Bulk Preparation Technologies in Food Packaging Applications: History
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Contributor:
Johan Stanley
,
Athira John
,
Klementina Pušnik Črešnar
,
Lidija Fras Zemljič
,
Dimitra A. Lambropoulou
,
Dimitrios N. Bikiaris

Active packaging has played a significant role in consumers’ health and green environment. Synthetic polymers, such as poly(ethylene terephthalate) (PET), polyethylene (PE), polypropylene (PP), polystyrene (PS), poly(vinyl chloride) (PVC), polycarbonate (PC), poly(lactic acid) (PLA), etc., and naturally derived ones, such as cellulose, starch, chitosan, etc., are extensively used as packaging materials due to their broad range of desired properties (transparence, processability, gas barrier properties, mechanical strength, etc.). The food packaging field has been challenged to deliver food products free from microbes that cause health hazards. However, most of the used polymers lack such properties. Owing to this, active agents such as antimicrobial agents and antioxidants have been broadly used as potential additives in food packaging substrates, to increase the shelf life, the quality and the safety of food products. Both synthetic active agents, such as Ag, Cu, ZnO, TiO2, nanoclays, and natural active agents, such as essential oils, catechin, curcumin, tannin, gallic acid, etc., exhibit a broad spectrum of antimicrobial and antioxidant effects, while restricting the growth of harmful microbes. Various bulk processing techniques have been developed to produce appropriate food packaging products and to add active agents on polymer matrices or on their surface. Among these techniques, extrusion molding is the most used method for mass production of food packaging with incorporated active agents into polymer substrates, while injection molding, thermoforming, blow molding, electrospinning, etc., are used to a lower extent.

  • food packaging
  • active agents
  • bulk preparation techniques
  • antibacterial and antioxidant properties

