Encapsulation is defined as the process to entrap one substance (active agent) within another substance, yielding small particles that release their contents at controlled rates over prolonged periods of time and under specific conditions. Antimicrobial active packaging has emerged as an effective technology to reduce microbial growth in food products increasing both their shelf-life and microbial safety for the consumer while maintaining their quality and sensorial properties.
1. Antimicrobial Food Packaging
In Europe, the food sector is a major sector that generates more than 750,000,000,000 euros each year
[1], representing 4.4% of the Gross Domestic Product to the European Economy
[2]. According to the latest data provided by FAO
[3], about one third of all food produced for human consumption is wasted each year, which corresponds to 1.3 Gtons of food; a global tendency that is expected to grow in the future
[4].
Given the economic impact of the food industry in our society, microbial contamination of foods can result in significant losses for the food industry due to food spoilage. Furthermore, the consumption of microbial contaminated foods can lead to serious public health threats such as foodborne diseases and outbreaks. Microbial food spoilage is mainly caused by non-pathogenic spoilage microorganisms that are responsible for alterations on the nutritional and sensory characteristics of food products, such as oxidation, generation of off-flavours and off-odours as well as undesirable changes in texture and colour
[5]. On the other hand, foodborne disease is caused by pathogenic microorganisms that are responsible, each year, for 600,000,000 cases of illness, with almost 420,000 deaths and 27,000,000 Years of Life Lost (YLL), according to World Health Organization (WHO)
[6].
The first attempt of the food industry to fight microbial contamination was based on the direct addition of antimicrobials (e.g., food preservatives) to food products. This strategy proved to be of limited action due to the rapid diffusion of the antimicrobial substance from the surface to the mass of the product
[7], with concomitant loss of efficacy, so the food industry had to search for new and innovative ways to introduce antimicrobials in food products. Given that 99.8% of all food and beverages have to be encased in some sort of packaging during their existence, the next logical step was to include these antimicrobial substances in the food packaging material, giving rise to antimicrobial food packaging technology. A clear advantage of this option would be that the packaged food would be protected without having edible preservatives added directly in its composition. Antimicrobial packaging has the main goal of reducing, retard or even inhibiting microbial growth by interacting with the packaged food (direct contact) or the package headspace (indirect contact)
[5]. By controlling microbial flora, antimicrobial packaging ensures microbial food safety, while maintaining food’s quality and sensorial properties and increasing products’ shelf-life
[8]. Nowadays, antimicrobial packaging can come in several forms such as sachets or pads containing volatile antimicrobials, polymer films with direct incorporation of antimicrobial substances (extrusion, casting) and coating, adsorption or grafting of antimicrobials onto the surface of the polymer
[7]. It is quite obvious that antimicrobials have to reach the cells to inhibit their growth or to kill them. This fact implies that the antimicrobial agents will have to be in contact with the food, either in vapour phase or by direct contact between the active packaging and the food
[8]. There is a wide and ever-growing list of antimicrobial agents that have been or are currently being for the development of antimicrobial food packaging. Although the list is vast, not all antimicrobials are suitable for every application, as the choice of the antimicrobial to be used depends on several factors. The primary factor is the antimicrobial activity against the target microorganisms, including specific activity and resistance development, and the regulatory status of its use in foods
[9]. Furthermore, one has to take into account whether controlled release approaches are necessary or not, given the chemical nature of the food, its storage and distribution conditions as well as the physical-chemical characteristics of the packaging material where the antimicrobial is going to be included
[9].
1.1. Antimicrobial Substances Used in Food Packaging
The list of antimicrobial substances used for the development of antimicrobial food packaging is quite vast and is continuously evolving as a result of changing consumer trends and legislation. These substances include chemicals such as organic acids, triclosan, antibiotics, chlorine dioxide, nitrites and ammonium salts that are slowly being replaced by “greener”, more natural alternatives such as bacteriocins, enzymes, phages, biopolymers, natural extracts and compounds, essential oils and their components and metal nanoparticles (Table 1).
Table 1. Antimicrobial agents used in active food packaging. NA-not applicable.
