Perspective Chapter: Development of Food Packaging Films from Microorganism-Generated Polyhydroxyalkanoates

Abstract

Petroleum-based packaging (PBP) materials cause environmental pollution and toxic substance accumulation because they cannot decompose in nature for a long time. To prevent these problems, a wide variety of food packaging materials emerge as alternatives to PBP. Researchers have already discussed how polysaccharides and biopolymer-based nanocomposites are used in the development of food packaging films. This chapter, we will introduce how the microorganism-generated biopolymer, polyhydroxyalkanoates (PHAs) to be specific, is used in food packaging. PHAs, have positive social and environmental impact when compared to traditional plastics in terms of production and recycling. Considering that industrial wastes contain high quality polysaccharides, essential oils and proteins, using them in the production of biodegradable packaging will both reduce environmental problems and provide economic gain by reprocessing the wastes into products with higher added value. However, it has some disadvantages in competition with synthetic plastics and applications as biomaterials due to some properties such as poor mechanical properties, high production costs, limited functionality, incompatibility with conventional heat treatment techniques and susceptibility to thermal degradation. In this chapter, we will discuss the future and potential difficulties that may be experienced in the production or dissemination of PHA as a packaging material.

1. Introduction

The packaging of foods plays an important role in the preservation of the product throughout the storage and distribution chain. The main purpose of packaging is to protect food from contamination and extend its shelf life [1]. To meet this purpose, packages are produced from different raw materials. Plastic packages are frequently used in food packaging due to their high quality, easy processing, and combination properties. However, today, the harmful effects of plastic waste on the environment have led the food industry to use biodegradable packaging materials [2]. Biodegradable packaging materials are obtained from renewable bio-based resources and are often referred to as “biopolymers” [3]. Biopolymers are strong alternatives to nonbiodegradable and nonrenewable plastic packaging materials due to their environmentally friendly nature [4]. Synthetic films used as packaging materials negatively affect the environment since they are not biodegradable, while biological-based packaging materials obtained from renewable materials draw attention to their biological compatibility, biodegradability, environmental protection, and reliability [3]. Biodegradable polymers are defined as polymers that can be broken down into simple molecules, such as carbon dioxide, methane, water, and inorganic compounds in a certain process in nature as a result of the enzymatic activity of microorganisms [5, 6]. Bioplastics are generally materials that can be produced from renewable resources such as sugar, corn, and potatoes, containing proteins, lipids, and polysaccharides or from certain microorganisms, algae, and fungi [7, 8]. Environmentally friendly biodegradable polymers have good mechanical and barrier properties and are thus seen as potential packaging materials to replace traditional plastic packaging materials [4, 6].

2. Bioplastics and their sources

Bioplastics which are biodegradable materials made from renewable resources are the new materials of the twenty-first century and are of great importance [9, 10]. Starches from corn, potatoes, wheat, rice, barley, and oats; fibers obtained from pineapple; jute, hemp, banana stems, cassava, newspaper pulp, waste paper, and citrus waste are renewable sources for bioplastics. The use of new techniques for bioplastic production that promotes sustainable solution and reduces plastic waste has been greatly promoted in recent years [11]. Bioplastics can be produced from the inedible parts of food. Food wastes, such as orange peel, pomegranate peel, banana peel, and potato peel, are used in bioplastic production. In recent years, cellulose, hemicellulose, starch, pectin, and these lignocellulosic raw materials are made useful for the bioplastic film production trend [11]. Biobased (biodegradable) packaging materials are a potential alternative to replace petroleum-based (synthetic) polymers. One of the reasons for this situation is the decrease in the demand for petroleum-based products and the movement toward renewable resources to produce plastics and the reduction of the amount of gases released into the atmosphere. Another reason is to reduce the solid waste problem by turning biodegradable materials into compostable organic residues after use [12, 13].

