Biopolymer-Based Multilayer Films and Coatings for Food Preservation: an Update of the Recent Development

Biopolymers present appealing properties such as gas barrier abilities and biodegradability, which can be used to develop multilayer films for food preservation applications. This article provides an update on the recent research progress on biopolymer-based multilayer films. Various multilayer films have been developed based on biopolymers and their combination with other biodegradable polymers (e.g., PLA, PCL, and PVA), organic compounds (e.g., lauroyl arginate ethyl, carvacrol, natural plant extracts, and essential oils), and inorganic particles (e.g., nanoclays, silver, and metal oxides). These multilayer films present enhanced properties and functions such as barrier performance against gas, water, oil, and UV light, antimicrobial and antioxidant activities, and pH indication. This article overviews the design principles and fabrication methods for multilayer films, their properties and functions, and specific food packaging applications. The current challenges and future perspectives in this area are also proposed.


Introduction
Food waste is a pressing issue nowadays since it not only generates an additional amount of greenhouse gas (GHG) emissions resulting from food production, transportation, and storage but also causes economic losses for producers, suppliers, and consumers [1,2].Moreover, food poisoning due to improper postharvest handling and storage of food and/or the lack of indication of its freshness poses serious health issues to humans [3].Given this, advanced food preservation and packaging technology are highly demanded to maintain food quality during storage and extend food shelf-life.
Suitable food packaging can protect packaged food from external environmental factors and reduce its physicochemical changes.These packaging materials must possess effective barrier properties, namely, preventing the entry/exit of different penetrants such as moisture (water vapors), oxygen, carbon dioxide, greases, microorganisms, and oils [4••].Moreover, active food packaging ensures such functions as antimicrobial and antioxidant [5•, 6].However, food packaging materials dominating the market are mostly non-degradable materials such as polyethylene (PE) and polyethylene terephthalate (PET) [7•], which may be problematic to the natural environment or end-of-life processing.Typically, flexible food packaging materials like PE are non-biodegradable and difficult to recycle [8].Given this, it is interesting to develop fully biodegradable or compostable alternatives for food packaging.
To develop environmentally friendly food packaging, different biodegradable polymers can be utilized, including natural biopolymers and some synthetic polymers.Biopolymers (natural polymers) are directly biosynthesized by living organisms (plants, animals, and microorganisms), consisting mainly of polysaccharides (carbohydrate polymers formed by connecting monosaccharide units via glycosidic linkages) and proteins (formed by amino acid residues linked via peptide bonds).Cellulose, starch, pectin, glucomannan, chitin/chitosan, hyaluronan, alginate, agar, carrageenan, pullulan, gellan gum, xanthan gum (XG), and curdlan are some common polysaccharides, and typical proteins include soy protein, zein, gluten, casein, collagen, gelatin (hydrolyzed collagen), whey protein, keratin, and silk fibroin [9].It is worth mentioning that polymers that can be biologically synthesized based on biomass carbon sources (e.g., sugars and lipids), such as polyhydroxyalkanoates (PHAs), bacterial cellulose, gellan gum, XG, and curdlan, can be considered as both biopolymers and biobased polymers.Besides, there are biodegradable polymers chemically synthesized from biobased monomers, such as polylactide (PLA), and those chemically synthesized from petroleum-based monomers, such as polycaprolactone (PCL).The use of these biodegradable polymers for developing food packaging films has been studied extensively, but there are still challenges in properties (e.g., barrier, mechanical and antimicrobial properties, and hydrophobicity), durability, and cost [7•, 10, 11].
Multilayer films are composed of two or more layers of different materials that are bonded together.Compared with traditional single-layer films, multilayer films can be superior in physical, mechanical, and gas barrier properties [12,13].This review focuses on the most recent progress in multilayer films and coatings based on biopolymers and other biodegradable polymers, including their design, properties/functions, and related food applications.