Antimicrobial Class
|
Antimicrobial Agent
|
Packaging Material
|
Main Microorganisms
|
Food Product
|
Ref.
|
Organic acids
|
Lactic acid
|
Polyamide
|
Escherichia coli O157:H7
|
Fresh beef cuts
|
[10]
|
Lactic acid
|
Chitosan pectin starch biocomposite
|
Bacillus subtilis
Listeria monocytogenes
|
NA
|
[11]
|
Sodium benzoate
Citric acid
|
Polyvinyl alcohol (PVA)
|
Staphylococcus aureus
Escherichia coli
Candida albicans
|
NA
|
[12]
|
Potassium sorbate
|
Fish collagen and polyvinyl alcohol (PVA) composite
|
Escherichia coli
Staphylococcus aureus
|
NA
|
[13]
|
Bacteriocins
|
Sakacin-A
|
PE coated paper
|
Listeria monocytogenes
|
Thin-cut meat
|
[14]
|
Sakacin-A
|
Cellulose nanofibres
|
Listeria monocytogenes
|
Smoked salmon fillets
|
[15]
|
Nisin
|
Starch-halloysite nanocomposites
|
Listeria monocytogenes
Clostridium perfringens
|
NA
|
[16]
|
Pediocin
|
Starch-halloysite nanocomposites
|
Listeria monocytogenes
Clostridium perfringens
|
NA
|
[16]
|
Nisin
|
Chitosan-carboxymethylchitosan composite films
|
Listeria monocytogenes
|
NA
|
[17]
|
Bacteriocin 7293
|
Poly (lactic acid)/sawdust particle biocomposite film
|
Listeria monocytogenes
Staphylococcus aureus
Pseudomonas aeruginosa
Aeromonas hydrophila
Escherichia coli
Salmonella Typhimurium
|
Pangasius fish fillets
|
[18]
|
Bacteriocin-like substances
|
Starch
|
Listeria monocytogenes
|
Cheese
|
[19]
|
Bacteriocin-like substances
|
Triticale flour films
|
Listeria innocua
|
Cheese
|
[20]
|
Bacteriocin-producer living bacteria
|
Poly (ethylene terephthalate) (PET) coated with polyvinyl alcohol (PVOH)
|
Listeria monocytogenes
|
Precooked chicken fillets
|
[21]
|
Enzymes
|
Lysozyme
|
Nonwoven regenerated cellulose with carbon nanotubes and graphene oxide
|
Micrococcus lysodeikticus
|
NA
|
[22]
|
Lysozyme+ lactoferrin
|
Carboxymethyl cellulose-coated paper
|
Listeria innocua
Escherichia coli
|
Veal carpaccio
|
[23]
|
Lysozyme
|
Polyamide 11 (PA11) with halloysite nanotubes (HNTs)
|
Pseudomonads
|
Chicken slices
|
[24]
|
Glucose oxidase
|
Whey protein isolate
|
Listeria innocua
Brochothrix thermosphacta
Escherichia coli
Enterococcus faecalis
|
NA
|
[25]
|
Lactoperoxidase
|
Chitosan
|
Shewanella putrefaciens
Pseudomonas fluorescens
Psychrotrophs
Mesophiles
|
Rainbow trout
|
[26]
|
Biopolymers
|
Chitosan
|
Chitosan/ethylene copolymer
|
Escherichia coli
Salmonella Enteritidis
Listeria monocytogenes
|
NA
|
[27]
|
Hydroxyethyl cellulose/sodium alginate
|
NA
|
Escherichia coli
Staphylococcus aureus
|
NA
|
[28]
|
Bacteriophages
|
ϕIBB-PF7A
|
Alginate
|
Pseudomonas fluorescens
|
Chicken fillets
|
[29]
|
vB_EcoMH2W
|
Chitosan
|
Escherichia coli O157:H7
|
Tomatoes
|
[30]
|
LISTEX™ P100
|
Cellulose membranes
|
Listeria monocytogenes
|
Ready-to-eat turkey
|
[31]
|
Other
|
LAE
|
Cellulose nanofibres
|
Listeria monocytogenes
|
NA
|
[32]
|
Sulphur nanoparticles
|
Chitosan
|
Listeria monocytogenes
Escherichia coli
|
NA
|
[33]
|
Chlorine dioxide
|
PLA
|
Staphylococcus aureus
Escherichia coli
|
NA
|
[34]
|
Quaternary ammonium salt
|
PVA/starch
|
Staphylococcus aureus
Bacillus subtilis
Escherichia coli
Pseudomonas aeruginosa
|
NA
|
[35]
|
2. Encapsulation Strategies for Antimicrobial Packaging
Encapsulation is defined as the process to entrap one substance (active agent) within another substance, yielding small particles that release their contents at controlled rates over prolonged periods of time and under specific conditions
[36]. In the antimicrobial food packaging area, the encapsulation of antimicrobial compounds provides more efficient packaging materials by (i) protecting the antimicrobial compounds from degradation, volatilization or undesirable interactions with packaging materials, (ii) improving the compatibility between the packaging polymer and the antimicrobial substance, (iii) increasing the availability of the antimicrobial and (iv) providing a controlled release or/and stimuli-responsive release to extend the activity of the active material, reduce changes in food sensorial properties or comply with the legal restriction limits.
