In the current state of the world market, consumers are demanding more natural foods that meet strict standards for excellent quality and safety. To store fresh fruits and vegetables, new food packaging has been developed (Oliveira et al., 2015). The most common materials used to produce films are often alginate, cellulose, chitosan, carrageenan, or pectins and their derivatives (Sabina Galus & Kadzińska, 2015). Because they are compatible with a variety of food products, cellulose- based packaging and wrapping films and coatings are of particular commercial importance (C.
Chang & Zhang, 2011). Additionally, it has been demonstrated that these films and coatings significantly lower moisture loss and the amount of oil that fried foods absorb (Wang et al., 2018).
Recent studies have shown that BC has numerous benefits for usage in food packaging. However, using pure BC to manufacture packaging has significant drawbacks. As a result, numerous modifications have emerged to enhance the desired qualities.
The promising application of BC has piqued the interest of this material to develop biodegradable materials, either using BC nanofibers or nanocrystals as a reinforcing agent for other matrices, which is the most ubiquitous study of BC. Another form of employing BC is as the main matrix of material, undergoing ex-situ or in-situ modifications to adapt its functionality to food preservation (Azeredo et al., 2019; Muthuraj et al., 2018). As listed in Table 2.1, BC composites are applied to various types of food to acquire antibacterial and antioxidant capabilities (Gao et al., 2014).
Table 2.2 Bacterial cellulose composites and application No. Application field Function Types of food References
1 BC/nisin
Antimicrobial food
packaging
Meat [151]
2 BC/polylysine
Biodegradable food
packaging
Sausage [62]
3 BC Emulsifier Surimi [102]
4 Carboxymethycellulose
Regulate gough rheology
Flour dough [155]
5 Hyroxypropyl methyl cellulose
Texture enhancer
Whipped
cream [157]
6 Methyl cellulose Enhancing
shelf life Egg [159]
7 Methyl cellulose Enhance
bioavailability Vitamin C [160]
2.3.2. BC films produced by impregnation
Due to the entangled structure and numerous vacant spaces present in BC membranes, other components can be trapped inside them [161]. Several investigations on food films have performed the impregnation technique. Although the process is straightforward technically, it is unsuitable for film production using the continuous method or coating formation on food surfaces.
Impregnation with other polymers
When BC is combined with other polymers, there may be benefits correlated to particular characteristics of the other polymer. For instance, some scientists created BC-chitosan films by impregnating BC into a chitosan solution, taking into account chitosan's antibacterial capabilities [162]. In addition to displaying activity against Gram-negative and Gram-positive bacteria, the resulting composite film also had a strikingly greater elastic modulus than neat BC films.
A highly intriguing approach for enhancing the tensile and barrier performance of packing sheets is cross-linking. Proteins are more readily implicated in crosslinking processes than polysaccharides because of their highly nucleophilic amino groups [163]. In 2012, coated BC sheets with gelatin and submerged the composite sheet in various protein crosslinking chemicals to stimulate crosslinking reactions [164]. Compact interpenetrating BC/gelatin networks were created, mostly at greater gelatin levels, and these networks resulted in fostered tensile qualities.
Impregnation with active compounds
Bacterial cellulose (BC) has been used to progressively apply antimicrobial agents on food surfaces, extending the microbial food stability in the process.
There was a development of antimicrobial BC membranes by nisin-impregnating BC sheets [165]. The discovery of nisin immobilization by Fourier – transform Infrared Spectroscopy (FT- IR) was attributed to the binding of nisin's amine groups to BC's C6 carboxylic acid groups.
Activity against both Gram-negative and Gram-positive bacteria was obtained by immobilization.
Nguyen’s team tested similar films on the surface of vacuum-packed frankfurters, and they were successful in preventing bacterial growth (Nguyen et al., 2008).
The production of active sausage casings involved coating BC tubes with -polylysine (-PL) [166]. In addition to having outstanding tensile and barrier qualities, the composite casings also displayed antibacterial activity, prolonging the shelf life of sausages. Furthermore, the casings demonstrated good thermal stability, maintaining inhibiting S. aureus activity following autoclaving at up to 121°C for 30 min.
When compared to free bacteriocins, BC membranes impregnated with antimicrobial peptides (bacteriocins) were found to be more effective at controlling the development of Listeria monocytogenes, indicating that BC protected the peptides.
Jebel and Almasi (2016) employed multilayer films with an antibacterial layer placed between two outer layers to regulate the antimicrobial agent's release rate (ZnO nanoparticles). The active layer was coated with wet BC membranes on both sides, dried, and then impregnated with a dispersion of nanoparticles. In addition to giving the films antibacterial activity, the introduction of ZnO nanoparticles improved their tensile properties and reduced their water vapor permeability.
Antioxidants, oxygen scavengers, and ethylene absorbers are just few examples of a huge number of active substances that can be loaded into BC membranes in addition to antimicrobial compounds. These substances act in various ways as carriers for substances that could assist in extending the stability of food against different phases of degradation.
Impregnation with reinforcing agents
Ul-Islam et al. (2012) have accomplished so to create BC nanocomposites containing montmorillonite (MMT) even though incorporating reinforcing materials into BC membranes via impregnation has not traditionally been a common approach. The BC membrane adsorbs the nanoclay, increasing the tensile strength and improving the thermal stability of the nanocomposite material [168].
