Applications of Nanotechnology in Food & Cosmetics Preservation

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This entry is adapted from the peer-reviewed paper 10.3390/nano12071196

Cosmetic and food products containing water are prone to contamination during the production, storage, and transit process, leading to product spoilage and degraded organoleptic characteristics. The efficient preservation of food and cosmetics is one of the most important issues the industry is facing today. The use of nanotechnology in food and cosmetics for preservation purposes offers the possibility to boost the activity of antimicrobial agents and/or promote their safer distribution into the end product upon incorporation into packaging or film constructions.

food preservation cosmetics preservation nanotechnology

1. Food and Cosmetics Packaging

According to the Commission Implementing Decision 2013/674/EU, «packaging material is the container (or primary packaging) that is in direct contact with the formulation» ^[1]. Primary packaging plays an important role in the preservation and safety of the cosmetic/food product not only because it protects it from microbial contamination and subsequent spoilage, but because at the same time it may also interact with the product, either through the migration of substances it may contain (including antimicrobials), or through the transport of atmospheric agents such as oxygen in the product. This is the reason why the cosmetic file should contain specific characteristics of the primary container such as composition, possible impurities, and possible migration. Moreover, compatibility tests with the cosmetic product and composition are mandatory. The EU Framework Regulation for food contact materials (Regulation (EC) No 1935/2004) requires that materials be manufactured according to Good Manufacturing Practices (GMP) and not release their constituents into food at levels harmful to human health, and provides rules for compliance documentation and traceability ^[2]. The European Union essentially relies on the relevant legislation on food packaging and acknowledges that once a packaging material has been accepted for food, then it is more easily approved for cosmetics as well ^[1].

1.1. Smart Packaging

There are four ways to improve food packaging, making it “smart”: (i) the improvement of its mechanical and barrier properties; (ii) the delivery of antimicrobials that slowly release into the product; (iii) the incorporation of sensors that can detect harmful substances, microbial spoilage, or gas; and finally, (iv) the development of a packaging made from biopolymers.

Active and Intelligent Packaging

Active packaging technology is used to extend products’ shelf life by incorporating preservatives, oxygen scavengers, moisture absorbers, carbon dioxide emitters, and ethylene scavengers into the packaging material. Active packaging creates a microenvironment between food and packaging materials that can scavenge oxygen or moisture and prevent the evaporation of volatile substances such as flavors and ethanol, and it may also offer antimicrobial activity ^[3]. During the last few decades, a variety of antimicrobial agents have been incorporated into packaging materials, films, and coatings to extend the shelf life of packaged products and avoid microbial spoilage. This is specifically called “antimicrobial active” packaging and concerns packaging systems that deliver antimicrobial agents and release them within the product, in some cases in a controlled manner. Among biodegradable polymers, those that are used most to deliver preservatives in food are polylactic acid (PLA), cellulose, carrageenan, starch, and chitosan. In some cases, two or more different polymers are applied as a mixture to take advantage of their different properties in the end product ^[4]^[5].

A plethora of antimicrobial agents have been introduced in antimicrobial active packaging systems: nisin, pediocin, sodium benzoate, potassium sorbate, propyl paraben, antimicrobial essential oils, and other plant extracts ^[6]^[7]^[8]^[9]^[10], and their applications are found in different types of products, including fresh meat, fish, nuts, fresh fruits, beverages, and others. Recently, the cosmetics industry obtained active packaging technology from the food industry to prevent microbial spoilage ^[11].

Along with the active packaging, “intelligent” packaging has also been developed, and together they compose the term “smart” packaging. Intelligent packaging is a container, coating, or film that can detect impurities of dangerous substances, as well as biochemical or microbial changes in the product. This is achieved using sensors in the packaging, and the science behind this achievement is now based in nanotechnology. Two types of nano-sensors can be used in food packaging: electrochemical and optical. The incorporation of NMs into sensing systems imparts properties such as optical, thermal, plasmonic, catalytic, and others, improving their performance. Copper NPs have been described in the packaging of soft drinks and detect carbohydrate oxidation ^[12], graphene nanoribbons find applications in detecting various antioxidants in mixed fruit juices ^[13], carbon NPs detect the presence of melamine in milk ^[14], and gold NPs can detect heavy metals in water ^[15], and when they are electrodeposited on graphene ribbons, they can detect the presence of tert-butylhydroquinine in edible oils ^[16].

In the paragraphs hereunder, the different types of NMs that find applications in the cosmetics and food industries are presented.