Encapsulating some types of antimicrobial substances has become essential to solve some problems that limit their use in packaging applications. In the case of EOs, for example, encapsulation is essential to reduce losses by volatilization or degradation during packaging manufacturing or storage, to improve the compatibility with biopolymer by increasing their solubility and/or to diminish the organoleptic impact in food products caused by their strong odour
[37][38].
A broad range of delivery systems or carriers have been developed to encapsulate bioactive compounds in the food and pharmaceutical sectors such as cyclodextrins, liposomes, emulsions, nanoparticles or microcapsules
[39]. However not all these available carriers can be applied in antimicrobial active packaging as they should be compatible with the packaging material and do not modify negatively their mechanical and physical properties in order to preserve their primary function of food protection.
2.1. Emulsions
Conventional emulsions consist of two immiscible liquids where one liquid is dispersed in the other in form of small droplets (
Figure 1). These colloidal systems can be used to encapsulate bioactive compounds at significant amounts. Lipophilic compounds can be encapsulated in oil-in-water (O/W) emulsions, while hydrophilic compounds can be encapsulated in water-in-oil (W/O) or oil-in-water emulsions. Multiple emulsions such as water-in-oil-in-water (W/O/W) and oil-in-water-in-oil (O/W/O) can also be used to encapsulate active compounds in order to improve delivery requirements
[39][40].
Figure 1. Schematic representation of a classical emulsion stabilized by surfactant and a Pickering emulsion stabilized by solid particles.
Regarding antimicrobial packaging, emulsions are used almost exclusively to incorporate essential oils or their chemical constituents into water soluble polymers, generally of natural origin, resulting in an O/W emulsion. The incorporation of EOs in emulsions improves their compatibility with water-based polymers, provides more transparent films while protecting EOs from volatilization and enabling a more controlled released
[41][42][43][44].
Emulsions with low particle size (nanometric or micrometric scale) present several advantages over systems containing larger particles
[45][46] such as better stability, decreased particle aggregation, increased transparency, added rheological properties and higher bioavailability of the encapsulated substances. Therefore, presumably, antimicrobial films containing emulsions of low particle size will be more homogenous, transparent and effective than those prepared with conventional emulsions. In fact, this hypothesis has been demonstrated by several authors dealing with the encapsulation of EOs and their major components in water-based films. For example, Guo et al. demonstrated that films containing allyl isocyanate (AIT) microemulsions revealed stronger antimicrobial activity and were more homogenous than those containing conventional emulsions
[47][48]. Similarly, Otoni et al. demonstrated that packaging films with nanoemulsions exhibited better uniformity and higher antifungal activity in packaged bread than those containing coarse emulsions
[49]. Oh et al. found that chitosan edible films containing lemongrass oil nanoemulsions showed better antimicrobial activity and produced less sensorial changes in coated grape berries than similar coatings with higher droplet size
[50].