Poly(L-lactic acid) infused with BC is another reinforced composite (PLLA). BC sheets were submerged in PLLA that had been chloroform-dissolved, and the chloroform was subsequently removed by letting the solution dry at room temperature for several days. The study's key finding was that the final transparent films achieved tensile strength that was twice of pure PLLA and increased nanocomposites crystallinity [169].
2.3.3. Films with disassembled BC
Bacterial cellulose (BC) in disassembled form employed in packaging production is the most common and extensively researched application of BC. It is easily converted into suspension or powder form microfibrils (BCMFs), nanofibrils (BCNFs), and nanocrystals (BCNCs) (Figure 2.7).
This strategy makes continuous film or coating production processes more feasible, and it is better suited for implementing them in industrial operations [9]. The enhanced BC dimension's large surface area encourages strong physical interactions, particularly hydrogen bonding, which can strengthen polymers' structural connections. Overall, better barrier, mechanical, and thermal properties are obtained [170-172].
A suspension of BCMFs or BCNFs in aqueous solution (referred to as a "BC slurry") was noted resulting in the obtention process of pure BC films, according to other authors [173].
Figure 2.7 Schematic process to obtain Microfibrils, Nanofibrils and Nanocrystals from Bacterial Cellulose (BC)
BCNFs were composed of a lengthy, network-like structure with a diameter size of 40–70 nm.
Mechanical procedures such as high-pressure homogenization, grinding, and refinement procedures can be used in BCNFs manufacture [174].
Most BCMFs are classified as nanofibrils since their diameters or widths are often in the submicrometer range, and they comprise nanostructures as a primary component [175, 176].
The most popular technique for isolating BCNCs is acid hydrolysis, while enzymatic hydrolysis is also an option [9, 177, 178]. Chains of cellulose crystals organized in paracrystalline areas make up cellulose microfibrils. As a result of acid treatment, rod-shaped cellulose nanocrystals are formed because of the probable removal of the paracrystalline regions. The average diameter and length of BCNCs produced by the acid hydrolysis technique are 20±5 nm and 290 ± 130 nm, respectively [170, 172]. Sometimes, the phrases BC nanocrystals and BC nanowhiskers are interchangeable [177].
In comparison to the impregnation method, disassembled BC is more versatile and can be utilized for formulation with a wide range of materials. In order to create films with other fruit purees (guava and mango), nanofibrillated BC (NFBC, synthesized by TEMPO oxidation and physical defibrillation), has been utilized (mixed or not to pectin, in varying amounts) [179]. The superior tensile and barrier qualities of NFBC in comparison to pectin were reflected by the outcome that higher NFBC/pectin ratios produced increased tensile properties, a barrier to water
vapor, and water resistance. Food wrappers or even fruit ribbons may be obtained from the films, which could also be employed for other uses where fruit flavors would be appreciated.
The combination of NFBC and copper nanoparticles (CuNP) has also been used to create antibacterial films with polyvinyl alcohol (PVA) [180]. For the controlled release of CuNP, the hybrid NFBC-CuNP material served as a prototype. The ability of NFBC to control copper release has been demonstrated to improve the films' long-term antibacterial capabilities against E. coli.
To create nanocomposite films, silver nanoparticles (AgNPs) and BCNCs were combined in a chitosan dispersion [172]. In addition to the antibacterial activity offered by the AgNPs, hydrogen bonds were established between chitosan and BCNC, and crystalline peaks showed up in X-ray diffractograms. This improved the barrier and tensile properties of the films.
Additionally, BCNCs were employed to strengthen thermoplastic corn starch sheets [181].
With a BCNC loading of 15 wt percent, the best results towards the barrier and tensile properties were obtained. When compared to the control film made of pure PVA, nanocomposite PVA films also displayed enhanced attributes (thermal stability, tensile strength, and elastic modulus) [182].
A way to combine the benefits of different materials is through the emulsion films made of polyssacaride-lipid or protein-lipid combinations. Particularly, the cooperated advantages are the tensile and gas barrier properties of polyssacarides (or proteins) and the water vapor barrier of lipids. An intriguing method for food applications that demand a low water vapor permeability is the inclusion of lipids to hydrophilic film-forming compositions. Conversely, the addition of lipids typically reduces the transparency and tensile qualities of films [183, 184]. Therefore, it seems a novel and interesting strategy to employ BCNC as an emulsion stabilizer and reinforcement agent simultaneously.
2.3.4. Future expectation
Due to its exceptional qualities and wide range of uses, bacterial cellulose is becoming a viable replacement for plastic materials in food packaging. Because of its large surface area, capacity for new bond formation and interaction, and water-insoluble nature, BC fibers or nanocrystals have come to dominate most research as reinforcing agents.
Numerous studies have shown that BC production costs tend to decline without sacrificing BC quality, particularly for food applications, which typically do not require a high level of purity compared to those for biomedical applications, even though they have not yet been properly investigated due to economic considerations. Additionally, new technology for BC production,
such 3D printing, which has already exhibited its viability for producing BC objects in any requested geometry, bode well for the future [185].
In order to assess their potential suitability as active packaging, current studies attempt to investigate the mechanical, permeability, interactions, and release rates in semisolid and solid food model media [174]. There are reasons to expect that BC has a possibility as a distinctive component for differentiated new foodstuffs with sensory and health appeals, including edible films and coatings, despite the difficulty in commercial production of BC and the need for validations for the majority of food applications.