2. Nanocomposites Used in the Cosmetics and Food Industries

As already mentioned, the packaging of food and cosmetics plays an important role in their preservation by preventing the entry of germs and delaying their spoilage. The evolution of nanotechnology and the multitude of properties that NPs have, have further advanced the packaging technology with the creation of active and intelligent packaging. The release of preservatives from the container to the product has made it possible to extend the shelf life of the latter, as well as to reduce the concentration of chemical preservative systems within the product, thus improving its safety. Consumer demands for and positive environmental impact of biodegradable packaging and sustainable waste disposal management have led to the development of packaging, including films and coatings, using only biodegradable materials, also called biopolymers. Among these biopolymers, cellulose, carrageenan, agar, starch, and chitosan have been investigated most. However, extended research concluded that packaging production using biopolymers has drawbacks, mainly in terms of strength and permeability, known as mechanical and barrier properties. Systems that have been shown to improve these properties are nanocomposites.

A nanocomposite is a heterogeneous system of two or more constituents with different characteristics, chemical and physical, and one of the components has at least one dimension in nanoscale. Nanocomposites in packaging are based on biopolymers as the continuous phase and nanofillers as the discontinuous phase. When fabricated in nanoscale, biopolymers have special properties that make them ideal materials for creating antimicrobial active films, such as the so-called “bacterial” nanocellulose. This refers to a natural nontoxic biopolymer synthesized from special bacterial species with unique nanostructured morphology, providing elastic properties in combination with high surface area, crystallinity, porosity, and resistance. Within its pores, bacterial nanocellulose can be the guest of several molecules held from its hydroxyl groups, such as antibacterial agents. The second most abundant polysaccharide resource in nature is chitin, which is derived from shellfish waste, and it differs from cellulose in that it has an amino group at the C-2 position instead of a hydroxyl group. Its partially deacetylated form is called chitosan, and it can be functionalized in derivatives that contain cationic or other moieties. Among structural properties, chitosan and its derivatives present antimicrobial activity that is dependent on its molecular weight, cationic charge density and position, degree of acetylation, hydrophobicity, and other functional groups it may contain ^[17]. Different modes of action are observed between Gram-positive and Gram-negative bacteria due to the differences in their cell envelope composition. In both cases, antimicrobial activity is based on the cationic charges in the chitosan backbone.

Nanofillers that have been incorporated into such composites include NPs, nanorods, nanotubes, and nanofibrils.

3. Carbon-Based NMs—Fullerenes

Carbon-based NPs include graphene oxide, carbon dots, carbon nanotubes, and fullerenes, and they have already found applications in tissue engineering, drug delivery, imaging, diagnosis, and cancer therapy. In the cosmetics industry, carbon nanotubes are used as delivery systems for bioactive compounds. Fullerenes are valuable skin-rejuvenating agents due to their ability to scavenge free radicals about 172 times more than vitamin C ^[18]. Indeed, scientists are investigating their cytoprotective therapeutic potential for several dermatological recovery treatments ^[19]. Fullerenes are usually dispersed into squalene or wrapped in polyvinylpyrrolidone. Yet, the applications of both type of ingredients are limited. Regarding fullerenes, the main obstacle is their poor solubility; however, this can be overcome either by using their derivatives or by encapsulating them into liposomes ^[20]. The most commonly used fullerenes in the cosmetics industry are fullerene C60 and fullerene C70, and although they are approved as antimicrobial and skin-conditioning agents, they are also applied in anti-ageing, whitening, and sun care products ^[21]^[22]^[23]. Very recently, on 6 July 2021, the European Commission requested a scientific opinion on fullerenes and their derivatives from the Scientific Committee for Consumer Safety. This was triggered from 19 requested notifications for new cosmetic products containing fullerenes. The deadline for the results was set at six months ^[24].

Carbon dots, nanotubes, and graphene oxide are already finding applications that improve antimicrobial activities of active packaging. Regarding fullerenes, such application has not been reported yet. However, a promising study is examining the antimicrobial activity of the fullerene C60 and three types of fullerenols. C60(OH)12, C60(OH)36. 8H20, and C60(OH)44. 8H2O were evaluated, and it was reported that although the pristine C60 did not manage to kill microorganisms, the fullerenols showed good antimicrobial activity against P. acnes, S. epidermidis, C. albicans, and M. furfur ^[25]. Although the preferred final application refers to anti-acne or anti-dandruff cosmetics, this study indicates that they could be tested as preservatives for the final application, especially C60(OH)44. 8H2O, which showed broad-spectrum antimicrobial activity.