Considering the advantages, most of the works carried out in recent years have focused their attention on the use of emulsion of lower particle size, namely microemulsions and nanoemulsions. Microemulsions are defined as oil and water colloidal dispersions stabilized by an interfacial layer of surfactant molecules with particles sizes ranging from 1 to 100 nm, usually 10–50 nm. This type of emulsions presents some advantages such as thermodynamic stability and transparency, which make them good vehicles to incorporate antimicrobial hydrophobic compounds into different polymeric matrices. However, they need a large amount of surfactant to be stable
[51]. Nanoemulsions are defined as conventional emulsions containing very small particles, typically lower than 200 nm. Like conventional emulsions, they are thermodynamically unstable, but their lower droplet size endows them long-term stability, higher bioavailability and transparency. These nanoemulsions also required surfactants, but in a lower surfactant-to oil ratio than microemulsions. As disadvantages, they have low stability in acidic conditions and are usually prepared by high-energy methods such as high-pressure valve homogenization, ultrasonic homogenization or high-pressure microfluidic homogenization
[51]. Nanoemulsions are, by far, the most used dispersions to encapsulate antimicrobials in active packaging.
Despite that, as can be seen, packaging materials containing emulsions as encapsulation strategy are based on polymers of natural origin. Most of the approaches used emulsifiers of synthetic origin, particularly, polysorbates such as Tween 20
[50][52][53] or Tween 80
[54][42][44][55][56][53][57][58][59][60][61]. Natural emulsifiers such as lecithin
[62][63][64][65], soy protein isolate
[64], arabinoxylan
[47] or sapindus extract
[66] have been scarcely used and generally in combination with polysorbates. Consequently, further research on the use of natural emulsifiers in bio-based packaging materials is on demand in order to satisfy the growing demand in food industry for natural ingredients.
Besides classical emulsions, Pickering emulsions have been used to encapsulate bioactive compounds with antimicrobial properties. These emulsions are stabilized by solid particles instead of the surfactants used in classical emulsions (
Figure 1). As in the case of surfactants, stabilization of emulsion droplets takes place by adsorption of small solid particles at the surface of the emulsion droplets, although the mechanism of adsorption is very different to the one of surfactants
[67]. This type of stabilization adds specific properties to Pickering emulsions which make them more suitable for certain applications. Particularly valuable for antimicrobial packaging applications is their higher stability and absence of surfactants
[51][67]. Conversely, the main disadvantages of Pickering emulsions are their opacity and the limited number of stabilizing particles that can be used in food applications
[51].
Additionally, it has been demonstrated that the use of this type of emulsions can improve some film characteristics when compared to those that incorporate classical emulsions. Almasi et al. compared pectin films incorporating oregano EO using nanoemulsions or Pickering emulsions
[68]. The results showed that both have similar antimicrobial activity but the film containing Pickering emulsions present more suitable mechanical and water barrier properties. Moreover, oregano EO release is slower from films containing Pickering emulsions than from those containing nanoemulsions.
Despite the potential advantages of using Pickering emulsions, to date, few antimicrobial packaging materials have been developed using this technology. Like in classical emulsions, Pickering emulsions are used as EO carriers and their components using solid stabilizing particles of natural origin. The antimicrobial activity of these new materials has been tested with good results in vitro, but only Fasihi et al. demonstrated their in vivo activity, namely, the inhibition of fungal growth in bread slices packaged in active films containing Pickering emulsions of rosemary essential oil
[69].
2.2. Core-Shell Nanofibers: Emulsion and Coaxial Electrospinning
Electrospinning is an effective, low cost and versatile technique used to produce continuous sub-micron or nano-scale fibrous films from various biopolymer materials such as chitosan, alginate, cellulose, dextran, gelatine or zein among others
[70]. This technique is based in the use of high voltage electrostatic fields to charge the surface of a polymer solution droplet, thereby inducing the ejection of a liquid jet through a spinneret to form a nanofibrous film
[70]. Electrospinning, particularly emulsion and coaxial electrospinning, can be used to produce nanofibers with core–shell morphology. Using this structure, bioactive compounds can be directly incorporated in the core protected by the shell layer minimizing their volatilization or oxidation and reducing their release ratio
[71][72]. In emulsion electrospinning, a stabilized emulsion (W/O or O/W) can be used as spinning solution using the conventional electrospinning technology to obtain core-shell nanofibers (
Figure 2). It has been shown that core-shell fibres produced by emulsion electrospinning are able to yield a more sustainable controlled released than fibres obtained by coaxial electrospinning despite the later having a more organized core-shell structure
[73]. In coaxial electrospinning, two solutions (core and shell) are delivered separately through a coaxial capillary and drawn by electric field to generate nanofibers with core-shell morphology (
Figure 2), meaning that this technique requires a more complex design than emulsion electrospinning and a precise control of different parameters such as interfacial tension and viscoelasticity of the two polymers
[71][74].
