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Polymer Science
Biopolymer-based packaging materials have become of greater interest to the world due to their biodegradability, renewability, and biocompatibility. Numerous biopolymers—such as starch, chitosan, carrageenan, polylactic acid, etc.—have been investigated for their potential application in food packaging. Reinforcement agents such as nanofillers and active agents improve the properties of the biopolymers, making them suitable for active and intelligent packaging.
- biopolymers
- food packaging materials
- bioplastics
- biopolymer-based materials
1. Introduction
The use of biopolymers as packaging materials is becoming an emerging trend worldwide due to their major benefits over plastics, such as biodegradability, eco-friendly nature, nontoxicity, and biocompatibility. These natural biopolymers have excellent film-forming cohesive structures and thin protective layers of film [1].
Biopolymers used as food packaging materials are mainly polysaccharides, proteins, and aliphatic polyesters, which can maintain food quality and increase the shelf-life of the product. These packaging materials [1] have barrier properties that control the exchange of gases, moisture, aroma, and lipids from the external environment and vice versa, [2] possess antimicrobial activity that can protect the food product from the external environment, and [3] prevent the loss of desirable compounds such as flavor and texture [2,3,4]. Biopolymers such as starch, cellulose, and polylactic acid (PLA) are currently used for food packaging materials. However, the main limitation of using biopolymers in food packaging is their weak mechanical strength and high sensitivity to moisture. The merits and demerits differ depending on the type of biopolymer used for food packaging. To overcome the weaknesses of biopolymers, many studies have been performed with the addition of reinforcing agents such as nanofillers, biopolymers, plasticizers, and natural agents such as essential oils. Furthermore, biopolymer matrices act as carriers for antimicrobial substances, antioxidants, flavor agents, vitamins, or nutrients, thereby aiding in improving food quality, safety, nutritional value, and sensory properties. An overview of biopolymers in food packaging is presented in Figure 1. Due to the numerous advantages, biopolymers have been proposed as an alternative to synthetic polymers such as plastic, which reduces the harmful impact on the environment [5]. As the use of biopolymers in food packaging materials is increasing, it is important to upgrade the biopolymer industry to a large scale. Strategies for sourcing biopolymers include utilizing agricultural waste, instituting efficient cultivation practices, and researching innovative biopolymer production technologies.
Figure 1. An overview of biopolymers in food packaging.
2. Current Food Packaging Materials and Associated Issues/Challenges
Plastic, a petroleum-based, diverse, and ubiquitous material, is widely used in food packaging due to its lightweight, cost-effective, transparent, versatile, and easy-to-process properties [1,5]. These synthetic polymers possess excellent mechanical, thermal, and barrier characteristics [1,5], while ultra-thin layers extend the shelf-life of packaged products and reduce food waste [5]. Consequently, plastics provide direct economic benefits by lowering transportation costs.
Global plastic production has increased significantly, with 40% of all produced plastic being used for packaging, and nearly half of that for food packaging [47,73,74]. Europe’s plastic distribution demand is dominated by packaging at 39.6% [75]. However, plastic’s high production volume, short usage time, non-biodegradable nature, and inadequate management have raised concerns worldwide, with recycling challenges arising from multilayer plastics [5,47,76].
Plastics account for about 6% of global oil consumption, projected to increase to 20% by 2050 [5]. Plastic waste damages terrestrial environments and pollutes aquatic ones, accumulating due to prolonged degradation. Landfill plastics release harmful substances during abiotic and biotic degradation, contaminating soil and water [77]. Chlorinated plastics leach toxic chemicals, polluting ecosystems, while plastic degradation in water releases chemicals such as polystyrene and Bisphenol A, causing water pollution [77]. Methane and CO2 emissions during plastic microbial digestion contribute to global warming [77]. Animals are exposed to plastic waste through ingestion and entanglement, with detrimental consequences.
Countries are addressing plastic pollution through waste reduction, production reduction, recycling, and alternatives [77]. Governments have adopted policy initiatives to reduce plastic pollution, with global legislation focusing on protecting territorial and marine environments. The United Nations Convention on the Law of the Sea (UNCLOS) in 1982 was the first international legislation agreement on plastic waste [77]. Other conventions include the International Convention for the Prevention of Pollution from Ships (MARPOL) in 1973, the London Convention (1972), the Global Program Action for the Protection of the marine environment from land-based activities (GPA) in 1995, and the Global Partnership on Marine Litter (GPML) formed in 2012 [77].