3.1. Graphene-Based NMs

Among the various allotropes of carbon, graphene, a planar graphitic sheet of graphite, presents the most biofunctional properties due to the ability to enrich its surface with several functional groups.

Graphene-based NPs can damage bacterial cells in two ways: either via the production of ROS or by trapping microorganisms in its aggregated sheets ^[26]^[27]. The main advantage of graphene oxide when used in packaging is that it can increase packaging hardness and strength while decreasing its weight. For example, carbon nanotubes were incorporated into a film composed of chitosan and polylactic acid for strawberry preservation. Results show that the film had low permeability and increased tensile strength and demonstrated antimicrobial efficacy ^[28]. In another study, graphene oxide nanosheets and clove essential oil were incorporated in a polylactic acid-based film and researchers proved its activity against Gram-positive and Gram-negative bacteria, and oxygen permeability also decreased due to the planar configuration of nanosheets ^[29]. Graphene can additionally be applied for the coating of the antibacterial agent iron oxide, and with the use of chitosan, produce a nanocomposite useful in both the biomedical and food industries ^[30]. Hemolysis tests indicated no toxicity of this nanocomposite. Due to safety arguments regarding the extended use of preservatives, the possibility to have a preservative in packaging films not in contact with the product but available on demand drove researchers to study the introduction of the phenolic compound salicylaldehyde to graphene oxide platforms ^[31]. The successful release of the preservative stimulated by synthesized acids at the ripening stage led to the conclusion that these nano-composites can be applied for the preparation of eco-friendly “fruit switches”.

3.2. Carbon Dots

The preparation of carbon dots can be achieved either using “bottom-up” or “top-down” methodologies. Interestingly, they can be extracted through a one-step physical method from carbon black, the coloring agent commonly used in cosmetics and listed in the catalogue of NMs used in the European cosmetics market ^[32].

The properties of carbon dots that make them valuable in most applications are the tunable photoluminescence, high quantum yield, low toxicity, small size, biocompatibility, and low-cost sources ^[33]. In cosmetics, carbon dots were recently proposed for hair-dying applications, taking advantage of the interactions between carbon dots and hair proteins. On this assumption, various carbon dot species were doped on hair during dying with commercial colorants. Three species of carbon dots were found to make hair glow under 365 nm light, and those were the ones with the lowest absolute zeta potential ^[34].

Although the photoluminescent mechanism has not been fully investigated, this study proves their potential application both in hair and nail colorants. In addition, some recent studies investigate the introduction of carbon dots in sun care products as promising non-toxic, broad-spectrum, and eco-friendly ultraviolet absorbers ^[35]. In the food industry, they are currently used for the detection of food quality and safety, usually using fluorescence recovery or fluorescence quenching as detection principles to detect quality-related biomolecules such as ascorbic acid, tannic acid, and melamine in fruit juices, wine, and milk, respectively, or microorganisms and their metabolites such as Salmonella Typhimurium and aflatoxin B1 ^[36]. Due to their properties, carbon dots can be adsorbed on the bacterial cell surface and induce ROS production when exposed to light. This gives them antimicrobial characteristics. In a recent study, Kousheh et al. introduced carbon dots in a cellulose matrix and prepared a nanopaper that offered both antimicrobial and ultraviolet protection ^[37]. Carbon dots were produced from beneficial bacteria biomass, constituting a beneficial green approach. Generally, carbon dots can be produced from natural sources, including plant extracts rich in phytochemicals and fructose. Very recently, researchers prepared a nanopaper based on bacterial cellulose doped with carbon dots and supported its potential application in food preservation ^[38]. In addition, both studies reinforce the production of carbon dots from natural sources not only due to environmental concerns (green approach), but because of the high cost of using organic material as a precursor.

3.3. Carbon Nanotubes

Carbon nanotubes are cylindrical molecules that consist of rolled-up sheets of single-layer carbon atoms. They can be single-walled, with a diameter of less than 1 nanometer (nm), or multi-walled, consisting of several concentrically interlinked nanotubes with diameters reaching more than 100 nm. Nanotubes have 8 nm-diameter cavities that can encapsulate various functional materials in the food or cosmetic articles. A couple of patents have been filed in the field of hair colorants and cosmetic compositions ^[39]^[40]. Single-walled nanotubes have also displayed bactericidal activity against both Gram-positive and Gram-negative bacteria, but not much work has been done in this direction. Their microbial toxicity is mainly due to the oxidative stress that interrupts the continuation of the cell membrane or due to their adhesion to the microbial surface ^[41]. The application of carbon nanotubes as nanofillers in gelatin films has been successfully demonstrated, showing good results regarding the improvement of tensile strength and mechanical, thermal, and antimicrobial properties ^[42]. For instance, multiwalled carbon nanotubes were incorporated in a composite based on polylactic acid and chitosan to extend the shelf life of strawberries ^[28]. These researchers added carbon nanotubes in the composite, knowing that chitosan lags behind in heat stability and mechanical properties, and suggested a way to overcome these deficiencies.

