Sophorolipid Butyl Ester: An Antimicrobial Stabilizer of Essential Oil-Based Emulsions and Interactions with Chitosan and γ‑Poly(glutamic acid)

The life-threatening side effects of synthetic food preservatives have brought about an increased demand for safe and bio-based antimicrobials. Sophorolipid (SL) esters, which are naturally derived and have the potential to both stabilize oil/water (o/w) emulsions and provide potent antimicrobial activity, are promising candidates. The interactions of SL-butyl ester (SLBE) with different oil (oregano/olive oil) mixtures and biopolymers [γ-poly­(glutamic acid) (γ-PGA) and chitosan (CH)] introduced in the aqueous phase was investigated. The mean emulsion droplet size as a function of olive oil content in the oregano oil phase (total oil phase of 5 wt %) with 0.5 wt % SLBE reaches a minimum (810 nm) at 20% olive oil. SLBE stabilizes o/w emulsions over 30 days with oil concentrations at least 10 times that of the surfactant. For emulsions containing biopolymers, ζ-potential values across γ-PGA concentrations (0–2 wt %) changed little (6.8–8.4 mV), indicating that anionic γ-PGA is not adsorbed onto the SLBE-stabilized oil/buffer interface. At 0.5 wt % γ-PGA, the emulsions separate into an oil-enriched upper phase and a lower transparent γ-PGA-rich phase due to depletion flocculation. In contrast, CH is a highly effective emulsion stabilizer with apparent favorable interactions with SLBE. Incorporation of CH (0–2 wt %) in SLBE-stabilized oregano/olive oil (4:1 w/w) emulsions leads to highly positive ζ-potential values (40.7–52.3 mV), indicating that CH is adsorbed onto the SLBE-stabilized oil/buffer interface. An increase in the CH concentration from 0.1 to 2 wt % leads to a regular decrease in creaming and oil separation due to enhanced electrostatic/steric repulsion that further stabilize emulsions. These promising results will be used as a platform for developing antimicrobial films/coatings for wound-healing and food preservation.