Petrochemical-based plastic materials have been widely employed for the packaging of food materials owing to their good mechanical, barrier and optical properties. Despite having many advantages of petrochemical plastic, the major disadvantage of this material is that it is non-biodegradable which is today’s burning issue around the globe.
In the last 15 years, half of the plastics ever produced have been manufactured. Plastic output expanded at an exponential rate from 2.3 million tons in 1950 to 448 million tons in 2015, with production predicted to double by 2050
(Parker, 2019). According to a Central Pollution Control Board study for 2018-2019, India generates 3.3 million metric tons of plastic garbage every year. Concerns about the environmental impact of petroleum-based plastic packaging materials, such as natural resource depletion, energy crisis, global warming and ecological challenges, have sparked interest in the development of bioplastics materials based on biopolymers. Biopolymer is an organic polymer produced by living organisms and plants from natural resources. Biopolymers have the characteristics of being fully capable of biodegradation at accelerated rates and breaking down cleanly into simple molecules such as carbon dioxide, water and methane under the enzymatic action of microorganisms
(Nase et al., 2014). They are categorized based on their production method or source 1) Polymers taken or removed directly from biomass, such as polysaccharides and proteins 2) Polymers produced using traditional chemical synthesis from sustainable bio-based monomers such as polylactic acid (PLA) 3) Microorganisms or genetically engineered bacteria create polymers such as polyhydroxyalkanoates and bacterial cellulose
(Nase et al., 2014).
Film-forming biopolymer materials
Film-forming biopolymers can also be differentiated based on the type of film-forming material, which forms cohesive or continuous matrices. Biopolymers are differentiated into composite films, protein, lipid and polysaccharides and they are shown in the Fig 1.
Composite films
Composite films are made up of protein, lipid and polysaccharides blends and are noted for their mechanical qualities. Lipid-based coatings have poor mechanical characteristics and are less permeable to water vapor while permeable to oxygen. The characteristics of the composite film can be improved by adding the optimum number of components.
Kim and Ustunol (2001) invented the film by combining butterfat, candelilla wax with protein and plasticizer. Water solubility (20°C, 24 h) and moisture sorption isotherms (0.18-0.90 (aw), 25°C) of the films were determined. The film’s solubility and equilibrium moisture content increased rapidly, however, the addition of lipid lowered the solubility and equilibrium moisture content of sorbitol and glycerol plasticized films
(Kim and Ustunol 2001). It has been reported that adding lipids (50% oleic acid), commercial vegetable oil and lecithin of sodium caseinates films resulted in an increase in water vapor permeability with increasing fatty acid concentration (4.88-22.56%, 12.20-48.14% and 61.59-15.85%) of the dry weight of casein
(Hammam et al., 2019).
Lipid-based film
Lipids are good moisture barriers. Herbal wax, acetylated monoglycerides and surfactants are among the lipid-based substances used as film/coating. Paraffin and beeswax are the most basic lipid compounds. Because of their low polarity, lipid-based films and coatings are thought to be particularly effective at preventing moisture delivery. Lipid films and coatings are typically more brittle and thicker because of their hydrophobicity. Water vapor permeability decreases as the concentration of the hydrophobicity phase increases
(Hassan et al., 2018). Different types of waxes are utilized in the formation of films and their ability to retain moisture varies. As paraffin wax is made up of a mixture of long hydrocarbon chains, whereas beeswax is made up of a mixture of hydrophobic compounds, it is more effective than beeswax. Fatty compounds have a high moisture retention capacity, substances such as plant fats, animal fats and waxes have been employed in the manufacture of the biobased film
(Hammam et al., 2019). Microemulsion cast films made from glycerol plasticized mesquite seed gum and sonicated palm fruit oil emulsion was successful. The inclusion of palm fruit oil increased the hydrophobicity of the films. This film is suitable for a variety of food packaging applications that require low water vapor permeability and solubility
(Rodrigues et al., 2016).