Design and Fabrication of Multilayer Films
To produce multilayer films, techniques such as co-extrusion, compression molding, electrohydrodynamic processing (including electrospinning and electrospraying), and layer-by-layer (LbL) deposition can be used [14][15][16].LbL deposition, which involves stacking multilayer by repeatedly casting/dipping/spraying and drying film-forming solutions, has been used most in the studies of biopolymer-based multilayer films as it is easy to practice.Olmo et al. [17] applied LbL dipping-coating to create a multilayered assembly of chitosan and anionic β-cyclodextrin (β-CD) onto PLA (Fig. 1A).In this method, a hydrolyzed poly(l-lactide) (PLLA) film was ionized with a 0.1 M NaOH solution to activate the carboxyl groups, which can electrostatically interact with subsequent polyelectrolyte deposition.Similarly, LbL assembly was employed to create quadlayers of carrageenan + chitosan + montmorillonite (MMT) + chitosan (Fig. 1B) [18].In both cases, the interaction between different layers was favored by the reverse charges of the different layers.
Paper or paperboard, whose main ingredient is cellulose, is biodegradable and is widely considered a promising alternative to traditional non-biodegradable plastics for food packaging.However, as paper itself is waterand oil-sensitive [19,20], coating or lining it with other biopolymers or biobased polymers is usually needed for food packaging applications.Wang et al. [21] reported the use of solution-coating to coat paperboard with a layer of carboxymethyl chitosan (CMCS) and carboxymethyl cellulose (CMC) and then a layer of PLA/ZnO (Fig. 1D).The multilayer coating showed excellent oxygen and water vapor barrier performance and oil resistance, along with excellent antimicrobial activity [21].Nanocellulose has excellent barrier performance against grease, mineral oils, and oxygen, while its poor tolerance against water vapor can be overcome by laminating with PLA [22].Considering this, Koppolu et al. [22] reported coating paperboard with a nanocellulose (microfibrillated cellulose (MFC) or cellulose nanocrystals (CNC)) layer using a custom-built slot-die and then a PLA layer using a pilot-scale extrusion coater (Fig. 1D).In both studies [21,22], the paperboard was precoated with cationic starch before coating nanocellulose or CMCS/CMC in order to improve the adhesion between the paperboard and the polysaccharide layer.
For multilayer films, strong adhesion between neighboring layers is necessary because weak adhesion might alter the characteristics of the materials and limit their practical use.Strategies must in place to ensure the adhesion between natural biopolymers, which are hydrophilic, and polyesters (e.g., PLA, PCL, and PHA), which are hydrophobic.Li et al. [23] used SiOx prepared via a plasmaenhanced chemical vapor deposition (PECVD) technique as a transition layer between PLA and chitosan to form a three-layer composite film of PLA + SiOx + chitosan.Oxygen plasma irradiation was demonstrated to be effective in enhancing the hydrophilicity of the SiOx surface and, consequently, the adhesive strength between SiOx and chitosan, especially for a longer irradiation time (60 s).To produce multilayer films, Heidemann et al. [24] treated PCL or PLA using atmospheric air cold plasma to improve its adhesion with starch layers.The treatment increased the PCL and PLA films' surface roughness and decreased their water contact angle (WCA), and the resistance to delamination grew with treatment time (10 min resulted in no delamination).The multilayer films (PCL + starch and PLA + starch) exhibited water vapor permeability (WVP) significantly lower than of the starch film (decreased by 94.9% and 94.8%, respectively) but similar mechanical properties to those of the PCL and PLA films.Moreover, the adhesion between PLA and starch can be improved by corona-treating PLA [24].In another study, corona-treated PLA was shown to have better adhesion with wheat gluten [25].Wang et al. [21] corona-treated a CMCS/CMC  [17] with permission from Elsevier, Copyright 2019.B is reproduced from Ref. [18] with permission from Elsevier, Copyright 2019.C is reproduced from [21] with permission from Elsevier, Copyright 2022.D is reproduced from Ref. [22] coating layer to enhance its adhesion with a subsequent PLA coating (Fig. 1C).Similarly, Koppolu et al. [22] showed that corona-treating nanocellulose-coated paperboard prior to the extrusion coating of PLA improved the adhesion of PLA to the surface of nanocellulose (Fig. 1D).

