Nanoemulsion of sweet orange and eugenol essential oils: optimization and viability over time under different temperatures

Compound delivery systems have been optimized to maximize the bioactivity of natural substances, as well as their bioavailability, improving the compounds’ dispensability into different matrices and their stability over time under the external stimulus. Among the delivery systems, it can be highlighted nanoliposomal structures, which are vesicles composed of one or more lipid bilayers and sizes that do not exceed 100 nm. In this context, the study focus on the optimization of the total phenolic contents and kinetic stability of nanoliposomes composed of two different natural bioactives such as essential oil of sweet orange Citrus sinensis L. Osbeck and eugenol, plus Tween 80 as emulsifier compound. The combination of the compounds was tested according to a mixed design, and the nanodroplets were produced through ultrasound tips (DES500, Unique Group, Brazil) at 90 kHz, for 40 min. Among the formulations studied, the composition containing 2.29% sweet orange essential oil, 1.93% eugenol, 0.5% Tween 80, and 95.27% water exhibited approximately 105.45 ± 2.4 nm of size, 0.130 ± 0.01 of polydispersity index, -11.44 mV ± 0.5 of zeta potential, and 35.03 ± 0.01 mg/g (gallic acid/sample). Besides, the optimized nanoemulsions were stored at refrigeration (6 ± 2 ºC), ambient (25 ± 2 ºC), and heating (40 ± 2 ºC) temperatures to evaluate the stability of the system over time. Size, polydispersity index, and zeta potential of nanoliposomes increased over time (30 days), but the increasing was higher for systems stored at heating temperatures, ranging from 100 nm to 450 nm, 0.1 to 0.4, and -12 to -16 mV. As expected, the size of the droplets increased over time due to the Ostwald ripening and coalescence phenomena, which were higher at 40 ºC temperature. In principle, the coarsening of a small particle on a substrate can occur due to binary collision accompanied by liquid-like coalescence of the particles and by interparticle transport of single atoms through Ostwald ripening. High temperatures can increase the kinetic energy and consequently the particles’ Brownian motion due to the irregular thermal movements, causing more collisions between the particles. Once this phenomenon is not homogeneous, as the size increments, the polydispersity index increased too. However, still maintained the ratio considered homogeneous, 0.2 to 0.5. The increment of zeta potential was slight and the charges involved were not enough to maintain stable the nanoemulsion through the electrostatic repulsion, above 30 mV or below -30 mV. After 30 days, the antioxidant capacity of the nanoemulsion reduced, ranging from 450 to 300 µmol/g (ferrous sulfate/sample), when stored at the refrigerator and ambient temperatures, and from 450 to 125 µmol/g ferrous sulfate/sample) under heating temperatures. The properties of the nanoemulsion manufactured with essential oil of sweet orange and eugenol compound changed over 30 days, however, the system still had enough antioxidant capacity and stability to be used as a bioactive system when stored in ambient temperatures, reducing costs with refrigeration. Therefore, this natural bioactive system delivery has the potential to be applied in the food industry to deliver stable antimicrobial and antioxidant compounds.