Starch is the most widely used source in the production of biopolymer packaging materials due to its edibility and ease of raw material supply. The main source of starch used for these purposes is usually corn the fact remains that the mechanical properties, such as strain rate and tensile and flexibility strength of films produced from starch are not sufficient. Therefore, starch can be chemically modified or mixed with other substances. Plasticizers, such as glycerol, polyether, and urea, are used to reduce the fracturability of starch [14]. Since starch shows hydrophilic properties, it is not suitable for liquid food products with high moisture content, however, starch-based films have good oxygen barrier properties [15], as well they are used as an alternative to petroleum-derived materials because it is inexpensive and biodegradable [16, 17]. The ability of starch to be hydrolyzed by microorganisms, and used as a carbon source and whether they have the ability to produce α-amylase enzyme is strain specific feature. For this reason, an external source of this enzyme is needed in order for starch to be hydrolyzed by microorganisms and used as a carbon source [18]. Figure 1 shows bioplastics and their sources.

Figure 1.

Bioplastics and their sources.

Cellulose consists of glucose monomer units linked by glycosidic bonds and is an inexpensive source because it is found in all plants. However, its hydrophilic properties, low solubility, and high crystal structure create difficulties in its use in packaging, and due to the successive hydroxyl side chains, it causes low moisture barrier properties in cellulose-based packages. Also, the packaging material formed due to the high crystalline structure is brittle and has poor flexibility and tensile strength [14, 19]. For these reasons, research now focus on cellulose derivatives for packaging purposes. Cellulose-based biopolymer cellophane films, known as candy wrappers, are highly transparent and colorful. Cellophane treated by lamination, injection, and extrusion molding exhibits good film-forming properties [20]. Because it is insoluble and has excellent dimensional stability, it is also used in packaging products ranging from laminate, flower wrapping, and cheese to coffee and chocolate [19]. Researchers stated that starch/PVA, which is a composite biodegradable film, reinforced with cellulosic fiber is suitable for use in food packaging [21]. Cellulose derivatives are obtained by the reaction of cellulose in the presence of an aqueous solution of sodium hydroxide and an esterifying reagent. Cellulose derivatives, such as hydroxypropyl cellulose, hydroxypropyl methylcellulose, carboxymethylcellulose, and methylcellulose, are used for edible films/coating. Suspensions of these substances have thermogelation properties that form gels when heated and, on the contrary, regain their original consistency when cooled. Such films are poor water barriers and show poor mechanical properties due to the hydrophilic nature of the molecules [22]. The quality of the moisture barrier can be improved by adding hydrophobic compounds, such as fatty acids, to the cellulose matrix [23]. Due to the high production cost, the use of cellulose-based plastic is limited in the market.

Chitosan, the second most abundant natural polymer after cellulose, is obtained by partial deacetylation of the natural polysaccharide chitin [24]. Chitosan is an important waste of the fishing industry. It has many functions, such as antimicrobial effect against bacteria, molds, and yeasts, moisture adsorbing, precipitation, film formation, and enzyme immobilization [25]. It also exhibits good oxygen and carbon dioxide permeability, but a major drawback is that it has poor solubility in neutral solutions. Sun et al. [26] determined that chitosan films combined with apple polyphenols can be used as bioactive packaging material to increase the shelf life of foods. Pectin, which is a heteropolysaccharide found in the cell walls of plants, is frequently used to thicken jams and jellies [27]. In the industry, mostly apple pulp and citrus peels are used as a source. These types of edible films are used for certain food-related functions, such as food packaging materials, anti-browning, flavor enhancer, and antimicrobial functions, their production usually uses casting method or extrusion [28, 29]. Pectins, which are frequently used to produce biodegradable films, can be supported with nano-structured fillers to compete with commercial polymers because they show poor physical and barrier properties. The nanocrystals obtained from cellulose, a natural component, impart rigidity and strength to the films [30].