Properties and Functions of Multilayer Films
Depending on the sequence of layers (outside and inside, and top and bottom), multilayer films can be designed to achieve complementary properties or functions while minimizing the disadvantages of participating components.
Yan et al. [26] reported that the multilayer films composed of PVA and chitosan exhibited better overall performance in tensile strength, water resistance, UV light blocking, and antioxidant activity than the blend film.Shan et al. [27] fabricated LbL multilayer films comprising a layer of gelatin added with green tea extract (GTE) as both a crosslinking agent and active ingredient and another layer of alginate, which showed better UV light blocking capability, mechanical strength, oxygen and water vapor barrier properties, moisture resistance, and thermal stability than the pure gelatin film, the alginate film, and the gelatin/GTE/alginate blended film.
However, multilayer films do not always show better properties than their blend counterparts.For example, a bilayer film made of chitosan and gelatin, with the addition of lauroyl arginate ethyl (LAE) as an antimicrobial compound, proved to be an effective UV light barrier.Compared to its blended version, the bilayer film exhibited reduced transparency, while also showing lower tensile strength and elastic modulus, but higher WVP [28].Wang et al. [29] reported that for a chitosan film, the introduction of an additional alginate layer improved the mechanical properties and water vapor resistance but decreased the antibacterial effect.
Typical properties and functions of biopolymer-based multilayer films are summarized in Table 1 and discussed below.

Barrier Performance Against Gas, Water, and Oil
While natural biopolymers exhibit excellent gas barrier properties, most of them (especially polysaccharides) are hydrophilic and present high WVP, which limits their applicability in food packaging.However, research has shown that creating multilayer films with combinations of different biopolymers, combinations of biopolymers with polyesters, or the addition of inorganic particles or other hydrophobic ingredients may overcome the poor moisture resistance of biopolymers.
Chen et al. [30] reported that a bilayer film of zein + starch presented a better barrier against water vapor than a starch monolayer film.In another study, a double-layer film composed of raspberry anthocyanins/low-acyl gellan gum + chitosan displayed good structural compatibility and lower WVP (1.57± 0.19 × 10 −11 g/m s Pa) than a single-layer film (anthocyanin/gellan gum) (6.82 ± 0.41 × 10 −11 g/m s Pa) [31].A bilayer of gelatin/GTE + alginate showed improved water vapor and oxygen barrier properties compared with gelatin and alginate monolayer films and even their blends, likely resulting from a compact multilayer structure with strong interlayer interaction [27].
Wax, due to its hydrophobic nature, can effectively increase the water resistance and decrease WVP of biopolymer-based multilayer films.Pasquier et al. [33] achieved oxygen, water vapor, and UV barrier capabilities using a stepwise assembly of cellulose nanofibers, a biobased wax (carnauba), and lignin particles supported by chitin nanofibers (LPChNF).The multilayer design comprising cellulose nanofibers (CNF)/wax/LPChNF enabled high oxygen (OTR of 3 ± 1 cm 3 /m 2 day) and water vapor (WVTR of 6 ± 1 g/ m 2 day) barriers at 50% relative humidity (RH) and was also effective against oil penetration [33].
As mentioned in the "Design and Fabrication of Multilayer Films" section, the paper itself is water-and oil-sensitive, which may be overcome by coatings.Considering that nanocellulose is an excellent barrier against grease, mineral oils, and oxygen, Tyagi et al. [34] coated a bilayer of CNF + CNC (containing MMT and soy protein) onto paper, enabling high oil and grease resistance (a kit rating of 11).Compared with uncoated paper, the coated one also showed a significant reduction in air resistance (by a factor of ~ 300), oxygen transmission rate (OTR) (by a factor of ~ 260), and water vapor transmission rate (WVTR) by 30%.
PLA is resistant to water vapor, although being a rather poor oxygen barrier.Given this, Koppolu et al. [22] coated paperboard with either MFC or CNC, which was subsequently coated with PLA to realize a packaging paper with overall excellent barrier performance (Fig. 1D).Similarly, multilayer films with a combination of PLA or PCL with starch showed significantly higher water vapor barrier performance than a starch film, while the mechanical properties were similar to those of PCL and PLA films [24].Rocca-Smith et al. [25] demonstrated that a trilayer film comprising a wheat gluten layer sandwiched between two PLA outside layers had significantly increased oxygen barrier performance (up 20 times) than PLA alone.Wang et al. [21] reported that the barrier characteristics to oxygen and soybean oil of a CMCS/CMC + PLA multilayer coating on paperboard (Fig. 1C) were superior to those of a single coating (CMCS/CMC or PLA) layer.Additionally, with a 1.5% content of ZnO added in the PLA phase, the barrier characteristics to oxygen, water vapor, and heptane vapor all doubled, and the oil resistance time increased to 235 h.Although corona treatment reduced the number of polar components in the CMCS/CMC coating and marginally impaired the coating's heptane barrier performance, the coating was still successful at repelling oil.Also for coating paper, Li et al. [18] used quadlayers of carrageenan + chitosan + MMT + chitosan, followed by carnauba wax treatment (Fig. 1B).Compared with the original paper, this superhydrophobic paper sample showed reduced WVTR and air permeability, especially with a larger number (up to 3) of quadlayers.

