Abstract
The environmental concerns created as synthetic petrochemical-based packaging materials had used in the past decades. Several biopolymers have been exploited to improve materials for environmentally friendly food packaging. Nevertheless, the use of biopolymers is inadequate due to their natural poor mechanical and barrier properties, which may be enhanced with the addition of reinforcing composites such as fillers. Nanotechnology interventions lead the evolution of nanocomposite films incorporated with nanomaterials. Nanoparticles favour the filler–matrix interactions and the performance of the resulting material. Besides nano-reinforcements, nanoparticles can have other roles when added to a polymer, such as antimicrobial activity, oxygen scavenger activity, etc. In this review paper, the structure and properties of leading nanostructured materials which have been studied to use as nanofiller in biopolymer matrices are overviewed, as well as their effects and applications.
1.0 Introduction
The production and use of plastic materials in food packaging has significantly increased in the past decades. These types of materials are expanding 12 per cent per year (1) due to advantages such as cost-effectiveness and mechanical properties like tensile strength and barrier properties (2). Yet, they pose a high-risk problem in waste disposal and environmental concerns as it usually derived from petroleum products and practically non-degradable (3, 4). G. Venkateshwarlu and K. Nagalakshmi estimated that 500 billion to one trillion plastic bags are used and wasted annually worldwide (2). In order to meet the increasing demand with the least environmental impact and sustainability, continuous attempts in research have been directed towards the development of food packaging materials that could rapidly degrade and completely mineralise in the environment (5, 6).
Biopolymers have been one of the favourable substitutions to be exploited and manufactured into eco-friendly food packaging materials due to their biodegradability (7). Used food packaging materials produced from biopolymers are disposed into the bio-waste collection for further composting, leaving behind organic by-products such as carbon dioxide and water. Presently, biopolymer-based packaging materials form about 1-2 per cent of the food packaging market although food packaging accounts for about 40 per cent of the US$460bn global packaging industry (8). Though biodegradable polymers are eco-friendly, they have not been in use as expected due to their inherent poor properties such as poorer mechanical, thermal, and barrier properties as compared to the conventional non-biodegradable materials made from petroleum.
From this situation, many research efforts were made to improve the properties of the biopolymers, which include the use of nanocomposites concept (9).
From the studies, nanocompositing was recognised as a promising route to improve the mechanical and barrier properties of biopolymers. Nanotechnology exploits the fact that material properties could be very different at a nanometre scale as compared to at a macro level (10). Lately, bio-nanocomposites, a new class of materials, has proven to be an improved mechanical and barrier properties (11). Bio-nanocomposites are multiphase materials consisting of two or more constituents which are a continuous phase or matrix particularly biopolymer, and discontinuous nanofillers on the scale of 1 to 100 nanometers (12). The main advantage is obtaining a functionally uniform material that combines the properties of all constituents.
The development of bio-nanocomposite materials for food packaging is essential not only to reduce the environmental problem but also to improve the functions of the food packaging materials. This review highlights several types of nano-sized fillers utilised in bio-nanocomposites materials for food packaging. Some recent results discussed the properties and characteristics of nanofillers and bionanocomposites. The unique features of Montmorillonite, MMT can result in bionanocomposites with improved properties, which can potentially replace conventional packaging materials such as plastics. The bionanocomposites could be either thermoformed into trays and containers for food service or cast into films for food packaging applications (1).
2.0 Biopolymer-based Food Packaging Materials Biopolymers are polymeric materials, which increasingly introduced as renewable packaging materials and substitutions for petroleum-based polymers. The prefix bio denotes that biopolymers are biodegradable. Moisture, temperature, and oxygen leads to destruction or breakdown of the plastics with no toxic residue (13).
Biopolymers generally divided into different groups based on the source of the raw materials and their manufacturing processes as shown in Figure 1.
Biopolymers can directly be obtained from bioresources such as plant-derived material (starch, cellulose, other polysaccharides and proteins) and animal products (proteins and polysaccharides). Biopolymers also can be produced synthetically by microorganism production or fermentation and chemical synthesis using renewable monomers or varied sources of biomass and petroleum.
Figure 1: Biopolymer’s Categories (10, 14).
In general, the most common biopolymers for food packaging applications are starch and derivates (15, 16). Starch and derivates are usually edible; therefore, they are safe to use as food packaging materials. Research has revealed that starch is degradable and can enhance the biodegradability of non-biodegradable materials when mixed. Poor mechanical properties of the starch material, which can improve with additives such as plasticiser and nanofillers (17). Other common biopolymers researched include cellulose such as cellulose acetate, chitosan, gelatin, as well as synthetic biopolymers such as polylactic acid, PLA (4, 14, 16).
The most common synthetic biopolymer is polylactic acid. PLA is derived from lactic acid which is a thermoplastic, biodegradable aliphatic polyester often used in food packaging (17). PLA mostly commercialised produced 150,000 tonnes/year due to excellent transparency and relatively good water resistance. Water permeability of PLA is relatively low compared to proteins and polysaccharides, but it is still higher than the conventional polyolefins and PET. There are still lots of drawbacks of PLA regarding performance which related to its low thermal resistance, stiffness, heat distortion temperature, excessive brittleness and barrier properties compared to other benchmark packaging polymer such as PET. Thus, it is an excellent industrial interest to enhance the thermal and barrier properties of this material while maintaining its inherently good features such as transparency and biodegradability (18, 19, 20).