The 17 Sustainable Development Goals (SDGs) by the United Nations General Assembly in 2015 aim to promote sustainability, protect ecological life support systems, and reduce waste and pollution by 2030 [77]. The Basel Convention (1989), Rotterdam Convention (2004), and Stockholm Convention (2004) address the safe disposal and management of hazardous substances associated with plastic disposal [77]. Legislation on global warming includes the United Nations Framework Convention on Climate Change (1992) (UNFCCC) and the Montreal Protocol (1987) [77].
The European Union (EU) combats plastic pollution through strategic legislation, including the EU action plan in 2015, the Regional Strategy for Plastics in a Circular Economy in 2018, and the directive on the reduction in the impact of certain plastic products in 2019 [78,79]. The latest update in 2020 focuses on the regulation of recycled content, waste reduction, and product labeling [75].
3. Possible Solutions for Current Food Packaging Materials
The growing environmental concerns surrounding plastics have prompted research into alternative food packaging materials [79]. Biodegradable materials, such as biopolymers, bioplastics, bio-nanocomposites, and edible coatings, are being developed to replace plastics.
Biodegradable polymers are renewable, nontoxic, biodegradable, biocompatible, reproducible, versatile, abundantly available, and boast a low carbon footprint [3,47]. However, issues such as viscosity, hydrophobicity, crystallization activity, brittleness, water sensitivity, thermal stability, gas barrier properties, mechanical strength, processing difficulty, and cost have hindered their widespread industrial adoption [2]. Biodegradable polymers can be classified as polysaccharides (starch, cellulose, chitosan, etc.), proteins (soy protein, collagen, zein, etc.), and aliphatic polyesters (polybutylene adipate terephthalate (PBAT), PLA, etc.) [1].
To address these issues, biodegradable polymers can be blended with other biodegradable polymers, plasticizers (e.g., glycerol), and compatibilizers (e.g., essential oils) [2,3,80]. The biopolymer packaging market in Europe increased from 1743.9 million m2 in 2016 to 2427.1 million m2 in 2021 [78]. Bioplastics are bio-based and/or biodegradable plastics that share properties with traditional plastics and offer additional benefits such as renewability and biodegradability [81].
Bio-nanocomposites, which consist of a bio-based polymer matrix and an organic/inorganic filler with at least one nanoscale material, are suitable as active and/or intelligent packaging materials due to their enhanced mechanical, thermal, barrier, antimicrobial, and antioxidant properties [82,83,84]. These materials focus on extending shelf-life and reducing microbial growth in food products [83,84].
Biopolymer-based edible films, formed from polysaccharides or blends of polysaccharides containing proteins, lipids, and food-grade additives, are suitable for human consumption and can increase the shelf-life and quality of food products [85,86]. Despite their potential, these packaging techniques confront obstacles such as poor elongation, safety and health concerns, high cost, processing difficulties, lack of awareness, cultural concerns, and customer acceptance [87].
4. Degradation Chemistry of Biopolymers
During biopolymer biodegradation, the polymers are first converted to monomers, and they are then mineralized. The mineralization of the organic material takes place by microorganisms (e.g., fungi, archaea, and bacteria) eventually resulting in carbon dioxide, water, and biomass. The reactions occurring during biopolymer biodegradation are as below:
Biodegradable polymers → CO2+ H2O + biomass
The biodegradation of the large molecules of the biopolymers takes place by extracellular enzymes in microorganisms, while the smaller molecules are transported into the microorganism digestion by endoenzymes. For the biodegradation of a biopolymer substrate, most microorganisms use multiple enzyme systems. The biopolymer biodegradation takes place either through oxo-biodegradation or hydro-biodegradation. Oxo-biodegradation takes place in natural polymers such as rubber, humus, and lignin. During this process, loss of the mechanical properties of carbohydrate polymers takes place by the peroxidation process, which is initiated by heat/light, resulting in oxocarboxylic acid molecules, aldehydes, ketones, and alcohols. After that, the biopolymers undergo bio-assimilation with the aid of enzymes of microorganisms into the water, carbon dioxide, and biomass. The hydro-biodegradation process takes place in cellulose, starch, and aliphatic polyesters. The biopolymers are converted into monomers through the enzymatic digesting of microorganisms. The hydrolysis of ester bonds in monomers is performed by the extracellular enzymes of microorganisms. The aliphatic polyesters and carbohydrate polymers are hydrolyzed and bio-assimilated rapidly in an aqueous medium [88].
