Effect of formulation on properties, stability, carvacrol release and antimicrobial activity of carvacrol emulsions

The structural design of essential oil emulsions can be exploited to modulate their antimicrobial activity, through the effect that the main formulation parameters (oil phase composition and type of emulsifier) have on the release of encapsulated antimicrobial compounds. In this work, different emulsions containing carvacrol, selected as model essential oil component, were characterized in terms of emulsions size, stability, and carvacrol release and solubilization, determined in Franz cells, and tested for minimum inhibitory and microbicidal concentration against P. fluorescens, S. epidermidis, and S. cerevisiae. The results showed that carvacrol fraction in the oil phase significantly affected oil viscosity, density, and O/W interfacial tension. Carvacrol solubilization in the aqueous phase, in equilibrium with the oil mixture, increased with the concentration of carvacrol in the oil phase and with the presence of an emulsifier/stabilizer in the aqueous phase. However, when encapsulated in emulsions carvacrol solubilization exhibited a weak dependence on carvacrol fraction in oil phase because part of the emulsifier/stabilizer was adsorbed at the O/W interface. Higher carvacrol solubilization was observed for WPM Pickering emulsions, followed by WPI and T80 emulsions. The antimicrobial activity was proportional to carvacrol solubilization, suggesting that emulsion droplets act as micrometric tanks for carvacrol, which is steadily released over time in the aqueous phase. The high carvacrol solubilization in the aqueous phase at higher carvacrol fractions in the oil phase (≥75% w/w) was also responsible for lower T80 and WPI emulsion stability because of coalescence, whereas all WPM emulsions exhibited signs of flocculation.