1. Introduction

Food packaging is an integral component of the food processing industry that aims at achieving the safe transportation and distribution of the products in wholesome conditions to the consumers. Currently, polymers play a significant role in the manufacture of packaging materials due to their desirable properties including resilience, stability, and ease in production. Nevertheless, the increased usage of plastics has caused serious environmental issues because of their resistance to biodegradation. Biopolymers on the other hand are a sustainable solution to the issues posed by plastics as they easily degrade in the environment and imitate the properties of traditional polymeric materials. [1]. However, a significant problem in the food packaging sector remains a challenge. Microbial spoilage threatens numerous food products such as meats, cheeses, baked products, poultry, fruits, and vegetables [2], leading to an increased demand for antimicrobial-based food packaging materials during the 21st century. Hence, various techniques have evolved in the food packaging industry to extend the shelf life of foods and provide germ-free foods to consumers. For that reason, various active agents have been incorporated into the polymeric substrates to improve their mechanical, barrier, antioxidant, and anti-microbial properties. “Antimicrobial agents” have been widely used to disinfect the microorganisms present on packaging surfaces, in households, on personal protective equipment, etc. At the same time, food packaging industries have also been focused on reducing the environmental impact caused during the disposal of packaging products, with green packaging playing a crucial role in the waste disposal system [3][4].
Active agents have been extensively incorporated into both synthetic and biobased polymers to improve the shelf life and food quality. Specifically, nanoparticles have been facilitated onto a large scale as active agents in modern packaging to produce nanocomposites, and nano-based sensors able to detect changes in food products [5]. However, active agents for packaging must be carefully selected based on their activity against targeted microorganisms and their sustainability after incorporation into the packaging substrate [6]. According to the European Commission’s regulation regarding the active and intelligent materials intended to come into contact with food, a maximum of 0.01 mg/kg migration of nonauthorized substances through a functional barrier is acceptable in food packaging. This regulation was proposed after considering infants and other particularly susceptible persons [7]. The effect of the active agents also depends on their release kinetics. If the release rate of an antimicrobial agent is high enough, the food spoilage could be inhibited before it even starts. A Fickian diffusion model and non-Fickian diffusion model are the two most commonly used mathematical models to determine the release profiles [8][9][10]. The degradation rate of polymer composite materials depends on the nature of the materials, how strongly the fillers and matrix are bonded to each other, and the environmental conditions, such as temperature, moisture, pH of soil, microbial population, and nutrient supply [11]. The area and properties of the exposed to environmental interface plays an important role in the degradation of polymer composites. A composite with a rough surface and polar hydrophilic functional groups tends to have a faster biodegradation than a smooth, hydrophobic, and inert one [12]. Most of the natural active agents are hydrophilic in nature and tend to provide a good adhesion to the surface of the composite materials and microorganisms in the environment, which favors biofouling [13]. The active agents must meet certain requirements when used as coatings: they should adhere well to the coating surface, the concentration of release agents, and the equivalence of properties provided by the conventional passive packaging [14].
Currently, synthetic antimicrobial active agents are widely used in the field of food packaging. The most well-known “antimicrobial agents” and antioxidant active agents in food packaging are organic acids, such as benzoic acid, propionic acid, and their salts, inorganic metallic ions, such as silver ions and titanium ions, natural compounds, such as bacteriocins and lysozyme, and plant extracts, such as essential oils, allyl isothiocyanate (AIT), etc. Moreover, synthetic antioxidant agents such as butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), and tert-butylhydroquinone have been phased out and replaced by natural antioxidants, such as chitosan, alginate, gelatin, galactomannans, and cellulose derivatives. When it comes to commercialized antimicrobial active agents, there are only few available due to the strict hygienic regulations concerning food packaging in different countries. Several of the commercially available products on the market, such as silver (Ag)-substituted zeolite or zirconium used in poly(vinyl chloride) (PVC), linear low-density polyethylene (LLDPE), polyethylene (PE), rubber matrix for wrap films, etc., are sold under the trade name Agion® (Wakefield, MA, United States), Zeomic™ (Nagoya, Japan), and Cleanaid™ (Cardiff, United Kingdom). Chlorin dioxides used in the polyolefin matrix for packaging films are sold under the trade name MicroGrade™ (Barnet, United Kingdom), Knick’n’clean® (Hannover, Germany). Triclosan used in polymers, rubber, etc., and used in sheets or pads is sold under the trade name Uvasy™. Wasabi (Cape Town, South Africa) (Japanese horseradish) extract encapsulated in cyclodextrin used in coated poly(ethylene terephthalate) (PET) films is commercially available under the trade name Wasapower (Tokyo, Japan) [15][16].
Polymer processing technologies are used in the mass production of products with various colors, designs, and complicated shapes to meet the long-life applications for use in industries such as food packaging industries to produce cling films, laminated pouches, plastic wraps, bubble warps, plastic containers, tins, cans, tetra packs, laminated tubes, etc. Generally, petrochemical-based plastics, such as PET, PVC, PE, PP, PS, and polyamide (PA), have been preferably used in food packaging industries due to their low cost and good mechanical performances. In recent years, due to their environmental impact and nonbiodegradability, the synthetically based polymers have tended to be replaced by ecofriendly biopolymers, such as aliphatic–aromatic copolymers (PET/biodegradable polyester sold under the trade name Bio-max by DuPont Tennessee), aliphatic polyesters (trade name: Nodax produced by Procter and Gamble Co. P&G, trade name: Eastar bio produced by Eastam Chemical Company UK), polylactide aliphatic copolymer (CPLA), polycaprolactone (PCL), poly(lactic acid) (PLA) (Natureworks™ PLA produced by Natureworks™ LLC Blair, NB), polyhydroxyalkanoates (PHA) and starch-based polymers (trade names: Biopur® from Biotec GmbH, Eco-Foam® from national Starch & Chemical and Envirofill™ from Norel) have been emerging in food packaging industries [17]. During the processing of polymers, the mechanical, thermal, optical, and other properties have been improved by using optimized processing parameters [18]. The innovative processing technology of the polymer composites has been well developed in recent years. Schmidtchen et al. invented a novel semidry extrusion approach to produce a 100% algae-based packaging material [19]. This process tended to be a cost-efficient processing method since it reduced the moisture content by 80% and prevailed over the drawbacks of a solution-casting method. González et al. combined both extrusion and compression techniques to produce thermoplastic starch nanocomposite films with excellent transparency and better mechanical and barrier properties [20]. Electrospinning is a newly emerged and versatile process to produce polymeric nanofibers with a large surface area, high porosity, and high molecular orientation, thus making it suitable for a food packaging application [21].