Whey, which is a by-product of the cheese and casein industry and makes up 90% of the processed milk volume, is used for humans and animals, while some is thrown into the environment. Whey, which is an inexpensive substrate and carbon source for bacterial growth, is also preferred in the production of polyhydroxyalkanoate (PHA) [31]. Wheat bran contains high protein, carbohydrates, and minerals, and they are suitable for use as waste. Various studies have been carried out to evaluate wheat bran waste. One of them is the use by Van-Thuoc et al. [32] as a carbon source for bacterial growth and PHA production. They determined that growing Halomonas boliviensis LC1 resulted in biomass production of 3.19 g/l and PHB production of 1.08 g/l. Soy proteins are generally a by-product of the soybean oil industry. Generally produced by wet casting, soybeans are preferred in edible films and coatings because of their good film-forming properties. Although their water permeability is high, their oxygen permeability is good like other protein films [33, 34, 35]. Gelatin is a naturally occurring hydrocolloid polymer derived from animal skin, bones, and related tissues. Gelatin-based bioplastic films are widely used as packaging material in the food industry [36]. Many studies showed that the physical and mechanical properties of biofilms made of gelatin can be improved with biocomposite films, such as gelatin-starch or gelatin-starch-glycerol [37]. Lipids are used as protective coatings against transfer, but besides lipids, wax and other resin materials have several disadvantages as packaging materials, such as a waxy taste and texture, oily surface, and bitterness. Among its advantages in terms of packaging, the lipid component reduces water transmission, while the hydrocolloid component acts as a gas barrier and even contributes to the strength and structural integrity [29, 38].

In the food industry, large amounts of both solid and liquid waste occur due to the production, preparation, and consumption of food. These wastes cause problems while being destroyed, but they are valuable products as biomass and nutritional components [39, 40]. Especially fruit and vegetable industry is an area that generates a lot of waste. Fruits and vegetables are consumed fresh as well as processed into juice and jam, and vegetables into canned products, and as a result of all these processes, wastes such as shell, seed and pulp with high polysaccharide, protein, and lipid content are formed. These wastes are reusable for different processes. For this reason, a “zero waste approach” is being tried to be adopted. The zero-waste approach is based on the use of organic waste generated after processing, in a different field, such as chemistry, medicine, cosmetics, or as a raw material again in food production by subjecting it to various processes. Especially fruit and vegetable industry wastes contain plenty of pectin and different essential oils. These organic substances in the structure of waste are used not only to enhance the mechanical and barrier feature of packaging materials but also for the production of biodegradable films [41, 42, 43, 44]. In recent years, the production of biodegradable packaging materials obtained from fruit and vegetable wastes has gained importance. Biodegradable films obtained from these are unfortunately not at a level to compete with commercial polymers in terms of mechanical and barrier properties. However, it is possible to develop these properties with nanotechnology applications [45]. Considering the mechanical and barrier properties of commercial polymers, although they are suitable for use as packaging materials, interest in natural polymers is increasing because they are not sustainable and biodegradable. However, films obtained from natural polymers show poor barrier and mechanical properties. In this respect, biodegradable polymers are not yet competitive with commercial polymers. For this reason, developed biopolymers are obtained by supporting many biopolymers with organic or inorganic additives [46, 47]. By integrating nanoparticles into biopolymers, the negative mechanical and barrier properties of biodegradable films can be eliminated, and new materials with completely different properties can be developed. Polymer/clay composites improve the barrier properties of thin films [48, 49]. It is stated in the literature that bio-nanocomposites and nanoparticles have an inhibitory effect on the growth of some bacteria [50], act as a carrier of antimicrobial substances [51], and directly form an antimicrobial film [52]. Antimicrobial bio-nanocomposite films are formed by using fillers, such as chitosan [53], nano-silver [54], zinc oxide [55], and titanium dioxide. The use of food industry wastes in the production of biodegradable films has recently been one of the topics of interest in terms of the environmentalist approach. The components contained in the waste can add different properties to the packaging material, such as elasticity, strength, biodegradability, transparency, and antimicrobial activity.

Orange peels are some waste rich in pectin content, and when the films obtained by using the powder form of this waste were dried, seen that the films obtained from orange peel powder, had values close to the tensile strength of commercial polymers, such as low-density polyethylene, high-density polyethylene, polytetrafluorethylene, polypropylene, and polystyrene (16–32 MPa), were obtained [56]. Another industrial output evaluated for use in bioplastic production is soybean waste. Soybean is a raw material that is processed in large quantities and produces excessive waste. Therefore, it is obtained economically at a lower cost than other bioplastic-produced materials [57]. In a study on the evaluation of food industry waste, a film was obtained by mixing lemon peels and potato pulp at different rates. The amount of potato pulp released during the production of potato chips is between 12% and 20% of the total production. Potato pulp is rich in starch, cellulose, hemicellulose, and fermentable sugar content, and is a potential waste for biopolymer film production [58]. Lemon, on the other hand, is a product that is usually consumed fresh or processed into fruit juice, and its peel is very rich in flavonoids, pectin, and essential oils.

