Ultrasonic emulsification of food-grade nanoemulsion formulation and evaluation of its bactericidal activity

Abstract

Basil oil (Ocimum basilicum) nanoemulsion was formulated using non-ionic surfactant Tween80 and water by ultrasonic emulsification method. Process of nanoemulsion development was optimized for parameters such as surfactant concentration and emulsification time to achieve minimum droplet diameter with high physical stability. Surfactant concentration was found to have a negative correlation with droplet diameter, whereas emulsification time had a positive correlation with droplet diameter and also with intrinsic stability of the emulsion. Stable basil oil nanoemulsion with droplet diameter 29.3 nm was formulated by ultrasonic emulsification for 15 min. Formulated nanoemulsion was evaluated for antibacterial activity against Escherichia coli by kinetics of killing experiment. Fluorescence microscopy and FT-IR results showed that nanoemulsion treatment resulted alteration in permeability and surface features of bacterial cell membrane.

Introduction

Nanoemulsions are metastable submicron oil-in-water dispersions with droplet diameter in the range of 10–100 nm [1]. Potential advantages of nanoemulsions over conventional emulsions like high physical stability, high bioavailability and low turbidity make them attractive systems for application in food, cosmetics and pharmaceutical industry. Nanoemulsions serve as delivery agents for lipophilic bioactive compounds such as drug in the pharmaceutical industry [2], [3], for flavors [4] and antimicrobial agents [5] in the food industry, for solubilizing water-insoluble pesticides [6] in agrochemical industry and as vehicle for skincare and personal products in cosmetics [7], [8].

Ultrasonic emulsification is a high energy method to develop nanoemulsion. This method is documented as fast and efficient technique for formulating stable nanoemulsion with very small droplet diameter and low polydispersity [9]. It utilizes sound waves with frequency more than 20 kHz by using a sonotrode to cause mechanical vibrations followed by the formation of acoustic cavitation. Collapse of these cavities generates powerful shocks waves which breaks the coarse droplets [10]. Size of droplet diameter can be controlled by optimizing the process parameters such as oil concentration, emulsifier concentration, mixing ratio of oil and surfactant, viscosity of continuous phase, emulsification time and energy input [11]. Using megasonic irradiation (frequency in the range of mega Hz) surfactant-free transparent nanoemulsions have been reported which are stable even in the absence of surfactant [12], [13].

Basil (Ocimum basilicum) is traditionally used as a medicinal plant for the treatment of several diseases including headache, cough and constipation [14]. Basil oil is a source of phenol derivatives such as eugenol, methyl eugenol, estragole, chavicol and linalool [15] etc., which possesses antimicrobial, antioxidant and insecticidal activity [16], [17], [18].

Formulation of nanoemulsion by ultrasonic emulsification has been reported by several works. But it was stable for few weeks only [19]. Also, there are not many reports on nanoemulsion with droplet diameter below 50 nm [20], [21]. So, the objective of the present work is to optimize process parameters to formulate basil oil nanoemulsion with very small droplet diameter with long kinetic stability. We also have investigated the bactericidal activity of the formulated emulsion along with the mode of action.