Antimicrobial and Antioxidant Properties
Multilayer films can be created with antibacterial or antioxidant properties if relevant agents are incorporated into the formulation.Chitosan is a polysaccharide with antimicrobial and antifungal activities [35], so it is commonly used to design films with bactericidal activity.Additional antimicrobial or antioxidant agents that can be used include inorganic particles such as ZnO [17,29], silver nanoparticles (AgNPs) [36], Cu 2 O [26], and CuO [32] and organic compounds such as lauroyl arginate ethyl (LAE) [28], carvacrol [17], tea polyphenol (TP) [37], cinnamon essential oil (CEO) [38], oregano essential oil (OEO) [39], lingonberry extract (LBE) [40], green tea extract (GTE) [27], and citric acid (CA) [41].When these antimicrobial or antioxidant agents are incorporated into one or more layers of multilayer films, their sustained release may be realized, resulting in prolonged antimicrobial or antioxidant function.
For coating paperboard, a bilayer film of CMCS/ CMC + PLA/ZnO containing 1.5% ZnO nanoparticles reduced the growth of bacteria S. aureus and E. coli by over 99% [29].A trilayer of chitosan + rGO@AgNPs + chitosan, with a sandwich-like structure, was shown to realize the sustained and slow release of AgNPs (only ~ 1.9% released for up to 14 days), resulting in a durable antibacterial effect and good antibacterial activity against E. coli and S. aureus [36].
Andrade-Del Olmo et al. [17] designed a multilayer film composed of PLLA/ZnO + chitosan + (2-carboxyethyl)-βcyclodextrin (β-CD)/carvacrol (see Fig. 1A), which showed enhanced antibacterial and antioxidant characteristics thanks to the controlled release of the carvacrol included in the β-CD (over 95 wt% of the totally released carvacrol was achieved by 14 days).Haghighi et al. [28] demonstrated the excellent antimicrobial activity of a chitosan + gelatin bilayer films enriched with LAE in each layer against four primary food bacterial pathogens including E. coli, L. monocytogenes, S. typhimurium, and C. jejuni.Zhang et al. [38] suggested that multilayer films (alginate + chitosan/CEO and alginate + chitosan/CEO + alginate) exhibited more sustained release and a higher retention rate of CEO as an antifungal agent compared a chitosan/CEO monolayer film.Cai et al. [39] developed films containing multiple layers (up to 3) of zein/gelatin and found that, by concentrating OEO in the middle and/or top layers, the multilayer films showed greater retention rates and more desirable release characteristics, including a sustained release profile in an aqueous food simulant and a higher release rate and content in a fatty food simulant, and, in the preservation test on strawberries, a protracted antibacterial action was seen.Jamróz et al. [40] reported that for a bilayer of furcellaran/LBE + CMC/gelatin hydrolysate, LBE endowed the film with antimicrobial activity against E. coli, E. faecalis, S. aureus, S. enterica, and P. aeruginosa.Moreover, the films with the highest amount (40%) of LBE demonstrated antimicrobial activity against the tested bacteria and antioxidant activity (from 3.01 to 41.89 mM Trolox/mg via the ferric reducing antioxidant power (FRAP) method, and from 0 to 20.56% via the DPPH free radical scavenging method) [40].Similarly, a gelatin/ GTE + alginate bilayer showed high antioxidant and antibacterial activity [27].