In addition, the antioxidant and antimicrobial properties of lemon pulp are also mentioned in the literature [59]. Pomegranate is a raw material that is generally processed into products, such as pomegranate juice or jam, after processing, approximately 55% of pulp is produced. In addition to its antioxidant and antibacterial properties, it has a pulp rich in pectin, tannin, and moisture [60]. Studies are carried out to obtain a film from pomegranate peels [41, 42]. After the banana processing process, up to 30% of the fruit is exposed. Banana peel is a waste not only rich in moisture, protein, pectin, and potassium but also rich in dietary fiber (cellulose), antioxidant, and phenolic compounds [61]. In a study conducted by adding pectin and cellulose nanocrystals obtained from banana peels to the film formulation, it was desired to improve physical and barrier properties. This study shows that nanofillers can improve the mechanical and barrier properties of biodegradable films [62]. Pectins, which are frequently used to produce biodegradable films, can be supported with nanostructured fillers to compete with commercial polymers because they show poor physical and barrier properties. The nanocrystals obtained from cellulose, a natural component, impart rigidity and strength to the films. In the production of biodegradable films, not only fruit and vegetable wastes but also the shells of nuts are used. It has been stated in the literature that shells containing a high percentage of starch are a suitable material for the production of bio-thermoplastics [63]. The same is true for walnut shells. In biodegradable films, walnut shells act as absorbent [64] and reinforcing material [65]. In a study in which starch obtained from cashew shells was used as a film matrix, cellulose obtained from walnut shells was also used as a filler. This study proved that the cellulose obtained from walnut shells shows very good barrier properties against oxygen [66].

3. Polyhydroxyalkanoate

3.1 Structure and classification of PHA

Recently, Zhang et al. [67] have reported about 150 different types of PHAs. They have various types and structures, this diversity is due to the number of carbon atoms, molecular structure, and chain lengths they have [68, 69]. However, when the carbon chain length is taken into account, three types of PHAs emerge. These are short-chain (scl-PHAs), medium-chain (mcl PHAs), and long-chain (lcl-PHAs) [18, 69, 70]. The most common and known member of the PHA family is poly-3-hydroxybutyrate (P3HB). It polymerizes to give a polymeric chain and consists of (R)-3HB repeating unit (monomer) [69, 70, 71, 72, 73]. Poly(3-hydroxybutyrate), poly(4-hydroxybutyrate), poly(3-hydroxyvalerate), and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) are known scl-PHAs, and they have three to four carbon atoms, and they usually can be used in food packaging and disposable products [69, 74, 75, 76].

In the preparation of these PHAs containing 3-hydroxyvalerate (3HV) or 4-hydroxybutyrate (4HB) monomers, copolymers containing a mixture of four carbon chain length subunits and bacteria synthesizing these polymers with valeric acid are used. The incorporation of HV into the PHB polymers results in a less hard and brittle poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [70, 77]. Medium-chain length PHAs with 6-14 carbon atoms are considered medium chain length. They consist of homopolymers such as poly(3-hydroxyhexanoate), poly(3 hydroxyoctanoate), or P(3HO) [69, 75]. They are synthesized by various bacteria with β-oxidation or novo biosynthesis pathway [78]. mcl-PHAs are flexible and elastic, having low crystallinity with low tensile strength and high elongation-to-break ratios [70]. On contrary to short-chain (scl-PHAs), they are rare, and less used in process of bioplastics [69]. The difference between the two classes is mainly due to substrate specificity. While an eutrophus can polymerize 3HAs consisting of 3–5 carbon atoms, Pseudomonas oleovorans can only use 3HAs consisting of 6–14 carbon atoms in PHA synthesis [79, 80].