Another type of carbon nanotube, called Halloysite nanotubes, have been introduced in nanocomposites to develop functional packaging for the extension of the shelf life of food and cosmetic products. Halloysite nanotubes are naturally occurring tubular clay NMs made of aluminosilicate kaolin sheets rolled several times. Biomaterials can interact with their surface due to aluminol and siloxane groups and form hydrogen bonds. Clay nanotubes have been studied for the deposition of hair dyes or anti-hair-loss active ingredients on hair ^[43], and have recently been used in nail polish and nail care products. The effectiveness of some essential oils as preservatives has already been mentioned, but their incorporation into active packaging has important drawbacks. High temperatures required for the formation of packaging can degrade such sensitive compounds and make them lose functionality. Halloysite nanotubes can protect the active molecules, keeping their quality and functionality in food-packaging systems ^[17]. Researchers studied the entrapment of carvacrol and thymol within halloysite nanotubes and concluded that using this method, the sensitive bioactive compounds retain their antimicrobial properties even in temperatures up to 250 °C ^[44]. The synergistic effect of the two essential oil components to preserve hummus spread was well demonstrated. Interestingly, the release of the two compounds within the product did not alter the organoleptic characteristics of hummus spread. Halloysites can be loaded both with liquid- and solid-phase ingredients and manage to slow down the release rate of those ingredients either by the admix of small NPs to make the end of the tube narrower or by coating the halloysite with higher-molecular-weight polymers like chitosan or gelatin in order to provide an additional barrier for the released ingredient ^[45]. In cosmetics, they have been applied to control the release of glycerol and solid sunscreen filters with success. Although no applications have been reported for the delivery of preservatives in a cosmetic product, it is expected that they could be a promising delivery system for them and a way to minimize their toxicity if their sustained release within the product for a long period is managed.

4. Nanoclays

Nanoclays are nanosize fluky soft silicate particles with a characteristic platelet form, with nanopores. These clays can be classified into four major groups: the kaolinite group from zeolite or halloysite, the montmorillonite/smectite group, the illite group, and the chlorite group. The montmorillonite group has gained more attention in the packaging sector due to the advantage of a high surface area with a large aspect ratio (50–1000) and good compatibility with most of the organic thermoplastics ^[46]. Many nanoclay materials are commercially focused on the development of improved food and cosmetics packaging ^[47]^[48]^[49]. They can be manufactured in many forms, such as rigid containers for beverages andflexible films for bread ^[50]. The incorporation of nanoclays into polymers depends on the type of polymer, the process, and the application, which determines packaging attributes, and these nanocomposites give unique mechanical and barrier properties to the container ^[49].

Except for improving packaging properties, nanoclays can be used as functional materials for active and intelligent packaging. They can provide antimicrobial or controlled release activity to the system and in the case of intelligent packaging, they can be used as colorimetric indicators. Several reports were found in the literature that demonstrated their antimicrobial function. For example, the addition of montmorillonite nanoclay in chitosan films applied on Gouda cheese increased their antimicrobial activity against Escherichia coli, Staphylococcus aureus, molds, and yeasts ^[51]. The minimum antimicrobial activity was observed for molds and yeasts. It is observed that this type of nanoclay is more effective against Gram-positive bacteria than Gram-negative ones. An explanation for this is the fact that Gram-positive bacterial envelopes are less complicated and the cationic groups of nanoclay, ammonium, or pyridinium, which are responsible for the antibacterial activity, bind on cells surface more easily.

The ability of nanoclays to entrap active ingredients and slowly release them into a product makes them valuable delivery systems for antimicrobial agents for the preservation of products on demand. Nanocomposites loaded with Salvia macrosiphon seed mucilage, Rosmarinus officinalis extract, carvacrol, potassium sorbate, and grapefruit seed extract were found to improve the antibacterial effect of active packaging, while at the same time maintaining food quality ^[52]^[53]^[54].