2. Bulk Preparation Technologies

The different types of polymer processing technologies widely used in packaging industries are compression molding, injection molding, extrusion molding, blow molding, thermoforming, and electrospinning.

2.1. Compression Molding

Compression molding is the oldest material processing technique used in the industrial, commercial, and consumer parts manufacturing process for high production volume for both thermoplastic and thermoset polymers. Thermoplastics such as ABS, nylon, polycarbonate, polyethylene, polypropylene, polystyrene, and various blends are used for compression molders. In addition, thermosets such as epoxy, phenol-formaldehyde, and elastomers are also processed in smaller quantities [22]. This process is mainly used to produce screw caps for bottles, sheets for food containers, and other liquid containers in the food packaging industries. This process delivers products with increased mechanical resistance, insolubility, and thermal stability in the curing or cross-linking process due to the presence of both pressure and heat [23]. Bulk molding compound (BMC) and sheet molding compound (SMC) and produced from thermosetting polymers, while glass mat thermoplastics (GMT) are produced from thermoplastic compounds through compression moldings. Mold A produces uniform plates due to its flat surface in the upper and lower molds, whereas Mold B produces nonuniform plates with a thin section, stepped section, and thick section [24]. In both types of compression molding, a hot treatment may be applied, which requires both pressure and temperature, as well as a cold treatment, which requires only pressure. [25]. Xie et al. produced highly flexible starch-based films with [Emim][OAc] as plasticizer, which inhibited the bacterial attack using a simple melt-compression molding process [26]. The main advantage of a compression molding process is that it can be performed even when the viscosity of the final formulation is not very low [27].

2.2. Injection Molding

Injection molding is the oldest and most widely used manufacturing technique to fabricate products with complex shapes and surface smoothness. Polymers such as PET, PP, high-density polyethylene (HDPE), and LDPE are widely injection-molded to produce preforms, jars, and containers in packaging industries [23]. Pavon et al. successfully produced rigid food packaging materials with poly(butylene adipate-co-terephthalate) (PBAT) blended with two pine-resin derivatives (gum rosin (GR) and pentaerythritol ester of GR (UT)) by an injection molding process [28]. During this process, the polymers were heated up to an elevated temperature and pushed into a mold cavity with a high pressure to obtain the desired shape. Clamping, injecting, cooling, and ejecting are the important processing steps of injection molding [29]. This method can be used for the incorporation of reinforcement agents or other additives into polymer granules. In composite manufacturing, the additive is incorporated into the polymer matrix through extrusion or internal melt technique and finally processed by injection molding to obtain the desired shape [30].

2.3. Extrusion Molding

Extrusion molding is the most preferable option in the food packaging industry due to its great design flexibility, low production costs, and its postproduction operations, since it does not necessitate any additional curing time [31]. Polymers such as PET, PLA, PE (low- and high-density), PS, PP, and cellulosic derivatives are commonly used in extrusion molding to manufacture food and beverage containers as well as packaging films [32]. It is the most popular process for continuous production of fixed cross-sectional shaped objects in the plastic industry [33]. The polymer, in the form of pellets or flakes is heated and forced into a long tubelike shape-molding machine using a screw extruder. The screw is a determinant factor of the extruder’s performance. The length and diameter (L/D) ratio of the screw is sectioned into feeding, compression, and metering zones. It also determines the mixing and uniformity of the process. The screw rounds per minute (RPM), screw configuration, temperature, and melt viscosity play an important role in the output rate of the extruder. Finally, the extruded object is cooled and ejected to produce shaped products such as films (blown or cast films), sheets, wires, tubes, drinking straws, synthetic and optical fibers, etc.