UV Barrier Performance and Transparency
Lipid oxidation can be easily and quickly brought on by UV radiation, degrading food quality [42].Therefore, increasing the UV barrier performance of food packaging films is necessary.Proteins like zein perform better as UV barriers than polysaccharides, and thus, they can be combined with polysaccharides to form multilayer films to address the shortcomings of polysaccharides.For example, a bilayer film of zein + starch can shield UV light better than a starch film, although being more transparent [30].
Besides, more organic and inorganic substances can be incorporated into films to increase their UV barrier performance.For example, a bilayer film of chitosan/LAE + gelatin/LAE was demonstrated as an effective barrier against UV light and showed lower transparency values, ascribable to the presence of LAE [28].A bilayer film of gelatin + gelatin/epigallocatechin gallate (EGCG) showed reduced UV transmission and improved light barrier performance due to EGCG [43].
For a carrageenan/curcumin/anthocyanins film, an additional emulsified layer of konjac glucomannan (KGM)/ camellia oil (CO) was also found to improve the UV resistance [44].

Hydrophobicity
As natural biopolymers are usually highly hydrophilic, increasing the hydrophobicity of multilayer films containing natural biopolymers would be necessary for food packaging applications.Multilayer films can be designed with components being more water-resistant to form outer layers.For example, paper modified with a wax-treated double quadlayer of carrageenan + chitosan + MMT + chitosan gave a water contact angle (WCA) of 151.4° [18].The addition of an Ala-Tyr peptide layer onto a furcellaran/gelatin hydrolysate layer reduced the water solubility of the latter layer [45].Compared with a carrageenan/curcumin/anthocyanins monolayer film, a bilayer containing an additional emulsified KGM/CO layer showed improved water resistance (reflected by lower moisture content and total soluble matter) [44].
Strong interactions between different components in multilayer films could also result in increased water resistance, as exemplified by a PVA/ferulic acid (FA) + chitosan/FA bilayer film (reflected by lower WVP and water solubility and higher WCA) and a gelatin/GTE + alginate bilayer film (reflected by lower moisture content, total soluble matter, and water uptake and higher WCA) [27].Besides, Yan et al. [26] found that compared with the PVA/FA + chitosan/ FA bilayer film, adding PVA/FA/Cu 2 O nanoparticles supported by chitosan nanoparticles (Cu 2 O@NCs), which should be more hydrophobic, to the PVA/FA layer of the bilayer film could further enhanced water resistance.

Mechanical Properties and Thermal Stability
Research has indicated that multilayer films could show modified mechanical properties depending on the ingredients.For example, compared with a starch monolayer film, a bilayer film of zein + starch displayed better tensile strength (TS) and Young's modulus (YM) but reduced elongation at break (EB) [30].For a bilayer film of chitosan/CMCS/ ZnO + alginate/CMCS/ZnO, the addition of CMCS/ZnO was found to increase the TS but reduced the EB [29].Compared with a furcellaran/gelatin monolayer film, the addition of an Ala-Tyr dipeptide layer enhanced the mechanical properties [45].Yan et al. [26] suggested that for a bilayer film of PVA/ ferulic acid (FA) + chitosan/FA, the interaction between PVA and chitosan led to enhanced TS, and the addition of Cu 2 O@ NCs improved the TS further.Zhou et al. [44] reported that for a carrageenan/curcumin/anthocyanins film, adding an emulsified layer of KGM and CO improved the mechanical properties.Fu et al. [32] reported that a double-layer film of PLA + PVA/hydroxypropyl trimethyl ammonium chloride chitosan (HACC)/TiO 2 nanoparticles/CuO@ZIF-8NP exhibited an enhanced EB value, up to 17.13%, about 2.4fold that of a PLA film.Shan et al. [27] suggested that for a bilayer film of gelatin/GTE + alginate, high intermolecular hydrogen bonding and good compatibility between the filmforming substances led to a compact structure and improved mechanical strength and thermal stability.