3.2 Properties of PHA

Hydrophobicity, melting point, glass transition temperature, degree of crystallinity, and some mechanical properties differ depending on the composition of the monomer [80, 81]. Structural differences in the monomers that make up PHAs cause them to differ chemically as well. For example, poly3-hydroxybutyrate (PHB) has good moisture resistance compared to polypropylene, and barrier properties against gases. They also have a high degree of crystallinity, about 55–80%, and form fine crystals with melting points of about 175°C [82]. Considering its tensile and impact strength, UV resistance, and oxygen permeability, PHB is similar to isotactic polypropylene. This shows that it has the potential to be a packaging material [82]. Having good resistance to hydrolytic attack, PHAs are insoluble in water and resistant to UV [18]. In addition to these properties, they are biodegradable in nature [83]. The main reason why the degradation of PHAs depends on their species and composition is that they have chiral molecules [84]. The type of polymer, its composition, environmental conditions, and microorganism species are effective in the biodegradation of PHAs. It is known that microorganisms produce different PHA-depolymerase enzymes to decompose PHAs and thus they are effective on them [85].

Some thermal and mechanical properties, such as melting temperature, glass transition temperature, crystallinity, tensile strength, and percent elongation, determine the quality of PHA [69]. Some thermal properties of PHAs are crystallization and heat resistance, and they affect the polymer quality [86]. The glass transition temperature (Tg) for the amorphous phase and the melting temperature (Tm) for the crystalline phase are expressed [69]. Increasing the number of carbons in the side chain from one to seven causes Tg to decrease and Tm to increase. As a result, the melting temperature rises from 45 to 69°C [69, 74, 87]. Medium-chain length PHAs are crystalline, have more tensile strength, and have high elongation at break. Due to this feature, they show different mechanical properties as compared to short-chain length PHAs, which have a high crystallinity usually 60–80%. Short-chain length PHAs are more brittle and stiff compare to medium-chain length PHAs [69, 80]. The addition of different monomers or mixing with PHA are methods used to improve the fracture flexibility and elongation of the polymer [69, 88]. One of the purposes of these processes is to reduce the difficulties created by the lack of flexibility of PHAs in terms of food packaging uses [89]. Blending with other polymers can decrease brittleness but it is not enough to be competitive with fossil fuel-based polymers used for food packaging [89]. PHBs have similar water vapor permeability to thermoplastics, such as PVC or PET, and their properties are seen as potential for food packaging applications. The fact that they do not swell and have lower hydrophilicity is seen as an advantage compared to various biopolymers, such as starch and cellulose [89, 90]. In addition to these properties, PHAs also have good barrier properties to some organic solvents. They show relatively high permeability to the moderately polar solvents chloroform, acetone, and toluene, while they have lower permeability to methanol, n-hexane, and isopropyl ether [90, 91].

3.3 The microbial production of PHA

The first discovered PHA, P3HB, is produced by Bacillus magaterium, and French researcher Maurice Lemoigne isolated it between 1923 and 1927 [69, 92, 93], also more than 300 species have been reported to produce these polymers. These species include various gram-positive and gram-negative bacteria, fungi, and microalgae [80, 94, 95]. Biodegradable PHAs can be produced by many bacterial species and their different strains [69, 96]. Although many bacterial species are capable of producing a variety of biopolymers, only a few have high productivity and high production rate [69, 97, 98]. Among these bacteria; Wauteria eutropha, Azotobacter spp. Bacillus sp., Pseudomonas putida, Pseudomonas fuorescens, P. oleovorans, Ralstonia eutropha, Cupriavidus necator, Burkholderia sp., Halomonas sp., Haloferax sp., Aeromonas sp., Thermus thermophilus, Hydrogenophagobacter, Saxogradanobacteria de Saxogradia Erwinia sp., and recombinant E. coli [80, 97, 98]. Buhwal et al. [99] isolated bacteria that accumulate polyhydroxyalkanoates (PHA) from pulp, paper, and wastewater. The isolates Enterococcus sp. NAP11 and Brevundimonas sp. NAC1 showed maximum PHA production between 79.27% and 77.63%, and they are considered good candidates for industrial production of PHB. Preusting et al. [100] investigated to high concentration of PHA and high productivity with P. oleovorans by fed-batch and continuous culture, and they were reached in a continuous mode, and the culture productivity was 11.6 g/l and 0.58 g/(l h), respectively.

