Antimicrobial Potential of Essential Oil Nanoemulsions

Antimicrobial action of essential oil nanoemulsions is stronger and wide in spectrum compared to free essential oils. A lot of studies are reported in the literature regarding the antimicrobial efficacy of essential oil nanoemulsions. The self-assemble potential of sodium caseinate was used for nanoemulsion preparation after neutralization of alkaline thyme oil and caseinate mixture (Zhang and Zhong, 2020). Encapsulated thyme oil was found more effective against E. coli 0157: H7 and Staphylococcus aureus compared to free oil in milk samples, and it was therefore concluded that thyme oil nanoemulsions can be effectively used as natural preservatives. Clear and transparent nanoemulsions of lemon myrtle essential oil (extracted from Backhousia citriodora) prepared using ultrasonication showed very good antibacterial activity against gram-positive (S. aureus-ATCC 33591, L. monocytogenes-ATCC 19111) and gram-negative (E. coli-ATCC 11775, Pseudomonas aeruginosa-ATCC 9626) whereas anise myrtle essential oils nanoemulsions were not found effective against previously mentioned microorganisms (Nirmal et al., 2018). Antimicrobial efficiency of nanoemulsions having oregano, lemongrass, thyme, and mandarin essential oils and high methoxy pectin against E. coli and L. innocua indicated that it was dependent on type of oil instead of droplet size and gradually reduced during storage period owing to loss of volatile components (Guerra-Rosas et al., 2017). Table 9.2 compiles the information about the use of nanoemulsions having essential oils with their targeted microorganisms.

A food-grade nanoemulsion (droplet diameter, 29.6 nm) of basil oil (Ocimum basi- licum) prepared using Tween 80 and ultrasonication changed permeability and surface features of E. coli in a very impressive way (Ghosh et al., 2013a). In another study, Ghosh et al. (2013b) demonstrated that cinnamon oil nanoemulsion (droplet diameter, 65 nm) prepared in a similar manner as that of basil oil. possessed bactericidal action against Bacillus cereus and therefore can be used for preservation of minimally processed food. Nanoemulsions containing eugenol were found effective against S. aureus in orange juice samples (Ghosh et al., 2014). Antimicrobial essential oil nanoemulsions of lemongrass (Cymbopogon citratus) prepared with low-energy methods were stabilized with chitosan and oleic acid, along with enhancement of antibacterial activity against 9 bacterial and 10 fungal strains (Bonferoni et al., 2017). Stable cinnamon bark oil nanoemulsions can be successfully formulated using a combination of Tween 80 and lauric arginate surfactants without influencing their antimicrobial potential and can be used in complex food systems like milk (Hilbig et al., 2016). In addition to this, the combined use of nanotechnology and synergistic influence of systems containing more than one antimicrobial component (D-limonene and nisin) showed very good physical stability and antimicrobial properties (Zhang et al., 2014). Zein nanoparticles encapsulated with essential oil were observed to have more water dispersion (14-fold) which increased their preservative action against human disease-causing bacteria like E. coli (Wu et al., 2012). A nanoemulsion of clove essential oil was also proved as effective antibacterials by Anwer et al. (2014). Likewise, thymol nanoemulsions prepared using spontaneous emulsification process were also act as antimicrobial agents against some common and foodborne pathogens and provide a different sanitizing process which can be used as agricultural sprays, aerosols, and washes (Li et al., 2017). Tra/tx-cinnamaldehyde nanoemulsion (0.8 wt%) retarded the activity of S. typhimurium and 5. aureus in watermelon juice (Jo et al., 2015).

Biopolymer-based (basil seed gum) edible films prepared with an added nanoemulsion of Zataria multiflora essential oil were found effective against gram-positive and -negative bacteria and increased the shelf life by reducing the release of volatile compounds (Gahruie et al., 2017). Anise oil (Topuz et al., 2016), Citrus medica L. var. sarcodactylis essential oil (Lou et al., 2017), negatively charged food-grade clove oil (Majeed et al., 2016), black cumin essential oil (Sharif et al., 2017), peppermint oil (Liang et al., 2012), tea tree oil (Li et al., 2016), Thymus daenensis essential oil (Moghimi et al., 2016a), sage (Salvia officinalis) essential oil (Moghimi et al., 2016b),

TABLE 9.2

List of Purpose of nanoemulsions Containing Essential Oil with Surfactant Used for Preparation and their Targeted Microorganisms

Source: Adapted from Pathania et al. (2018).

oregano oil (Bhargava et al., 2015), Pimpinella anisum oil (Hashem et al., 2018), thyme oil (Chang et al., 2015), and citral essential oil (Lu et al„ 2018) were found effective against various microorganisms studied. Therefore, it is cleared that essential oil nanoemulsions are promising tools for food preservation. In contrast to the earlier- mentioned studies, some controversial reports are also presented in the literature. In a study regarding nanoemulsions of thymol and eugenol, which were со-emulsified by lecithin and arginate, Ma et al. (2016) determined that gram-negative bacteria (E. coli 0157:H7) was not affected by mixture of lauric arginate with lecithin in milk. However, this combination was a good idea for nano-emulsifying essential oil components for addition to foods which require much transparency.