5. Metallic NPs and Their Nanocomposites

Metallic NPs, such as silver NPs, titanium dioxide NPs, and zinc oxide NPs, are extensively applied to make nanocomposites due to their antimicrobial properties, and most of their applications are found in the food industry. Bhuyan et al. presented the various antimicrobial mechanisms of metallic NPs ^[55]. Silver NPs release silver ions (Ag⁺) that cling to the microbial cell surface and disorganize the cell membrane, leading to the destruction of DNA and ending in cell death. More specifically, the uptake of free Ag⁺ ions forces the production of ATP, and in combination with the disruption of the DNA replication, ends up creating reactive oxygen species (ROS) ^[56]. Regarding the mode of action of titanium NPs, this is triggered through exposure to UV irradiation, which causes the production of ROS, leading to the lipid peroxidation of phospholipids in the cell membranes and bacterial death ^[57].

This photocatalytic activity as the starting point of titanium NP activation is the major handicap in the selection of this nanoparticle as a preservative. Even though zinc NPs possess high photocatalytic efficiency among all inorganic photocatalytic materials, it has been observed that the antibacterial activity of ZnO can be demonstrated likewise in the dark, inhibiting the bacterial growth ^[58].

5.1. Silver NPs

Concerning cosmetics, among the registered nano-ingredients in Europe, those that claim antimicrobial properties are NPs based on colloidal gold, platinum, and silver. The term “colloidal” implies that the particles are in aqueous suspension. In spite of their claimed antimicrobial activity, none of these metallic NPs have been approved as preservatives. Nano-colloidal gold and platinum suspensions are currently used in antiaging products to stimulate cell turnover and natural healing. This ability is based on the property of promoting electron transfer between metal ions naturally found in the skin. About 19 trade ingredients of colloidal gold and eight of colloidal platinum are registered ^[59]^[60]. None of them are listed in the preservatives list. Nano-colloidal silver is intended to be used as an antimicrobial agent in cosmetics, including toothpastes and skin care products, with a maximum concentration limit of 1%. It is commonly used in cosmetic products with bactericidal effects. It is well known that silver ions and silver-based compounds have strong antimicrobial effects ^[61]^[62]. However, these silver-based compounds gradually precipitate in solutions, resulting in the diminishing of their efficacy. On the other hand, silver NPs do not present these limitations and are more effective as cosmetic preservatives. The reduction of the particle size results in more stable silver solutions, with higher efficiency at killing both bacteria and fungi without penetrating human skin ^[63]. A recent comparative study between silver and gold NPs reported differences in the structure of the skin care product where they were applied. More specifically, silver NPs created agglomerates, whereas gold NPs did not ^[64]. In the same study, it was also reported that the fungicidal properties against A. niger and S. cerevisiae of both NPs were different. Sensory profile, smell, and color assessments of the tested creams demonstrated that the 200 mg/kg gold nanoparticle cream had a better performance.

Silver NPs and their nanocomposites are among the most widely used NMs in the food industry ^[65]. Silver has been authorized by the European Food Safety Authority (EFSA) as food additive for coloring food (E174). According to the EFSA, approximately 20% of the E174 used is from confectionery pearls. This is the reason why Narciso et al. conducted a study to investigate the accumulation of silver NPs in the gastrointestinal system and its possible toxicological effect. According to this study, silver NPs did not cause tissue damage or genotoxicity even though it was accumulated on the emptied duodenum due to the fact that they never passed into the cell nucleus ^[66]. Colloidal silver is still banned as food ingredient. On the other hand, colloidal silver proteins are consumed in the USA as a functional food. The bacteriostatic and bactericidal concentrations of electrically generated silver were determined in 1976 ^[61]. Since silver is not approved for direct addition to food, most of its applications are found in food packaging. A recent study demonstrated that a polyethylene composite film containing nanosilver showed great potential in developing an antibacterial and acidic food packaging system, while at the same time enhancing its barrier properties ^[67]. As can be seen, this kind of technology finds applications in a wide range of food, such as raw chicken fillets, bread, and nuts.

Instead of using silver NPs as preservatives alone, a wise application is to boost their activity with the combination of conventional preservatives by conjugation. For example, a recent study applied conjugated silver NPs on the surface of sodium benzoate (E211) and created a stable antimicrobial composite with significant efficiency against the food-borne pathogens Salmonella typhimurium type 2, Shiga toxin-producing E. coli (STEC), B. cereus, and S. aureus ^[68]. Given the fact that sodium benzoate-functionalized silver NPs were produced in water and the requested amount of sodium benzoate was reduced, this application offers a green and probably safer alternative for food preservation.