Figure 2. Emulsion electrospinning and coaxial electrospinning techniques.
Despite the attention drawn to electrospun core-shell nanofibers containing bioactive compounds in last years, the vast majority of research works are focused on pharmaceutical and biomedical fields while food applications have been less explored. However, the incorporation of antimicrobials in the core-shell nanofiber has shown a great potential to be used in active packaging materials, demonstrating a higher controlled-release and a strong antimicrobial action.
2.3. Cyclodextrins
Cyclodextrins (CDs) are a family of cyclic oligomers of α-
d-glucopyranose linked by α-1,4 glycosidic bonds (
Figure 3A) that can be produced due to the biotransformation of starch by certain bacteria such as
Bacillus macerans [75]. The more common natural cyclodextrins are α- cyclodextrins (6 glucose subunits), β- cyclodextrins (7 glucose subunits) and ɣ- cyclodextrins (8 glucose subunits), being β-CD the cheapest and, therefore, the most used. CDs present a truncated conical cylinder shape with an inner non-polar cavity and a polar external surface that makes them capable to encapsulate hydrophobic substances (
Figure 3B). The complex created between the CD and the loaded compound is called inclusion complex where CDs are the host molecules
[75][76].
Figure 3. (a) Chemical structure and (b) geometrical shape of cyclodextrins.
The use of CDs and modified CDs are one of the strategies most used in the food packaging area to encapsulate active compounds as indicated by the high amount of publications in the last fifteen years regarding this topic. Using this encapsulating strategy, the bioactive molecules improve their water solubility, can be protected from volatilization, oxidization or heating and can be released in a more controlled manner
[77][78][79][80]. Moreover, the low price, semi-natural origin and non-toxic effects
[75][76] of CDs explain the great interest of both research and industry in their use.
In last years, several of the publications dealing with cyclodextrins as encapsulation method in antimicrobial packaging have explored novel strategies to develop improved materials such as the incorporation of inclusion complexes in electrospun nanofibers.
As explained above, electrospinning is an effective and low cost technique to produce nanofibers mats. The fibrous film produced display high porosity, small pore size and high surface-to-volume ratio that make them more suitable to load high amounts of active substances
[81]. The combined use of electrospun nanofibers with cyclodextrin inclusion complexes aim to combine the benefits provided by each technique at the same time. Wen et al. produced and tested polylactic acid film electrospun nanofibers containing cinnamon EO/β-CD inclusion complexes. The inclusion of cinnamon in the cyclodextrin improved its thermal stability and its antimicrobial action, probably due to a higher solubility. Moreover, the electrospun fibres containing the inclusion complex exhibited better antimicrobial activity and retain the EO better than those films prepared by casting
[82].
Another recent strategy developed to encapsulate antimicrobial in CDs is the use of nanosponges
[83]. Nanosponges are cross-linked cyclodextrin polymers nanostructured within a three-dimensional network that offer some advantages in respect to monomeric native cyclodextrins such as a higher loading capacity, increased protection of encapsulated compounds and better controlled released
[84][85]. This novel approach has been used recently to encapsulate cinnamon and coriander essential oil demonstrating antimicrobial activity against foodborne Gram positive and Gram negative bacteria and a controlled EO release
[84][85]. However, the incorporation of these novel structures in packaging materials has not been tested yet.
2.4. Halloysites Nanotubes
Halloysite nanotubes (HNTs) are a type of natural occurring aluminosilicate clay minerals which are available in abundance in many continents including Asia, North America, Europe, Oceania, and South America
[86][87][88]. These substances display a characteristic two-layered (1:1) aluminosilicate structure similar to kaolin that usually adopt a hollow tubular nanostructure with a typical size of 500–1000 nm in length and 15–100 nm in inner diameter
[89] (
Figure 4). Owing to their tubular structure, HNTs can be used to load and release bioactive molecules, including antimicrobial agents. Furthermore, their low price, abundance, non-toxicity and eco-friendly features as well as their biocompatibility make them an attractive alternative to other tubular materials such as carbon nanotubes or TiO
2 nanotubes
[88][89].
