MICROBIOLOGICAL, MICROSTRUCTURAL AND RHEOLOGICAL ASPECTS OF EMULSIONS (O/W) WITH LEMON ESSENTIAL OIL

The Tahiti lemon essential oil (Citrus latifolia Tanaka) (LEO) is rich in bioactive, primarily limonene, which degrade when exposed to the environment, requiring the microstructures development for their protection. Bilayer emulsification, caused by electrostatic interactions between biopolymers, is effective in encapsulating and stabilizing bioactive. Antimicrobial activity, stability, microstructure, and rheological behavior of oil-in-water (O/W) emulsions obtained with soy protein isolate (SPI) and pectin (PEC) at different LEO concentrations were evaluated. The emulsions were prepared with 2% (w/w) biopolymers at ratio SPI:PEC 2:1 with 1%, 3%, and 5% oil, and kept at rest for 24 hours and 7 days. Antimicrobial activity against Staphylococcus aureus and Penicillium chrysogenum was assessed for pure oil and emulsions using the microdrop, agar diffusion, and disk methods, while stability was evaluated by creaming index (CI). Microstructure and droplet size were observed with optical microscopy and rheological properties were assessed through flow curves with a 300 µm gap and parallel plate geometry (ϕ50mm) at 25°C, as well as heating-cooling ramps (5-72°C/72-5°C). All emulsions were stable, without CI during resting, with small (<24.5 µm) and well-distributed droplets, indicating an ideal biopolymer ratio. The emulsions antimicrobial activity was limited across all three methods, with inhibition zones observed only on control plates. Many factors may influence the antimicrobial efficacy of emulsions, such as the culture medium, concentration of tested substances, conditions imposed during emulsification or the bilayer efficiency for protection and release of bioactive compounds. Flow curves showed Newtonian behavior with a good fit to the Newton Model (R²=0.99), indicating stability. Heating-cooling ramps showed a reduction in viscosity with increasing temperature, irreversible behavior, and oscillation of curves above 45°C, possibly due to limonene volatilization. The analyses indicated that the bilayer method and the biopolymers ratio were effective for emulsions stabilizing over time. The antimicrobial activity of LEO emulsions was limited.