5.2. Titanium Dioxide NPs

Both titanium and zinc NPs are extensively used in the cosmetics industry, but not for preservation reasons. Titanium dioxide is applied mainly in sunscreens for ultraviolet protection and in color cosmetics to neutralize color pigments. Even though it does not seem to present any health risk, the Scientific Committee on Consumer Safety (SCCS) of the European Commission does not recommend its use in sprayable formulations. In the food industry, titanium dioxide is no longer considered safe when used as a food additive. Due to its ability to scatter visible light, it was added in food formulations to make products look white and bright. As already mentioned, the mode of action of titanium NPs is triggered from the exposure to UV irradiation, which causes the production of ROS, leading to the lipid peroxidation of phospholipids in the cell membranes of the bacteria and their subsequent death.

Titanium NPs are usually among the key ingredients constructing multilayered nanocomposites added in active packaging to extend the shelf life of specific food products. A nanocomposite film consisting of glycerol, cellulose nanocrystals, TiO2 NPs, and wheat gluten was coated over a kraft paper in three layers and was found to exhibit excellent antimicrobial activities against S. cerevisiae, E. coli, and S. aureus after 2 h of exposure to UVA light illumination ^[69]. In another study, a shelf-life extension of 6 days was observed following the application of a nanocomposite film composed of TiO2 NPs and rosemary essential oil on lamb meat during storage under refrigeration ^[70]. Similarly, a packaging system made of silver and titanium nanocomposite was able to significantly extend the shelf life and improve the microbiological safety of bread in comparison with bread packed in high-density polyethylene (HDP) and bread not subject to packaging ^[71].

5.3. Zinc Oxide NPs

Zinc oxide has been considered a valuable active ingredient for the treatment of various skin disorders since ancient times, and it is the key ingredient in diaper rash ointments. In addition, zinc NPs are the broadest spectrum UVA and UVB filters approved for use by European and USA authorities. Being a promising preservative for the pharma and cosmetics industries, zinc oxide has been studied in various sizes and concentrations against different microorganisms. A recent study explained the effect of the size of zinc NPs in their antimicrobial efficacy and demonstrated that this increases by decreasing their size ^[72]. This activity can be enhanced by γ irradiation maybe because of the effect on the nanoparticle diameter after irradiation ^[73].

Zinc NPs display high migration phenomena when they are in contact with an acidic environment. This was shown in a recent study where a homogeneous dispersion of such NPs was incorporated into a starch-based flexible coating for food-packaging paper ^[74]. Even though migration is a negative issue, whether it is within legislative limits can be measured and confirmed ^[75].

6. Nanoencapsulation and Delivery Systems for Preservatives

Encapsulation is the entrapment of molecules within a system and aims to protect them from the effects of environmental factors without losing any of its properties. Encapsulation can be promoted in micro or nanoscale. Nanoencapsulation in particular offers the advantages of increasing the solubility of some specific molecules and promoting their slow release. This can be exploited for the delivery of antimicrobial agents for the preservation of cosmetic and food products through the production of nanoemulsions, nanoliposomes, lipid carriers, nanofibers, and polymeric NMs.

6.1. Nanoemulsions

Nanoemulsions are oil-in-water or water-in-oil colloidal dispersions where the diameter of the droplets of the inner phase ranges from between a few nanometers to 200 nm ^[76]. This small droplet size gives them unique properties, including enhanced solubility, optical transparency, stability against sedimentation, and creaming for the delivery of a wide range of bioactive compounds. In comparison to macroemulsions, they are kinetically stable. Their preparation method requires high or low energy, and the final polydispersity index is typically low. Surfactant concentration and length as well as ultrasonication time control the final droplet size. Nanoemulsions are used in the food industry to create processed dressings. The use of nanoemulsions as antimicrobial agents is a very promising innovation. Their mode of action relies on the electrostatic attraction between the cationic charge of the emulsion and the anionic charge of the bacterial membrane. Due to the attractive forces, nanoemulsion particles fuse with lipids of the bacterial cell membrane, and when enough particles are fused, a part of the trapped emulsion energy is released, resulting in cell lysis.

Several companies of raw materials have used nanoemulsions as delivery systems to preserve bioactive compounds. For instance, vitamins, plant extracts, and essential oils are facing oxidation or solubilization problems and need a vehicle to deliver them effectively on skin, increasing their bioavailability. The replacement of synthetic preservatives with natural alternatives is a growing demand in both the cosmetics and food industries. Essential oils contain terpenes, terpenoids, phenylpropanoids, and other molecules that demonstrate antimicrobial properties. For instance, a comparative study between essential oils of Lavandulla officinallis, Melaleuca alternifolia, and Cinnamomum zeylanicum with methylparaben, which is used as a cosmetic preservative, showed that all those essential oils could replace the synthetic preservative ^[77]. Another study investigated specific essential oils when combined with common cosmetic preservatives and in certain cosmetic preparations, and showed that the essential oils increased the effectiveness of the preservatives against P. aeruginosa and S. aureus ^[78]. The incorporation of essential oils as preservatives in cosmetics or food products has limitations, especially regarding their volatile aromatic compounds, which may give an undesirable smell and taste to the final product. Because of such limitations, essential oils cannot be added in formulations in high concentrations, leading to a loss of effectiveness. The entrapment of essential oils or their bioactive compounds in nanoemulsions to improve their preservation function has found applications in many different products.

