Fabrication of Monarda citriodora essential oil nanoemulsions: characterization and antifungal activity against Penicillium digitatum of kinnow

Postharvest fungal pathogenic invasions are the major root cause of reduced shelf life of kinnow fruit, thereby contributing to the postharvest losses. Development of eco-friendly alternates are the need of the hour owing to health safety concerns for replacing the ongoing synthetic fungicide use. Essential oils with promising antimicrobial activities offer a promising solution but their hydrophobicity poses a big hindrance for exploiting the same. Present work was planned to explore their antimicrobial potential by developing their hydrophilic formulation with the use of nanotechnology. An in vitro study was conducted to assess the efficacy Monarda citriodora essential oil (MCEO) and its emulsions against major postharvest fungal pathogen of Kinnow; Penicillium digitatum. Both micro and nano formulations were prepared for different ratios of MCEO (0.5 to 3%) with different surfactant combinations and oil-surfactant-ratios (OSR) of 1:1 to 1:3. The influence of several process factors such as surfactant and oil phase concentrations, as well as sonication time intervals on emulsion stability was investigated by assessing attributes such as droplet diameter, Polydispersity index (PDI), zeta (zeta) potential and rheology. An emulsion formulated with 1% oil and 1:1 OSR treated with ultrasonic waves for 15 min was optimized with droplet diameter of 52.2 nm, 0.245 PDI and - 21 mV of zeta potential with consistent stability till 1 month. Further, in vitro antifungal activity of the optimized MCEO nanoemulsion exhibited the best efficacy with 100% inhibition at 200 mg L-1.