Formulation and Characterization of Essential Oil-Based Nanoemulsions for Antioxidant and Antimicrobial Applications

Abstract

Carvacrol and cinnamaldehyde are essential oils with proven antimicrobial activity; however, their application is restricted due to its low aqueous solubility and poor bioavailability, which greatly limits its efficacy and potential uses. To overcome this challenge, different combinations of soy lecithin and Tween 80 stabilized essential oil nanoemulsions were formulated using ultrasonication. Carvacrol is a natural monoterpene essential oil and cinnamaldehyde is a natural compound isolated from the essential oil of cinnamon (Cinnamomum spp.) bark. Two nanoemulsions; carvacrol and cinnamaldehyde were tested for bioavailability and in vitro antioxidant and antimicrobial activity. Carvacrol nanoemulsion containing 2% carvacrol, 0.5 mg/mL soy lecithin, and 2% Tween 80 showed the best results with a size ~248 nm, the zeta potential of -32.32 mV encapsulation efficiency of 75% and loading efficiency of 73%. Disk diffusion assays indicated that the carvacrol nanoemulsion at 2% (v/v) and free carvacrol showed inhibition zones of 18 mm and 9.75 mm, respectively, against L. monocytogenes V7. Moreover, a higher inhibition zone of 19.73 mm against Salmonella Poona 01A4754 was also recorded for carvacrol nanoemulsion as compared to the free carvacrol (12.75 mm). Similarly, 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging activity revealed three-fold higher antioxidant activity compared to free carvacrol. A dialysis release study revealed the maximum release of carvacrol from nanoemulsion within 2 to 8 h. Similarly, cinnamaldehyde nanoemulsion containing 2% cinnamaldehyde, 0.5 mg/mL soy lecithin, and 1% Tween 80 showed a droplet size around 398 nm with -26.47 mV zeta potential, an encapsulation efficiency of 79% and a loading efficiency of 77%. Disk diffusion assays showed inhibition zones of 12.12 and 16.37 mm against Listeria monocytogenes V7 (serotype 1/2a, raw milk iii isolate) and Salmonella Poona 01A4754 (cantaloupe outbreak isolate), respectively, which was higher than the inhibition obtained with free cinnamaldehyde (i.e., 7.25 mm and 9.75 mm, respectively). Cinnamaldehyde nanoemulsion treatment immediately reduced bacterial populations on spinach by 2 log CFU/cm^2 as compared to control. Moreover, 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging activity of cinnamaldehyde nanoemulsion was higher than that of free cinnamaldehyde. Our results suggested that carvacrol and cinnamaldehyde nanoemulsions can be conveniently delivered and may provide higher bioavailability and bioactivity against food-borne pathogens.