The rate of biodegradation depends upon different factors such as (1) polymer characteristics (chemical bonds, branching, hydrophilicity/hydrophobicity, stereochemistry, molecular weight, chain flexibility, crystallinity, interactions with polymers, coatings, surface area, mobility, and addition of plasticizers/additives/active agents), (2) microorganism type (aerobic and anaerobic facultative, co-metabolism, nature, enzymes, enzyme level, enzyme location, enzyme kinematics, and inhibitors/ inducers), and (3) environment conditions (temperature, humidity, oxygen, salts, metals, trace nutrients, pH, redox potential, stability, pressure, alternate carbon, and light). When the above conditions are present appropriately, the rapid degradation process occurs. During industrial composting, the bioplastics are biodegraded in approximately 6–12 weeks [89].
To access the biodegradability of a biopolymer, laboratory tests, simulation tests, and field tests are carried out. The laboratory tests applied include enzyme tests, clear zone tests, Sturm tests, and synthetic environment-defined conditions. Stimulation tests are performed using laboratory reactors, water, soil, compost, and material from landfills with complex environments in defining conditions. Finally, field tests are performed in nature, water, and soil/compland fill under a complex environment in variable conditions [90].
5. Important Properties of Biopolymers in Food Packaging
The properties of the packaging materials, such as barrier, mechanical, chemical, and thermal properties, play an important role in increasing the shelf-life and maintaining the quality of the food products. The barrier properties of a biopolymer used in food packaging are the main parameter for extending the shelf-life of the packed food product. Barrier properties such as gas, water vapor, organic vapors, and liquids are essential for food packaging to separate the food product from the external environment. In addition, these products differ in the different biopolymers used in food packaging. Thus, the loss/gain of oxygen and water plays a major role in food deterioration. The barrier properties play a crucial role in packaging since gas/water vapor may pass through the walls of the biopolymer, resulting in changing the food product quality and shelf-life [91,92].
The gas permeability of the packaging material depends on the parameters; transmission rate, permeance, and permeability. However, the barrier properties of materials not only depend on these factors but also on environmental conditions such as temperature, pressure, and relative humidity. Further, the rating of the barrier properties also depends on the nature of the food products that are to be packaged. As a result, food packaging materials can prolong the shelf-life of food products by improving the barrier properties [93].
The oxygen barrier properties of a packaging material play an important role in the preservation of fresh food products. The oxygen permeability is quantified by the oxygen transmission rate and oxygen permeability [93]. This measures the amount of oxygen in the packaging system. When the oxygen permeability is reduced, the oxygen pressure in the packaging system drops, resulting in an extended shelf-life of the food product [93].
The water vapor barrier properties are of great significance for food products to maintain physical or chemical deterioration concerning the moisture content. The water vapor barrier properties are quantified by the water vapor permeability of the packaging material by the ASTM E-96-95 standard method and the water vapor transmission rate [94]. The water vapor permeability depends upon the solubility and the diffusion of the water in the polymer material. The shelf-life of some food products is directly related to the water exchange rate between the external and internal environment; thus, the water transfer should be reduced to protect the food product from moisture [95].
The UV barrier properties of packaging material are quantified by the optical properties of a film using a spectrophotometer [96]. The UV barrier properties are essential to prevent the loss of nutrient value and the change in the color of food [29].
Mechanical properties of the packaging system are essential to secure the food during stressful conditions such as storage, handling, and processing of the food. The architecture of the polymer matrix is the key factor that determines the mechanical properties of the biopolymer. The mechanical properties of packaging material are determined by tensile properties such as tensile strength, elongation at break, and elastic modulus [30,34,97].
Chemical resistance is important because the food in the package may be acidic and combine with the packaging material. For safety reasons, it is important to find out what the food is made of chemically before packing it. When these chemicals combine with and become absorbed by the biopolymer matrix, the mechanical properties of the material may change [92].
The thermal properties of the packaging material are determined by thermogravimetric and differential scanning calorimetry. Thermal properties and thermal stability are essential for the heat resistance of the packaging material. Thus, the thermal properties allow us to store and transport the food packaging at the temperature essential for the food products [57].
This entry is adapted from the peer-reviewed paper 10.3390/foods12122422
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