Development and optimization of thyme-pennyroyal essential oils based active nanostructured lipid carrier (NLC) containing saffron extract with antioxidant and antimicrobial properties

The saffron extract-loaded nanostructured lipid carrier (NLC) was synthesized by the double emulsion (W/O/W) method for application as a preservative to increase the oxidative and microbial stability of French dressing. The optimization of the NLC system was carried out using a combined D-optimal design methodology and effect of thyme (TEOs)/pennyroyal (PEOs) essential oils, and polyglycerol ester (PGE)/polyglycerol polyricinolate (PGPR) surfactants ratios (independent variables) were investigated on the mean particle size, polydispersity, zeta potential, and encapsulation efficacy (dependent variables). The optimal formulation (NLC opt) was obtained as 90/10 thyme/pennyroyal essential oils and 90/10 polyglycerol esters/polyglycerol polyricinoleate ratio, which resulted in the lowest PDI and particle size (0.38 and 66.87 nm respectively), highest zeta potential and encapsulation efficiency (−26.7 mV and %78.88 respectively). Different techniques including transmission electron microscopy (TEM), Fourier transform-infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) analysis were used for the characterization of structure and physicochemical properties of the different NLC systems and confirmed successful incorporation of SE into NLC. The DPPH test showed decrement of antioxidant activity during 30 days of storage and IC50 value was lower in the SE-NLC (0.68 mg/ml) than in the free extract. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) tests against S. aureus, E. coli, and P. aeruginosa indicated that the SE did not have antibacterial properties. However, NLC formulations showed antimicrobial effect due to the presence of TEOs and PEOs essential oils. Peroxide value of control sample was the highest (8/35 O2/kg) and it was the lowest (3.75 O2/kg) in French dressing with SE-NLC.