Ultrasound-assisted emulsion of laurel leaves essential oil (Laurus nobilis L.) encapsulated by SFEE

Highlights

• Ultrasound technique aid the formation of miniemulsions of laurel essential oil in Hi-Cap.

• Essential oil concentration and sonication time affected the emulsion hydrodynamic droplet size.

• Higher essential oil concentration in the emulsion favors the encapsulation efficiency by SFEE.

• Amorphous and solvent-free particles were formed by SFEE.

• SFEE was adequate to provide encapsulation of a complex volatile mixture, the laurel essential oil, in Hi-cap.

Abstract

Essential oil of laurel leaves presents, as the main component, the bioactive 1,8-cineole. The Supercritical Fluid Extraction of Emulsions (SFEE) is an alternative method for the encapsulation of natural substances, such as laurel essential oil, particularly allied with the ultrasound technique, which has been promising for the formation of emulsion. The objective of this work was to evaluate the influence of essential oil concentration and sonication time to reduce the hydrodynamic size of the emulsion droplets (Dg) in order to enhance the efficiency of encapsulation by SFEE. The essential oil concentrations in the emulsion varied from 6 to 12 mg/mL, for sonication time from 2 to 6 min. Stable miniemulsions were obtained up to 24 h assays, with Dg ranging from 239.5 to 357.0 nm. The encapsulation efficiency in modified starch was up to 75.5%, performed at 100 bar and 40 °C, favored by the increase in essential oil concentration.

Introduction

Laurel leaves are commonly used as flavorings in world cuisine. The 1,8 - cineole is the major compound responsible for the aroma of laurel leaves, followed by linalool and other benzene compounds, such as eugenol, isoeugenol and trace substances. The synergy of these compounds contribute to the striking flavor and overall quality of the laurel leaves [1].

The laurel leaves essential oil contains various bioactive properties. It presents attributes to prevent food deterioration, maintain the red color of meat products [2], perform as antibacterial, antimicrobial and antifungal agent [[3], [4], [5]], and acts as acaricidal and antioxidant [6,7].

The encapsulation of a lipophilic compound, such as laurel leaves essential oil, in a hydrophilic encapsulating agent, has the main advantage to make it available in aqueous medium. Besides that, the encapsulation contributed to the flavor preservation, and aid the reduction of degradation or loss of the aroma during the material processing for formulation and/or further consumption [8].

High-pressure methods, such the Supercritical Fluid Extraction of Emulsions (SFEE), emerge as an alternative for the encapsulation of natural substances, particularly due to the use of environmentally safe solvents, especially CO₂, which is widely used and has low cost [9,10]. The SFEE can be comprised in two main steps: the formation of the emulsion and encapsulation of the active compound, with the organic solvent removal and precipitation of the suspension [10].

According to the literature, the hydrodynamic size of the suspension droplets formed by SFEE method is dependent on emulsion droplets size [9,11,12]. Therefore, it is necessary to ensure the reduction of the droplets to achieve stability during the emulsification process.

Emulsions oil-in-water (O/W) are colloidal systems widely applied in the encapsulation, solubilization and controlled release of bioactive compounds [13]. The use of ultrasound to aid the emulsions formation provides advantages over other techniques, such as Couette cell and pressure homogenizer, providing good energy-efficiency, low production cost and ease of system manipulation [14].

Ultrasound formation of emulsion occurs due to cavitation. During this process, bubbles can collapse near the interface of the two liquids and the shock results in an efficient mixing of the two liquid phases [15]. The parameters that affect the efficiency of cavitation in emulsion formation are the sonication time [16], surfactant concentration [16,17], oil/water fraction [17,18], sonication energy [14,17] and the properties physical-chemical of oil [16,18,19].

The SFEE has been used for the encapsulation, in biopolymers, of several bioactive compounds such β-carotene, lycopene, quercetin and pepper oleoresin [10,12,17,20,21]. However, to the best of our knowledge, the essential oil encapsulation by SFEE has not been reported in the literature. Therefore, the main objective of this work was to evaluate the influence of the laurel essential oil concentration (C_eo) and sonication time (t_s) in the reduction of the hydrodynamic size of emulsion droplets (Dg) formed by ultrasonic probe. Finally, after the emulsion formation, the effect of laurel essential oil concentration on encapsulation efficiency by SFEE was also studied.