2.4. Blow Molding

Blow molding is a process of forming a hollow object by inflating it with air using an extruder or thermoplastic molten tube called a parison. The air-blown tube acquires the shape of the mold, and the formed parts are cooled in the blow mold taking the desired shape without wrapping. The blow molding consists of three types, namely, extrusion blow molding, stretch blow molding, and injection blow molding. Stretch blow molding has numerous advantages such as a low cost, a low gram weight, the optimization of the preform design, durability, and a high-volume production [34]. The extrusion blow molding is a workhorse process of polyolefins such as HDPE and PP, to produce plastic containers such as bottles, jars, and containers [35]. PET resins are not suitable for this method. It is classified into two major categories namely continuous extrusion and intermittent extrusion. Injection blow molding is a process where the resin is injected into a preform cavity through a horizontal screw and the parison is blown into the final shape and cooled. Resins such as HDPE, PS, PET, and PVC are used for producing bottles via injection blow molding [23]. The core rod carrying the product is heated to 120 °C and the strip bar removes the final product. Stretch blow molding is used to manufacture PET bottles for the soft-drink beverage industry. In this process, the parison or preform is stretched in both axial and hoop directions.

2.5. Thermoforming

Thermoforming is a rapidly emerging technique to produce plastic packaging due to its low tooling and operational cost. High-impact polystyrene (HIPS), ABS, PVC, PP, HDPE, and polycarbonate are used for manufacturing thermoformed cups and bowls. Polystyrene is thermoformed to produce salad bar containers, cups, cutlery, yogurt, cottage, and cheese containers. Crystallized poly(ethylene terephthalate) (CPET) is used to produce containers for baked products such as muffins and cakes. PS blended with poly(phenylene oxide) (PPO) is used in the manufacturing of thermoformed trays and containers [36]. During this process, the plastic sheets are heated slightly above their glass transition temperature or melt temperature against a rigid single surface mold followed by cooling and trimming processes to form a three-dimensional product with a high surface area. Forming time and temperature play a vital role in the surface quality and shrinkage. The thermoforming process is classified into three different methods: mechanical thermoforming, vacuum thermoforming, and pressure or external pneumatic air thermoforming. It is widely used in various industries such as food packaging, automotive industrial building, etc. The development in the processing technologies enables thermoforming to compete with injection and blow molding [37].

2.6. Electrospinning

Electrospinning is an eloquent technique used to produce polymeric fibers in several structures and morphologies. The electrospun fibers are optimized and produced with different morphologies such as normal electrospun fibers, spring/helical electrospun fibers, porous electrospun fibers, core-shell electrospun fibers, hollow electrospun fibers, Janus electrospun fibers, triaxial electrospun fibers, and others [38]. The antimicrobial agents are incorporated into the fibers to impart the antimicrobial activity of the fibrous mat. When antimicrobial agents such as silver or copper agents are loaded into fibrous mats, it creates thousands of loading spots due to their large surface area and high porosity. The resultant fibrous mats will exhibit antimicrobial activity [39][40]. The electrospun materials commonly used in food packaging industries are chitosan (antibacterial), corn protein (edible), poly(vinyl alcohol) (transparent), etc. Biopolymers such as PCL, PLA, natural cellulose, starch, and poly(propylene carbonate) (PPC) have also been electrospun and used in the food packaging field [21]. Briefly, Amna et al. produced an antimicrobial hybrid packaging mat using biodegradable polyurethane mixed with virgin olive oil and zinc oxide via an electrospinning process [41]. Vega-Lugo et al. fabricated antimicrobial electrospun fibers from a soy protein isolate (SPI)/poly(ethylene oxide) (PEO) blend and PLA with the addition of allyl isothiocyanate (AITC) into fiber-forming solutions for active food packaging applications [42]. The steps involved in the electrospinning process are: (1) the formation of a Taylor cone or cone-shaped jet by charging the liquid droplets; (2) the extension of the charged jet; (3) the thinning of the jet in the presence of an electric field and whipping instability; and (4) the solidification of the jet into nanofibers [43]. It is a thermodynamic process during which the liquid droplets are electrified to produce a jet, followed by stretching and elongation that result in the formation of fibers [44]. The electrospun fibers exhibit a very high surface activity which is required for active packaging applications. The possibility to vary the structure and the morphologies of the electrospun fibers results in improved mechanical and barrier properties. It also protects the active agents loaded in the electrospun fibers [45].