Indication
Incorporating pH-sensitive compounds such as curcumin and anthocyanins can endow films with indication functions for food freshness monitoring [31,44,46].Bilayer films of carrageenan/anthocyanins + KGM/CO and carrageenan/ curcumin/anthocyanins + KGM/CO used as labels presented more obvious color changes reflecting chicken meat freshness at 25 °C than a bilayer film of carrageenan/curcumin/ anthocyanins + KGM/CO [44].A double-layer film of raspberry anthocyanins/low-acyl gellan gum + chitosan had obvious color response correlated to meat quality during storage [31].

Potential Food Applications of Multilayer Films
Many studies have demonstrated the advantageous properties of biopolymer-based multilayer films for the packaging or preservation of different foods such as fruits [18,26,29,32,[37][38][39][40][41], meat [31,44], fish [45], chicken skin oil [43], mushroom [27], and crackers [33], as summarized in Table 1.The positive effects of multilayer films on food preservation could be mainly linked to their excellent barrier and antimicrobial and antioxidant properties.As discussed above, multilayer films incorporated with pH-sensitive compounds such as anthocyanins can monitor meat freshness.

Conclusion and Future Perspectives
Biopolymer-based multilayer films outperform single-layer counterparts in barrier performance against gas, water, oil, and UV light.They can be enhanced with antimicrobial or antioxidant agents for prolonged activity.By incorporating more hydrophobic ingredients into/as layers, water resistance and surface hydrophobicity improve.Multilayer films can also be designed to be pH-responsive to indicate food freshness.Moreover, these multilayer films showed comparable or even higher mechanical properties than their single-layer counterparts.Evidently, developing multilayer films from biopolymers combined with other polymers (e.g., PLA, PCL, and PVA), organic substances (e.g., carvacrol, LAE, LBE, GTE, CEO, and OEO), and inorganic nanoparticles (e.g., nanoclays, ZnO, Ag, CuO, and Cu 2 O) addresses hydrophilicity and mechanical issues, while making use of functional features like gas barrier performance and antimicrobial activity.And these biopolymer-based multilayer films are idea for food preservation due to biodegradability and food-contact safety.
Despite the potential of biopolymer-based multilayer films, challenges remain.Firstly, cost-effective production of multilayer films is essential.Current methods such LbL solutioncasting or coating are time-consuming or energy-intensive.Although continuous processes for coating paper or paperboard show promise, further research is needed to ensure cost-effectiveness and compatibility with industrial techniques.Besides, the development of multilayer films should prioritize cost-effective ingredients and processes.
The second aspect to tackle is enhancing the properties (e.g., water resistance and mechanical properties) and durability of biopolymer-based multilayer films compared to their non-biodegradable counterparts.Achieving this may involve designing composite materials with strong interfacial interactions and novel structures.
Furthermore, to cater to the diverse needs of active and intelligent food packaging, the functionality of biopolymer-based multilayer films should be enhanced.This calls for focused research on developing biopolymer-based multilayer films with functions of oxygen scavenging, the absorbance of carbon dioxide, moisture, and ethylene, the emission of carbon dioxide and ethanol [6], the indication/sensing of time-temperature, humidity, oxygen, pH, microorganism, and specific chemicals [47].
Last but not least, the practical efficacy of biopolymerbased multilayer films should be thoroughly tested for a wide range of food products.
By addressing these challenges and advancing research, these films can become a valuable solution for various food preservation applications.

Fig. 1
Fig. 1 Schematic representation of techniques for fabricating multilayer films.A Layer-by-layer (LbL) assembly by dip-coating of chitosan and anionic β-cyclodextrin (β-CD) onto hydrolyzed PLLA.B (a) LbL process to prepare quadlayers of carrageenan + chitosan + montmorillonite (MMT) + chitosan; (b) the structures of chitosan (CS), carrageenan (CR) and MMT; (c) the multilayer structure built.C (a) The process to coat paperboard with a bilayer of carboxymethyl chitosan (CMCS)/carboxymethyl cellulose (CMS) and PLA/ ZnO; (b) coating equipment; (c) heat-drying equipment; (d) the mold used to lessen cardboard deflection when drying inside the heater;

Table 1
Summary of biopolymer-based multilayer films for food packaging