Protein-based film
Proteins are found in nature as fibrous or globular proteins. Proteins that can be employed in film production include lactic serum, casein, collagen and seine, soya protein and zein protein. Because of their unique structure, protein films have better mechanical qualities than polysaccharides. Modification of stearic acid soy protein isolate can be made from the cake leftover after extracting soya oil. Soy protein isolate can be remanufactured into plastic, stearic acid was used to improve the water-resistance of soy protein isolate films using the bioconjugation method and it considerably enhanced the film’s water resistance
(Ye et al., 2019). Schmid et al., (2013) used glycerol as a plasticizer to both hydrolyzed and non-hydrolyzed whey protein isolate films. This study found that increasing the content of hydrolyzed whey protein isolate greatly modifies the mechanical characteristics. The tensile and elastic characteristics of the film decreased
(Schmid et al., 2013). Protein-based films outperformed lipid and polysaccharide-based films in terms of gas barrier and mechanical properties. The formation of a pumpkin oil cake protein isolate film with glycerol as a plasticizer at various pH values revealed that the film could be formed with a wide pH range (2 to 12)
(Popovic et al., 2012). Zein, a protein derived from corn, is found in the grain’s endosperm and is alcohol soluble. Zein films were created by combining oleic acid and glycerol as plasticizing agents in various ratios. The highest tensile strength was observed at an oleic acid to glycerol ratio of 3:1
(Xu et al., 2012).
Polysaccharide based films
Plants and animal cells, the outer structure of insects all contain polysaccharides. Biobased and biodegradable materials have seen increased use in packaging, agriculture, textiles, medicine and other fields in recent years. There are numerous polymers blends available to replace the current synthetic plastic. The most important polymer is starch. Gums, cellulose derivatives, carrageen, chitosan, alginate and pectin are some of the examples of polysaccharides. Polysaccharides are inexpensive, renewable and abundant in nature. They combine to form a film with superior mechanical properties. The polysaccharide-based packaging material is readily available and has some functional properties. Polysaccharide-based biodegradable films are an effective barrier to gas transfer, such as oxygen and carbon dioxide
(Caz et al., 2017). Starch is made up of amylose and amylopectin, which are found in grains like corn, wheat and sweet potato. Amylose is in charge of forming a film. Increasing the amount of amylose in the films increases their elasticity
(Hammam, 2019). Cellulose is the most common biopolymer found in nature. Because of its high hydrophilicity and crystal structure, it is not easily soluble in water. Cellophane films, which are used in food packaging, are made of cellulose
(Pellicer et al., 2017). Alginate is a polysaccharide extracted from brown algae such as
Laminaria,
Macrocystis and
Azotobacter vinelandii and bacterial alginate was extracted from
Azotobacter vinelandii. It is a binary copolymer with carboxyl groups in every constituent residue
(Daniela, 2012). Chitin is nature’s second most abundant polysaccharide after cellulose. Chitin can be found in the exoskeletons of crustaceans and several insects, which is why chitosan derived from chitin is commercially available from abundant renewable sources
(Caz et al., 2017). Chitosan is a biomaterial that is biodegradable, bio-functional, non-toxic and biocompatible
(Dutta et al., 2009). Chitosan has the disadvantage of not being a thermoplastic polymer and thus cannot be extruded or molded at high temperatures
(Caz et al., 2017). Pectin is a complex anionic polysaccharide composed of β-1, 4 linked d-galacturonic acid residues. Pectin contains the carboxyl group of uronic acid, based on the degree of esterification it is divided into two types: high methoxyl pectin DE>50% and low methoxyl pectin DE<50%. Low methoxyl pectins form a gel with divalent cations
(Galus et al., 2013). Batori et al., (2017) made a biofilm from citrus peel waste for food packaging by combining the gelling ability of pectin and the strength of cellulosic fibers.
Microbial sources of bio-film
To improve the degradation and other properties of the films, biodegradable polymer blends have been developed. PHA, or Poly-b-hydroxy alkanoates, is a biocompatible and biodegradable thermoplastic with potential applications that have received a lot of attention. Poly 3- hydroxybutyrate (PHB) is used in bulk shrink packaging and flexible intermediate bulk containers
(Jabeen et al., 2015). PHA and bacterial cellulose packaging materials have the potential. PHB is another natural polymer; both PHA and PHB are thermoplastic polyesters and have mechanical properties close to synthetically produced degradable polyesters for use in food packaging
(Gurgel et al., 2011). Bacterial cellulose produces by bacteria
Acetobacter xylinum and A.
pasteurianus. Its chemical and physical structure is similar to the plant cellulose
(Taylor et al., 2002).
