (Lactuca sativa L)

Abstract

Thymol is a monoterpene with broad antimicrobial activity and great potential for use in applications against pathogens and spoiling agents, as it causes changes in the permeability of microorganism cell walls. However, its low solubility in water can alter its bioavailability and antimicrobial effectiveness. The aim of this study was to evaluate the physicochemical and antimicrobial stability of emulsions made up of thymol (standard reference substance SRS-thymol) and Lippia sidoides essential oil obtained by primary emulsification and high energy shear methods and stored at temperatures of 5 and 25 °C for 30 days. The most stable emulsions were obtained by the high-energy method and showed a range of −28.26 to −24.63 mV for Zeta potential (ZP); 0.22 to 0.41 for dispersion index (PDI) and drop sizes of 145.46–193.40 nm. The treatment with the lowest Minimum Inhibitory Concentration (MIC) values was against the Staphylococcus aureus cocktail, being 0.50–1 mg/mL and 1–2 mg/mL for the SRS-thymol and L. sidoides essential oil emulsions, respectively. The application of the emulsions showed reductions of up to 3.5 logs of CFU/mL in lettuce samples by the challenge test with Salmonella Typhimurium, S. aureus and Escherichia coli. However, treatments with higher concentrations (2 × MIC) cause side effects related to changes in the texture and color of the leaves after 15 min of sanitization.