Essential Oil Nanoemulsions As Natural Antimicrobial Agents
Currently, in spite of the availability of a large number of preservation techniques, food poisoning and spoilage by microorganisms are major concerns in the food sector (Ghosh et al., 2013a). Consumer needs of natural, wholesome, and safe food free from microorganisms with greater shelf life have raised the interest in the use of natural antimicrobials to preserve food. Therefore, the use of natural antimicrobial substances (bioactive compounds which hinder the growth of spoilage causing microorganisms) as food preservatives, is the latest topic of research for the food scientists and technologists. During recent years, the use of essential oils as natural antimicrobial agents in food has gained much popularity which offers a good prospectus to meet the need of natural preservatives. The exponential growth rate (5% annually) of the essential oil industry supports their widespread use for the present scenario (Pavoni et al., 2020). Essential oils, which are commonly used in the food, cosmetics, and pharmaceutical industries, are plant-derived secondary metabolites containing a mixture of volatile and non-volatile components (Punia et al„ 2019). Due to the presence of biochemical compounds, these oils show antioxidant, antibacterial, antiviral, and antifungal properties (Martin-Pinero et al„ 2019). These are generally recognized as safe (GRAS) for humans and animals use in form of flavouring compounds in the United States (Hyldgaard et al„ 2012; Shah et al., 2012; Bhargava et al., 2015).
Owing to very high antimicrobial efficiency, essential oils have gained noteworthy consideration in the field of food preservation, and the working capacity of essential oils gets increased when encapsulated in proper delivery systems. There are many benefits of encapsulation of essential oils in nanoemulsions form, and recently, most of the essential oil nanoemulsions are prepared using synthetic surfactants. The uses of a nanometric delivery system for essential oil encapsulation have many additional benefits over conventional macroemulsions. Owing to their small droplet size, nanoemulsions possess good practical importance, as these appear transparent and are more stable as compared to conventional emulsions with respect to coalescence, cream, flocculation, and Ostwald ripening (Lu et al., 2018; Panghal et al., 2019). These are stable colloidal systems of very small-sized droplets (ranging 20-500 nm) found dispersed in a continuous phase which may be either aqueous (oil-in-water) or oily (water-in-oil), and these emulsions are stabilized by amphiphilic molecules called surfactants (Moreno-Trejo et al., 2019; Dhull et al., 2019). In nanoemulsions, the physical stability of essential oils is enhanced due to prevention of interaction with other food components, and it also overcomes dosage limitations problem with their low water solubility (Sessa et al., 2015). Additionally, the nanoemulsions increase the bioactivity of essential oils, refining the passive mechanisms of cell absorption, allowing the reduction of doses needed to show antimicrobial activity, therefore minimizing the influence on taste, flavour, and aroma (Donsi et al., 2011). This chapter mainly focuses on some important points regarding characteristics, formulation, antimicrobial efficiency, mechanism of microbial action, and safety regulations of essential oil nanoemulsions.
Major Challenges Associated With Food Application of Essential Oil Nanoemulsion
Even though the use of essential oils emulsions as natural antimicrobial agents provides a good alternative to chemical preservatives, high volatile nature, little water solubility, and strong odour are some major limitations linked with their use in food products. In addition to aforementioned problems, incorporation of oil-based substances in aqueous food products is also a major technological issue, owing to their chemical and physical instability and lipophilic nature (Amaral and Bhargava, 2015; Chawla et al., 2019, 2020). Thermodynamically, nanoemulsions are unfavourable systems as in generating oil-water interface; positive free energy gets escorted.
resulting in breakdown with time. There are some physicochemical phenomena, that is gravitational separation, flocculation, coalescence, and Ostwald ripening, associated with splitting of nanoemulsions. Some forces (inter-particle repulsive, attractive, gravitational, molecular, and flow forces) are related to these destructive processes (Karthik et al„ 2017; Prakash et al„ 2018). Therefore, nanoemulsions can be a better option for food use of essential oils, which is suggested by a lot of studies reported in the literature.