Tween 80 is the emulsifier used in most of the studies, and formulations vary depending on the application. The oil phase composition of nanoemulsions, ripening inhibitor type, and concentration can influence the antimicrobial activity of the essential oils ^[79]. The main destabilization process of such nanoemulsions is the Ostwald ripening phenomenon, where an increase in the oil droplet size is promoted mainly in the first 24 h after production. One of the proposed strategies to overcome such aging is the addition of a gelling agent or a gum into the dispersed phase. Aggregation is another phenomenon that can occur during the lifetime of a nanoemulsion. Polymer-coated nanoemulsions can delay the onset of this phenomenon, and recent studies suggest the use of the cationic biopolymer chitosan due to the generation of both electrostatic and steric repulsive interactions ^[80].

6.2. Nanoliposomes

Nanoliposomes are among the most investigated nanocarriers in the cosmetics and food industries. They are vesicular systems with an aqueous core and are surrounded by a lipophilic bilayer, offering the advantage to carry and deliver both hydrophilic and hydrophobic molecules. Nanoliposomes are produced through the assembly of amphipathic molecules, usually phospholipids, and the techniques of their preparation are divided into passive and active loading. In the cosmetics industry, liposomes find applications as delivery vehicles for various bioactive compounds such as vitamins, peptides, phytosterols, and phytocompounds. The encapsulation of cosmetic preservatives into liposomes has not yet been studied, but several studies demonstrate the benefits of using nanoliposomes as delivery systems of antimicrobial compounds not yet registered as preservatives. A recent such study investigated three essential oils distilled from Artemisia afra, Eucalyptus globulus, and Melaleuca alternifolia encapsulated in nanoliposomes based on diastearoyl phosphatidylcholine and diastearoyl phosphatidylethanolamine and tested them for their antimicrobial efficacy ^[81]. The E. globulus and M. alternifolia liposomes showed the lowest minimum inhibitory concentrations, but further coating with polymers improved their stability.

In contrast to the fate of nanoliposomes in the cosmetics industry, nanoliposomal formulations delivering antimicrobial agents have found many applications in the food industry. Some of their most promising applications are presented in Table 1, and many of them could have a potential use in cosmetics as well.

Table 1. Applications of nanoliposomes in the food industry.

Application	Targeted Microorganisms	Encapsulated Preservative	References
Food contact surfaces	S. aureus, L. monocytogenes, E. coli, and Salmonella spp.	Carvacrol	[120]
Food contact surfaces	S. aureus or S. enterica	Thymol, carvacrol and thymol/carvacrol	[121]
Milk, yogurt, spices, juice, processed meat, mayonnaise, and tahina	E. coli, Salmonella, and Candida	clove oil, black seed oil, thyme oil, garlic oil, rosemary oil, and green tea, tetracycline	[122]
Edible films	S. aureus	Nettle (Urtica dioica L.) extract	[123]
Functional foods, e.g., dairy products and beverages	S. aureus, L. monocytogenes, and E. faecalis	Nisin	[124]
Minas fresca cheese	L. monocytogenes	Nisin	[125]
Milk, dairy industry	L. monocytogenes, S. Enteritidis, E. coli, and S. aureus	Nisin and garlic extract	[126]
Tofu	S. aureus and E. coli	Clove oil	[127]
Milk containers	S. aureus biofilms	Salvia oil	[128]
Not reported	(S. aureus, E. coli, S. Typhimurium, and L. monocytogenes	Eugenol	[129]

It is well noticed that the entrapment of nanoliposomes delivering preservatives in edible films minimizes their antimicrobial activity due to the inhibition of their release from the matrix ^[82]. Another reason why nanoliposomal formulations can diminish the preservation activity of some molecules is the negative charge of the zeta potential. Thus, the electrostatic repulsion between the negatively charged nanoliposomes and the negatively charged bacteria results in lower interaction between the encapsulated preservative and the bacteria, and as a consequence, less antibacterial activity ^[83]. On the other hand, the surface modification of liposomes could improve the stability of the liposomal membrane while at the same time avoid lipid oxidation that can often affect liposomes ^[84]