Another study by Guo-Qiang et al. [101] found to Pseudomonas stutzeri 1317 synthesized a variety of PHAs when grown in glucose and/or fatty acids. The use of recombinant E. coli is common in PHA production, as in many areas. Recombinant E. coli has been used for PHA biosynthesis, to synthesize the biopolymer to extremely high intracellular levels, and to produce the P(3HB-co-3HV) copolymer. As PHB synthesis with recombinant E. coli, it depends on the amount of acetyl-CoA available. Some advantages of using recombinant E. coli for PHA production are rapid growth, high cell density, ability to use a few inexpensive carbon sources [79]. Masood et al. [102] investigated various parameters on the yield of PHAs produced by bacillus cereus. They determined that B. cereus was able to produce both PHAs copolymer and tercopolymer, and it is depending on the type of substrates. Some fungi, such as Aspergillus fumigatus, Saccharomyces cerevisiae, and Yarrowia lipolytica, can be producers [69]. Microalgae have the ability to produce pigments, carotenoids, proteins, enzymes, sugars, fatty acids, polysaccharides, and vitamins, as well as many bioactive compounds. They are ideal for PHA production, but the number of knowns is limited. Some of those are Nostoc muscorum, Chlorella minutissima, and Botryococcus braunii [103]. Cyanobacteria are known to produce PHA by oxygenic photosynthesis, also various studies show that some cyanobacteria have natural capabilities to store PHAs. Although species-specific, some cyanobacteria produce predominantly PHB [104, 105]. It is known to produce PHA in archaea, which needs salt to maintain its growth and can optimally tolerate 5% NaCl (w/v). First reported in 1970 from the Dead Sea, designated Halobacterium marismortui [105, 106] these halophilic archaea produce PHB under nutrient-abundant carbon sources [105, 107]. Different bacteria produce different types of PHAs [108], Fluorescent Pseudomonas species are preferred, because they have the ability of mcl-PHA synthases, and can synthesize of PHAs with 6–14 carbon atoms [109]. Figure 2 shows PHA biosynthesis process scheme.

Figure 2.

PHA biosynthesis process scheme.

3.4 Carbon sources for the production of PHAs

The substrates used in the biosynthesis of PHAs, which are synthesized by bacteria through a metabolic process, are generally small molecules. This is due to the rigid cell walls of bacteria. Large molecules cannot be transported into the cell, they must undergo an extracellular transformation by the microorganism or by a chemical process in order to be used. The substrates that can be used are simple sugars (monosaccharides), triacylglycerol, and hydrocarbons. Most microorganisms use simple sugars. Triacylglycerol and hydrocarbon metabolism are less common. Different bacteria for the same substrate can produce PHAs with different compositions [110]. Monosaccharides and disaccharides do not need any hydrolysis to be used in the production of PHA, while polysaccharides cannot be fermented unless hydrolyzed first [110]. It has been reported by various researchers that Azotobacter vinelandii, Alcaligenes latus, and Hydrogenophaga pseudoflava can hydrolyze sucrose, consisting of a glucose unit linked to fructose, extracellularly to glucose and fructose, both of which can then be used for cell growth, and thus have the ability to produce PHA [110, 111, 112, 113, 114]; (Jiang et al., 2016). Lactose, a disaccharide composed of galactose and glucose, can be used by microorganisms. Whey is preferred as a source [110].

Whey, rich media that is suited for microbial growth, is obtained by precipitation and removal of milk casein during cheese-making processes [115]. Being a good PHA producer does not mean that the microorganism can produce PHA directly from whey. Vandamme and Coenye [116] stated that C. necator, Wautersia eutropha, or Alcaligenes eutrophus can accumulate up to 80% of its dry weight PHA. Escherichia coli cells, which can consume lactose as a solution for microorganisms that can produce high levels of PHA but cannot obtain it from whey, have been modified to express PHA biosynthesis genes from microorganisms that produce high PHA [115, 117]. Starch, the main component of maize, rice, and potatoes, is a polymer of D-glucose and must be enzymatically or acid hydrolyzed to fine maltose and then to glucose for the industrial production of PHAs [110]. Triacylglycerols are the main components of animal fats and plant oils [110], they were considered a cheap and viable source for the biosynthesis of PHAs [102]. In order for bacteria to use triacylglycerols in the production of PHA, they must be able to secrete lipase. The use of triacylglycerol as a carbon source was determined by Shiotani and Kobayashi in 1993 with Aeromonas caviae [110, 118]. Several studies have reported that some gram-negative bacteria can produce PHA using waste glycerol [119], oleic acid [120], or palm kernel oil [121].