3. Active Agents incorporated in Bulk Preparation Technologies

Active packaging can be achieved by the addition of active agents into the polymer using several methods, such as the addition of sachets, a coating of the polymer, surface immobilization through covalent/ionic linkages, or direct incorporation. However, the most used method is the direct incorporation of active compounds into the bulk of the polymer through extrusion. It ensures the uniform distribution of the active compounds throughout the matrix of the polymer and avoids any direct contact of the compounds with the food [46]. However, the additives used in packaging must be wisely chosen so that they comply with the requirements of regulatory agencies, such as the Food and Drug Administration (FDA) and the European Food Safety Authority (EFSA). Therefore, active ingredients are included into food packaging if they support one or more of the following attributes, such as mechanical or barrier capabilities that increase shelf life or antibacterial, antioxidant, or antiviral properties. Additionally, the enhancement of qualities such as transparency would be advantageous because it would make food more accessible and appealing to customers.
Understanding the mechanism of action of antibacterial agents and antioxidants are of great importance while their addition in bulk preparation technology is considered. The release of ions, reactive oxygen species (ROS), and electrostatic interactions with the cell walls of bacteria are the prospective explanation for the active mechanism of antibacterial agents. Antioxidants can delay spoilage by slowing both lipid oxidation and protein denaturation by reducing the amount of oxygen in food systems. Additionally, they stop the oxidation chain reaction, hinder the further development of the oxidation reaction, and inhibit or deactivate the activity of the enzymes that promote the oxidation reaction. The best antioxidants for use in active packaging should be affordable, nontoxic, highly active at low concentrations, and highly permeable and should have a good stability. Additionally, they should not have a negative impact on the food’s quality, such as taste or odor. The active substances should have a low molecular weight because they are intended to diffuse through polymer macromolecules. As a result, the release rate is influenced by their physical size, with larger active compounds releasing at a slower rate than smaller ones [47].
Various active agents have been studied in the past few decades, ranging from metal ions and metal oxides, such as silver, copper, TiO2, and ZnO, to essential oils, bioactive components, such as thymol and carvacrol, and enzymes, such as nisin and lysozyme. Nevertheless, all the bulk preparation techniques involve high-temperature processing, which is why the fillers are also required to be thermally stable. Based on their origin, fillers could be broadly classified into natural and synthetic active agents [48]. A list of some of the active agents extensively used during the past decades in bulk preparation methods is listed in Table 1.
Table 1. Active Agents Widely Used for Bulk Preparation Methods.

Active Agents

Advantages

References

ZnO

Low-cost, high ultraviolet absorption capability, and strong antibacterial activity on a wide range of bacteria

[49][50][51]

TiO2

Enhances ethylene-scavenging activity

[52][53][54]

Nanoclays

Improved mechanical, gas barrier, optical properties at low filler content

[55][56][57]

Nanosilver

Antimicrobial activity

[58][59]

Catechin

Antioxidant and antimicrobial activities

[60][61]

Curcumin

Excellent antiviral, anticancer, antimicrobial, anti-inflammatory, and antioxidant activities.

[62][63]

Essential oils

Antioxidant, anticancer, and antimicrobial, cost-efficient

[64][65][66][67]

Lignin

Excellent antioxidant and antimicrobial activity attributed to the large number of phenolic groups; mechanical properties and fire-resistance

[68][69][70]

Tannin

Antioxidant and antimicrobial activities

[69][70]

Gallic acid

Oxygen-scavenging ability

[71][72]

This entry is adapted from the peer-reviewed paper 10.3390/macromol3010001

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