Aureobasidium pullulans synthesize pullulan, a microbial polysaccharide composed of maltotriose units linked by α (1,6) glycosidic units, from starch. Pullulan is water-soluble, has a low oxygen and oil permeable film, is odorless, colorless and heat-sealable
(Mohamed et al., 2020). The bacteria
Xanthomonas campestris produces xanthan gum, which is an exopolysaccharide. It forms a stable viscous solution in hot or cold water over a wide temperature and pH range
(Mohamed et al., 2020).
Binding agents
Binding agents are hydrocolloids that are added to improve the physical properties of the film; the addition of biomacromolecules increases the film’s cohesiveness. The chemical structures of the hydrocolloids are expected to have the ability to form films. As binding agents, pectin, cellulose, carboxymethylcellulose, methylcellulose, gelatin, collagen, chitosan, alginate and carrageenan are used. Sometimes more than one binding agent is used in films
(Otoni et al., 2017).
Plasticizers
Plasticizers are low molecular weight agents that are incorporated into polymeric film-forming materials to increase the polymers’ thermoplasticity. Plasticizers increase the free volume of polymer structures or polymer molecular mobility, as well as their flexibility and processability
(Han et al., 2014). Natural plasticizers with low toxicity and migration include epoxidized triglycerides, vegetable oils derived from soyabean oil, linseed oil, castor oil, sunflower oil and fatty acid esters
(Gurgel et al., 2011). The compatibility of the plasticizer and the polymer is important for effective plasticization. Polarity, hydrogen bonding, solubility parameters and dielectric constant are all examples of parameters. Glycerol, also known as vegetable glycerin, is a common plasticizer that is suitable for water-soluble polymers. Glycerol has a higher plasticizing efficiency than sorbitol
(Otoni et al., 2017). Water is the most common and effective plasticizer; however, the plasticizing effect of water in hydrophilic biopolymers is difficult due to environmental conditions such as relative humidity and temperature
(Parreidt et al., 2018). To overcome the brittleness of alginate films, some studies combine more than one plasticizer, such as glycerol, palmitic acid, β-cyclodextrin and glycerol monostearate used in film formulations
(Fan et al., 2009). Miller et al., (2021) investigated the effect of glycerol and sorbitol plasticizers on potato-based film and discovered that as plasticizer concentration increased, so did the film’s water vapor and oxygen permeability. A glycerol-containing film had higher water vapor and oxygen permeabilities than a sorbitol-containing film
(Miller et al., 2021).
Fillers
In comparison to petrochemical-based plastic, biobased packaging films have lower mechanical resistance, barrier and thermal properties. A biobased nanocomposite is a multiphase material formed by the combination of two or more components, including a matrix in the continuous phase and a matrix in the discontinuous phase at the nanometer scale
(Ramos et al., 2018). Polymer nanocomposites are polymer-nanoparticle mixtures; polymer nano reinforcement fillers are added to polymers to produce nanocomposites with improved mechanical and other properties. Fillers include polysaccharide nanoparticles such as cellulose nanocrystals (CNC), microcrystalline cellulose (MCC), montmorillonite (MMT) and nano clays
(Otoni et al., 2017). Eco flex includes 1.0 wt. % biobased silica/carbon hybrid nanoparticles (SCNPs). The TGA and tensile test analysis data revealed that the thermal and mechanical properties of the film have improved significantly
(Biswas et al., 2017). Clay can be used as a reinforcement agent because it improves the physicochemical, mechanical and barrier properties of several biopolymers. Clay is a natural, non-toxic and abundant resource
(Ramos et al., 2018).
Other additives
Additives used in biobased packaging film are cross-linking agents, antioxidants and antimicrobials. These additives are also known as functional additives, it provides improved characteristics to the packaged product or packaging material.