2.1 Benefits and Limitations
In general, though biodegradable polymers are eco-friendly, they provide comparatively poor mechanical and barrier properties, which currently limit their industrial use. The difficulties related to biopolymers are performance, processing, and cost. Although these factors are slightly interconnected, problems due to “performance and processing” are common to all biodegradable polymers despite their origin (21). In specific, material’s brittleness, high gas and vapour permeability, low heat distortion temperature, and reduced resistance to protracted processing operations have sharply limited their applications (22, 23). Table 1 displays the benefits and limitations of biopolymer-based packaging and the advantages of Nanofilled biocomposite. Due to the highly advantageous in nanofilled biocomposite material, researchers start to focus and look into this area.
Table 1: Benefits and Limitations of Biopolymer-based Packaging and The Improvement in Nanofilled Biocomposite.
BENEFITS LIMITATIONS
Biopolymer
Processability
• Can be made edible (4)
• No release of toxic substances (3) • low manufacture costs (3)
Environmental
• Biodegradable (1)
• Environmentally friendly (2) • Waste Utilisation (1)
Processability
• Low tensile strength and brittle (1) • Poor resistance to heavy processing condition (18)
• low heat distortion temperature which limits their industrial use
Transparency
• Poor barrier properties (23) • High permeability such as oxygen and moisture (22)
Bionanocomposites
Food Quality
• Lengthen the shelf life of food, enhanced food quality and properties (2) • Enables incorporation of active agents like antioxidants and antimicrobials (17) Transparency
• Improved barrier properties and migrations such as oxygen and moisture (22) • Protection against rancidity of lipid (1) • Supports the practice of biosensors for food quality assessment (4) Processability
• Potential use in multilayer food packaging materials (18) • Low cost and reduced packaging volume, weight and waste (3) • Protection against rancidity of lipid (23) Environmental
• Edible, Biodegradable and Environment-friendly (2) • Supports the practice of biosensors for food quality assessment (2)
3.0 Nanomaterials in Food Packaging Composites usually consist of a polymer matrix or continuous phase and a discontinuous phase or fillers. Generally, fillers are organic or inorganic materials and having different geometries such as fibres, flakes, particulates, etc. can be surrounded by the polymer matrix, which makes the composites more robust and flexible as well.
Some reinforced materials present poor adhesion at the interface of their components.
The use of nanosize fillers is prominent to the improvement of new composite product that is also known as “nanocomposites.” Polymer nanocomposites are the latest constituents intended to solve the problems mentioned above (24).
Bionanocomposites consist of a biopolymer matrix, reinforced with particles which have at least one or more dimension in the nanoscale ranging from 1 to 100 nm (1). However, there has been a variety of nanocomposite materials available for other applications such as in electrical industry, sensors, medical, etc., the development of nanocomposites for food packaging (2, 13). In specific, bionanocomposites show high possibilities in providing great barrier characteristics, featuring to the presence of the individual clay platelet layers which proficient of interruption the molecule path producing the more tortuous diffusive pathway as described in Figure 3.
Figure 3: Overall Procedure for Nanocomposites Preparation snd Improvement in Barrier Properties (28).
The use of nanocomposite formulations expected to enhance the shelf life of many types of food considerably. Specific examples include packaging for processed meats, cheese, confectionery, cereals and boil-in-bag foods extrusion-coating applications in association with paperboard for fruit juice and dairy products, together with co-extrusion processes for the manufacture of beer and carbonated drinks bottles.
The quality and freshness of frozen, seafood products and processed meat can be controlled by lowered down the moisture loss, improving product appearance, reducing the lipid oxidation and discolouration, also reducing the oil uptake by battered products during frying.
The significant development is the enhancement of food quality shelf life.
This improvement can lead to lower weight packages because less material is required to achieve the same or even better barrier properties. Fillers added to the polymer matrix, and they can reduce costs, decrease shrinkage, and reduce moulding cycles, increase thermal conductivity and lower resistivity (25). Reduction in raw materials, because of the improved stiffness, and cuts the packaging cost with less packaging waste also the cost of transportation, storage and recycling due to the lighter packaging.
The incorporation of nanoclays into packaging offers elimination of expensive secondary processes, such as laminations for barrier packaging and easier recycling due to the less complex structures nanocomposites may have. Improved shelf life and lower packaging cost are some of the reasons why nanotechnology pursued in consumer packaging. Besides, due to the modifications of the physical and thermal properties of polymers, machine cycle time and temperature reduced as well.
Therefore, incorporation of nanocomposites into several natural, biodegradable materials had investigated (29).
The application of polymer nanocomposites in the food packaging application is to reduce food losses and provide safe and healthy foodstuffs. To improve the properties of polymer nanocomposite as packaging materials, not only required low oxygen, carbon dioxide and water vapour permeability and mechanical properties, recyclability, smart antibacterial also a parameter that make food packaging unique among the greatest focused polymer bionanocomposites technology improvement.
3.1 Types of nanoscale particles (NSPs) Many types of nanosize fillers (less than 100 nm) used to enhance the performance of biopolymers. The nanofillers that commonly studied for food packaging applications can classify into nanoparticles, nanofibrils, nanorods, and nanotubes. As shown in Figure 4, NSPs classify into three categories depending on their dimensions as follows:
• Nanoparticles: 3-dimensions of particulates are in the order of nanometers, they denoted as equiaxed nanoparticles or nanocrystals such as silica and metal oxides like TiO2
• Nanotubes: 2-dimensions are in the nanometer scale while the third is greater, producing an elongated structure, which commonly referred to as “nanotubes” or nanofibers/whiskers, for example, carbon nanotubes and cellulose whiskers.
• Nanolayers: Particulates which are considered by only 1-Dimension in the nanometer scale are called nanolayers or nanosheets. This particulate is present in the form of sheets of one to a few nanometers thick and hundreds to thousands of nanometers long [e.g., clay (layered silicates), layered double hydroxides] (26, 27).