6.3. Niosomes

Niosomes can be mentioned as an advanced version of liposomes. They are delivery vehicles with a closed bilayer structure composed of non-ionic surfactants. They are recognized as safer and cheaper than liposomes and more stable, with a longer self-life ^[85]^[86]. Nanoliposomes are prone to oxidation due to sensitive phospholipids ^[87]. On the other hand, niosomes face leakage deficiencies, a phenomenon that can be corrected by the addition of cholesterol ^[88]. Applications of niosomes can be found in both cosmetic and pharmaceutical industries, taking advantage of their property of enhancing the bioavailability of bioactive compounds ^[89]. They are usually applied for the protection of sensitive compounds such as vitamin A and Ε or resveratrol ^[89]^[90]^[91]. Interestingly, niosomes provide a protective delivery system for antibiotics, making them remarkably effective in orthopedic, orthodontic, ophthalmological, and other treatments ^[92]. Applications in the food industry are very few, and their use in a preservation system has not been studied yet.

6.4. Solid Lipid NPs and Nanostructured Lipid Carriers

Solid lipid NPs (SLNs) are NPs composed of lipids with a solid lipid matrix. Their nanometer size offers them unique properties such as high drug-loading capacity and long-term stability. Their production does not require organic solvents; therefore, they can support green chemistry claims. Nanostructured lipid carriers (NLCs) belong to the second-generation lipid NPs and are the result of the combination of solid and liquid lipids. In comparison to SLNs, NLCs have a distorted structure, with spaces for the accommodation of biomolecules, and offer better loading capacity and stability. In the last 15 years, SLNs and NLCs have been the most common carriers of active ingredients used in the cosmetics and food industries and arose from the need to overcome the deficiencies of liposomes, nanoemulsions, and polymeric NPs ^[83]. Some of the current applications of lipid NPs in these industries are summarized in Table 2.

Table 2. Applications of SLNs and NLCs in the food and cosmetics industries.

Lipid Nanostructure	Incorporated Substance	Application	References
SLNs	Vitamin E	Cosmetic	[138]
SLNs	Quercetin	Food	[139]
NLCs	Mediterranean essential oils	Cosmetic	[140]
NLCs	Phenylethyl resorcinol	Cosmetic	[141]
NLCs	Retinol	Cosmetic	[142]
SLNs	Tretinoin	Cosmetic	[143]
SLNs	Coenzyme Q10	Cosmetic	[144]
SLNs and NLCs	Lycopene	Food	[145]
SLNs	Adenosine	Cosmetic	[146]
SLNs and NLCs	Resveratrol	Food and Cosmetic	[147,148]
SLNs	Citral	Food	[149]
SLNs	Mosquito repellent essential oils	Cosmetic	[150]
SLNs and NLCs	Alpha-lipoic acid	Cosmetic	[151]
SLNs	Carvacrol	Food	[152]
SLNs and NLCs	Butyl 4-hydroxybenzoate	Cosmetic	[153]

Lipid nanoparticle applications aim to protect the transported biomolecule and increase its bioavailability. Many different formulations have been tested to create the ideal SLN or NLC, and depending on the biomolecule, the target, or the type of the final product, different lipids make up the nanoparticle. In addition to that, several manufacturing procedures are applied for each formulation, such as double emulsion and melt dispersion ^[93], high pressure homogenization cold dispersion ^[94], high pressure homogenization hot dispersion ^[95], warm microemulsion ^[96], supercritical fluid ^[97], and solvent displacement ^[98]. Even though there is a plethora of different formulations and manufacturing processes for SLNs, scientists are still investigating new cost-effective and scalable production methods ^[99].

SLNs made with pure precirol and NLCs made of a mixture of precirol and almond oil are able to act as reservoir systems for parabens and their mixtures ^[100]. The sustained released of parabens minimizes product contamination during usage by the consumer, and at the same time reduces the toxicity of such preservatives in cosmetics. The prolonged antibacterial activity of nisin against L. monocytogenes and L. plantarum, when encapsulated in SLNs, was evident for more than 15 days in comparison to free nisin ^[101]. In that case, SLNs were formulated with the emulsifier glyceryl monostearate and the main surfactant poloxamer 188. Their manufacturing process was based on hot high-pressure homogenization. Interestingly, in another study, SLNs loaded with carvacrol were more effective against food spoilage bacteria when fabricated with propylene glycol monopalmitate and glyceryl monostearate in a ratio of 1:1 ^[102].

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