Crosslinking agent
Crosslinking of polymer matrices forms covalent bonds that connect adjacent chains. Crosslinking can be accomplished through two methods: physical, chemical, or radiation. Physical treatment entails the formation of a tridimensional network, whereas chemical crosslinking entails the addition of appropriate chemical agents
(Otoni et al., 2017). Minerals such as Ca2+, Mg2+, Fe3+, proteins, organic, amino and phenolic acids are the naturally occurring crosslinkers present in fruit and vegetable pomace, peels and seeds
(Otoni et al., 2017). When citric acid was used as a cross-linking agent in agarose bioplastics film, the mechanical properties of the film improved and the degradation rate of the crosslinked film was greater than that of the non crosslinked film
(Awadhiya et al., 2016). Chitosan film was thermocompression molded with citric acid as a crosslinking agent, a natural polycarboxylic acid. The reaction of crosslinking resulted in the formation of a homogeneous structure
(Guerrero et al., 2018).
Antioxidants
Antioxidants are substances that are used to preserve food by preventing deterioration, rancidity, or discoloration caused by oxidation
(Parreidt et al., 2018). Antioxidants, both natural and synthetic, are successfully incorporated into biobased films. Antioxidants that are used in the biofilm are butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), Tertiary butylhydroquinone (TBHQ), tocopherol, catechin, carvacrol, lignin, citric acid, sorbic acid and propyl gallate. The addition of active components varies between 0.02 to 12% mostly in the range of 1-5%
(Jariyasakoolroj et al., 2018). The stability of the active compound varies from compound to compound as a result of the various thermal and mechanical processes. Polymer coupling improves the thermal stability of the antioxidant. Catechin was used as an antioxidant in the soy protein isolate and carboxymethylcellulose film because CMC causes the film to become opaque. Catechin incorporation shows a synergistic free radical scavenging effect and this film can be used as a potent antioxidant packaging material in the food industry
(Han et al., 2015).
Antimicrobials
The biofilm contains antimicrobial agents of both natural and synthetic origin. The majority of essential oils are used as antimicrobial agents. The protein-based film contained bioactive agents, formic acid and oregano essential oil. Antimicrobial activity was demonstrated by bioactive agents
(Martinez et al., 2013). Cinnamaldehyde in concentrations ranging from 1.5 to 5% increases cross-linking and imparts antimicrobial activity to gliadin films. The films were composted after being subjected to simulated conditions of use in a food packaging application.
Pau et al., (2015) found that the disintegration profile of the most cross-linked gliadin film was very fast.
Methods of bioplastic formation
Casting
It is the most common technique, which entails spreading a film-forming solution on a suitable surface. The film-forming solution allows for surface drying and peeling. Rapid drying of the solution should be avoided; rapid drying reduces polymer chain mobility and the development of intermolecular interactions in the film
(Parreidt et al., 2018). Casting is divided into two methods: bench casting and continuous casting, with the former being more popular in the scientific community
(Otoni et al., 2017). Continuous film casting can be done on steel conveyors or coating lines. Continuous casting has the advantage of requiring less space and labor and is more suitable for industrial applications
(Otoni et al., 2017). Table 1 shows the materials used for biofilms formation. Starch-based thermoplastic materials have a broader range of industrial applications, including extrusion, injection molding and film blowing
(Jabeen et al., 2015).
Future of bioplastic
According to the global bioplastics production market, the compound annual growth rate of the global bioplastics packaging market is 15.2% over the forecast period (2021-2026). Fig 2 shows the global production capacities of bioplastics in 2018-2023. Bioplastic packaging materials are becoming more popular due to their low environmental impact, sustainability and growing consumer awareness. By report biobased building blocks and polymers-Global Capacity, Production and Trends 2018-2023, The total production of bioplastics biopolymers has reached 7.5 million tons, accounting for 2% of total synthetic polymer production and is expected to rise by 4% by 2023.
Application of bioplastic in food packing
There are numerous biobased materials available in the market, with PLA being one of the most widely used. Bioplastics can be used to pack food products with both short and long shelf lives
(Peelman et al., 2013). The following are some of the biopolymer applications for various food products. Table 2 shows the packaging applications of biopolymers as food packaging.