Figure 4. Halloysite nanotubes have an external surface composed of silanol (Si-OH) along with siloxane groups and an internal surface composed of aluminol (Al-OH) groups.
Given the advantages described above, HNTs have been also applied in the antimicrobial packaging area. Several studies have demonstrated that the incorporation of antimicrobial substances via halloysite nanotubes improves the retention of the active compound in the packaging material and enables a more controlled-release. For example, a more extended lysozyme release from poly (ε-caprolactone) or poly(lactide) films has been achieved through its incorporation in HNTs
[24][90]. Similarly, a slow release of rosmarinic acid from PLA films was obtained by including rosemary EO in halloysite nanotubes
[91]. The use of HNTs to control the delivery rate has made it possible to increase the shelf-life of materials containing volatile antimicrobial agents. For example, films containing halloysite nanotubes loaded with thyme oil showed antimicrobial activity against
Escherichia coli during 10 days after thymol was loaded into HNTs
[92]. Similarly, LDPE lipid containing thymol/carvacrol/halloysite nanotubes retained their initial antimicrobial activity during 4 weeks of storage
[93].
By being included in HNTs, antimicrobials can be protected from losses due to volatilization or other processes. For instance, in another study, carvacrol was encapsulated in halloysite nanotubes and subsequently incorporated into polyamide polymers by extrusion. The results showed that polymers containing halloysites retained approximately 90% of the initial carvacrol content; while for the control PA/carvacrol system, no residual carvacrol was detected due to carvacrol evaporation
[94]. Similar results were obtained for LDPE containing halloysite nanotubes encapsulating mixtures of carvacrol and thymol
[93].
Nonetheless, the incorporation of halloysites has also been related to negative effects as the incorporation of HNTs in starch films increased the opacity of the films and reduced the antimicrobial activity of the active starch
[16].
Modifications in halloysites have been performed in order to obtain some advantages. For example, halloysites treated with NaOH have been used to increase the loading capacity of thyme oil from 180.73 to 256.36 (mg thyme oil/g HNT)
[92]. Other studies demonstrated that the capping of HNTs both ends and/or the coating of the outer surface of the HNTs can be employed to modify the release rate of antimicrobial compounds. For instance, the capping of HNTs ends with sodium alginate and the coating of the surface with positively charged poly(ethylene imine) polymer using the layer-by-layer method, yielded a slower thyme EO release from HNTs
[92]. Likewise, the coating of allyl isothiocyanate loaded HNTs with sodium polyacrylate (both ends and surface) enabled a more efficient release of AIT comparing to non-treated HNTs
[95].
Halloysite-loaded film manufacturing has been made using different techniques that include classical methodologies as casting
[16][90][91], compression moulding
[90], extrusion
[93] or more innovative ones such as electrospinning
[24]. Besides, halloysites have also been incorporated in packaging materials as coatings
[16][96][97] or inks
[92].
The antimicrobials materials loaded with HNTs as carriers have demonstrated high in vitro antimicrobial activity
[16][91][92][95][98]; notwithstanding, not all works carried out have applied this novel technology to food applications.
2.5. Liposomes
Liposomes are microscopic spherical-shape vesicles composed of a wall of amphipathic lipids arranged in one or more concentric bilayers with a aqueous phase inside and between the lipid bilayers
[99] (
Figure 5). The ability of liposomes to encapsulate hydrophilic or lipophilic drugs have allowed these vesicles to become useful drug delivery systems, being one of most widely used carriers for antimicrobial agents
[99]. Besides, the development of nanoliposomes has added the benefits of the nanosized particles to the encapsulation, delivery and targeting of bioactive compounds
[100].
Figure 5. Liposome loaded with hydrophobic and hydrophilic antimicrobial substances.