3.5 Applications of PHA in the food industry

The packaging industry is responsible for the consumption of more than 40% of the total plastic produced worldwide. Food packaging accounts for about half of this percentage. The excessive use and unconscious disposal of disposable plastics cause large amounts of plastic waste. Therefore, packaging materials that are not easily recyclable, in particular, need to be converted from nonbiodegradable plastics to biodegradable plastics, such as PHAs [122, 123]. Figure 3 shows the different applications of PHAs. Having good barrier properties to gases and water, PHAs are hydrophobic, thermoplastic, and nontoxic to humans and nature. Because of these material properties, PHAs become suitable for packaging materials [124, 125]. The three most commonly used bio-based plastics for packaging are PLA, starch-based plastics, and cellophane [126].

Figure 3.

Varied applications of PHAs.

PHAs are an important biodegradable packaging material used as films, foils, etc., and such barrier-coated packaging keeps oxygen and moisture inside the packaging [127, 128]. The film-forming feature of PHA provides a protective barrier against oxygen and UV rays. PHA can be used as food packaging material and has similar properties to polypropylene. PHA is insoluble in water and resistant to water and hydrophobic [129, 130]. Today, boxes, paper, boards, and cardboard are available in the market as coated with PHA [131]. PHA is used in the food industry for interior surface coatings in beverage and milk packaging. Studies have been conducted to show that they can be used instead of PP and HDPE in the packaging of foods with high-fat content, such as ready-made sauces, mayonnaise, and margarine cream [5, 132, 133]. Many companies are focusing on food packaging materials that combine conventional packaging materials with nanoparticles, thereby improving the mechanical performance of the packaging material and providing antimicrobial properties [128]. Castro-Mayorga et al. [134] synthesized an antimicrobial PHA containing silver nanoparticles and an active food that inhibits the growth of Salmonella enterica. They developed active food packaging.

In recent years, PHAs have become available as packaging materials for food-related products, such as straws and bottles. Danimer Scientific, who developed the first fully biodegradable plastic straw made of Nodax^TM PHA in 2018. In addition, the Cove developed drink bottles, which were made completely of PHAs in 2019, California. Containers mix the soil in about 5 years; however, traditional PET plastic bottles can take around 500 years to decompose. Food and beverage manufacturing company Nestle has recently started work with Danimer Scientific to develop biodegradable plastic bottles [135, 136]. Yield10 agricultural bioscience company is the largest holding in the PHA industry with an annual production capacity of 50,000 tons [137]. Global biodegradable plastic materials are used in a variety of applications, and the packaging industry is the largest contributor, contributing 60% of global bioplastic production [138].

4. Conclusion and future outlook

The use of biodegradable packaging materials is becoming widespread day by day with the increase in environmental awareness, the desire to move away from the use of petroleum-derived packaging materials and the developing technology. The fact that natural resources, such as starch, cellulose, and protein, are the raw materials of a significant portion of biodegradable plastics increases the usability of these packaging materials. The most important reason why bioplastics cannot compete with plastics yet is high research and development costs and low production capacity. In order to reduce the production cost of PHA, it is important to use some cheap carbon sources and to make the best use of the waste. It has been revealed that biodegradable films have weaker properties in terms of barrier and mechanical properties, which are important in food packaging when compared to commercial plastics. In order to strengthen the barrier and mechanical properties of the films, montmorillonite, cellulose nanocrystals, nanoclay, and similar nanofillers can be used as well as biocomposite applications. It is clear that biodegradable packaging materials have potential for the food industry. It is thought that with the increase in crude oil prices in the future, renewable raw material sources will gain more importance and the production of environmentally friendly plastics will replace today’s plastics. For this reason, it is thought that the use of environmentally friendly plastics, which do not have raw material shortages compared to petrochemical plastics, will be produced in much larger quantities with the help of new processes to be developed and more detailed studies should be done on these materials. By evaluating the food industrial wastes together, it can be achieved to develop high-performance packaging materials with stronger mechanical and barrier properties. As biodegradable packaging materials develop, it is expected that the production and use of disposable materials, such as biodegradable cups, cutlery, and plates, will become widespread.