Using natural and non-toxic lipid molecules commercially available (generally lecithin and cholesterol), liposomes and nanoliposomes loaded with antimicrobial agents have been prepared and included in food packaging materials to obtain materials with improved properties. For example, the encapsulation of eugenol or cinnamon essential oils in lecithin liposomes led to chitosan films with higher retention ratio (40% − 50%) of volatile compounds when compared to what is retained when they are free incorporated by emulsification (1% − 2%)
[101]. Moreover, the incorporation of cinnamon essential oil nanoliposomes in gelatine films allowed for a lower antimicrobial release rate together with an improvement of the antimicrobial stability
[102]. Besides, coatings of chitosan loaded with
Satureja plant essential oil nanoliposomes exhibited a prolonged and consistent antimicrobial activity on meat pieces during their storage time in comparison with coating containing free EO
[103].
It is important to point out that liposomes can lead to negative changes in the optical properties of films due to the chromatic properties of lecithin or the occurrence of chemical reactions
[101][104].
Liposomes can also be further engineered to confer stimuli-responsive properties for drug delivery. Despite that these advanced structures have been widely applied in the biomedical area
[105], only few developments have been carried out for food applications
[106][107][108][109][110][111]. In the antimicrobial packaging field, only Lin et al. used this strategy to control the release of antimicrobials from the packaging material. In this work, cinnamon EO/β-cyclodextrin complexes were loaded into stimuli-responsive proteoliposomes, and subsequently incorporated in poly(ethylene oxide) electrospun nanofibers as strategy to control the growth of
Bacillus cereus in beef. The mechanism of activation of these proteoliposomes is based in the degradation of casein present in liposome walls produced by
B. cereus proteases
[112].
2.6. Other Encapsulating Particles
Besides the previously mentioned encapsulation particles, other micro- or nanoparticles such as microcapsules, nanocapsules, nanostructured lipid carriers, solid-lipid nanoparticles or nanoparticles among others have been used to encapsulate flavours, vitamins, antioxidants, food colorants or antimicrobials for food applications
[83][113]. However, not all these structures have been applied for antimicrobial encapsulation in active food packaging materials.
In the past years, responsive microcapsules and nanocapsules (
Figure 6) containing antimicrobials agents have been incorporated in polymers to control the release, and consequently, improve its effectiveness. For instance,
Cymbopogon citratus oil has been encapsulated in responsive microcapsules of gelatine-carboxymethylcellulose, gelatine-gum or melamine-formaldehyde walls. When these structures are subjected to mechanical stress, the wall breaks and the active compound is released. These responsive microcapsules have been incorporated in paper through coating, exhibiting antimicrobial activity against
Escherichia coli and
Sacharomyces cerevisiae in vapour phase after activation
[114]. Similarly, thyme EO has been incorporated in responsive capsules of lightly cross-linked polyamide shell. The shell is responsive to light due to the trans–cis isomerization of the photochromic azo-moieties, which causes a contraction of the polymer chains leading the release of the encapsulated content
[115]. These capsules have been incorporated in low-density polyethylene or poly(lactide) polymers by coating, releasing thyme EO with proved antimicrobial efficacy
[116]. An innovative responsive microcapsule for the delivery of chlorine dioxide (ClO
2) based on the reaction of NaClO
2 with water and tartaric acid was developed by Huang et al.
[34]. Poly (lactide) capsules were loaded with a gelatine core and NaClO
2 and, afterwards, incorporated in PLA film containing tartaric acid. In the presence of water, ClO
2 gas is produced and released from the film reducing the population of
Escherichia coli and
Staphylococcus aureus [34]. In a more recent work, this material was tested in vivo displaying a positive effect in food preservation by extending the shelf life of packaged mango
[117].
Figure 6. Microcapsule/nanocapsule and nanoparticle loaded with antimicrobial substances.
Nanoparticles (
Figure 6) have been also widely used in last years to encapsulate antimicrobials, generally EOs or their components, in diverse packaging materials. Antimicrobial-nanoparticle complexes of chitosan
[118][119][120][121], silica
[122][123], zein
[124] and polylactide
[125] have been incorporated into chitosan
[118][119][125], gelatine
[106][120] or cellulose
[124], among others, with the attainment of antimicrobial activity both in vitro and in vivo.
This entry is adapted from the peer-reviewed paper 10.3390/molecules25051134