1. Introduction
Food packaging is vital to the food industry, which could protect the food from physical damage, chemical hazards, and biological contamination [
1]. Synthetic polymer-based plastics [
2], paper and paperboard [
3], glass [
4], and metal [
5] are the most widely used packaging materials, and the choice of the packaging material is food-type-dependent. Nowadays, plastics derived from fossil resources are the most common food packaging materials. By far the most plastic, almost 40%, is used for packaging. Annually, approximately 500 billion plastic bags are used worldwide. After use, plastic products are generally incinerated or landfilled, causing serious environmental pollution. In addition, recycling and processing plastic packaging waste is estimated to cause an annual loss of USD 80–120 billion globally [
6]. Today, with the promotion of “sustainable development”, new functional food packaging materials that are green, environmentally friendly, biodegradable, renewable, and sustainable have become the cutting-edge research direction in the food packaging field.
Cellulose is the most abundant biopolymer on Earth. It can be synthesized by trees [
7], plants [
8], sea animals (tunicates) [
9], algae [
10], and certain cellulose-secreting bacteria [
11], and the properties of the cellulose are also found to be source-dependent [
12]. However, cellulose derived from trees is still the most commonly investigated due to its high abundance and easy availability. In fact, paper made from woody cellulose has been used as packaging material for decades, though its low wet strength and poor barrier properties inhibit its further applications as packaging material to replace plastics [
13]. More recently, cellulose fibers have been processed to nanoparticles through many different methods, such as acid hydrolysis [
14], homogenization [
15], grinding [
16], blending [
17], etc., which are normally terminated as nanocellulose [
18]. Though the nomenclature of nanocellulose is still not yet standardized, it is commonly categorized into three types in terms of the preparation method and the morphological profile: CNC (cellulose nanocrystals), CNF (cellulose nanofibrils), and BC (bacterial cellulose) [
19]. CNC with a rod-like shape with a width of 3–5 nm and a length between 50–500 nm is typically produced by mineral acid hydrolysis of lignocellulosic materials [
20]. Another type of nanocellulose is CNF, with a width of 4–20 nm and a length between 500 nm and a few micrometers. CNF is typically produced via a mechanical treatment of lignocellulosic materials in a high-pressure homogenizer or a microfluidizer, achieving defibrillation and breakage of fibrils. Normally, a complementary pretreatment such as enzymatic treatment [
21], carboxymethylation [
22], or 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO) oxidation [
23] is employed to facilitate the defibrillation, thus lowering the energy consumption [
24]. BC is biosynthesized by specific bacteria, with many characteristics differing from CNC and CNF. BC is composed of ribbon-like fibers, with a width in the range of 0.01–0.10 μm and a length between one hundred nanometers to the micron level, and the fibers interweave to form a 3D network [
25]. For all these three categories of nanocellulose, they generally have a high degree of polymerization (DP), high crystallinity, good thermal stability, and excellent mechanical properties, making nanocellulose a most promising candidate to develop bio-based nanocomposites for green packaging applications.
In this review, the sources, extraction, and properties of nanocellulose are summarized, and the potential of nanocellulose as the matrix, nanofiller, or coating materials to prepare advanced cellulose nanocomposites for food packaging is discussed: (i) the source-specific physicochemical properties of nanocellulose prepared by various cellulose sources, (ii) the inter-relation between preparation methods and the properties of the obtained nanocellulose, (iii) the fabrication strategies of cellulose nanocomposites aiming for potential food packaging applications, (iv) the performance of the thus-prepared cellulose nanocomposites as packaging materials, and (v) the state-of-the-art in the commercialization of nanocellulose on the market. We also discuss the opportunities and challenges of developing cellulose nanocomposites-based food packaging materials.
2. Source and Structure of Cellulose and Its Derived Nanocellulose
As one of the most widely distributed and abundantly available biopolymer materials in nature, cellulose has been widely used throughout human beings’ history. Cellulose is a linear natural biopolymer composed of β-D-glucopyranose, which has a chemical structure with repeating units of cellulose disaccharide (
Figure 1) [
26]. The repeating unit is then linked by a β-1,4-glycosidic bond by forming an acetal functional group between hydroxyl groups at the C
4 position of the glucose unit and a hydroxyl group at the C
1 position of the adjacent unit. The structural formula of cellulose is (C
6H
10O
5)
n, where n is the degree of polymerization (DP). There are hydroxyl groups (-OH) at the C
2, C
3, and C
6 positions of each glucose unit [
27]. The presence of these -OH groups render the cellulose a high chemical reactivity, which allows a series of chemical and physical modification of cellulose, so that the modified cellulose has the expected properties to meet the different needs for use.
The large number of -OH groups in the molecular structure of cellulose make it easy to form intramolecular and intermolecular hydrogen bonds, leading to the aggregation of cellulose molecular chains and the formation of crystalline supramolecular structure. The supramolecular structure of cellulose is divided into crystalline and amorphous regions [
28]. In the crystalline region, many intermolecular hydrogen bonds exist, and the molecular chains are orderly packed. However, molecular interaction is weaker in the amorphous region than in the crystalline region, and the molecular chain arrangement is disordered with a loose structure. The XRD analysis of the crystalline structure of cellulose shows that the intensity of the X-ray diffraction peak in the crystalline region is high, while no specific diffraction peak is observed in the amorphous region [
29].
In nature, cellulose can be found from many different sources, though the most common ones include wood, plants (cotton, wheat straw, sugarcane bagasse, ramie, hemp, flax, sugar beet pulp, etc.), tunicate, algae, and bacteria [
30]. The synthetic mechanism and the properties of the cellulose are source-specific, shown in
Figure 2, and the typical microscopic images of cellulose from different sources are presented in
Figure 3.
2.1. Cellulose Source and Structure
2.1.1. Wood
Trees are the major source of cellulose in nature, in which the cellulose is synthesized through photosynthesis by cells (
Figure 2a) [
31]. As shown in
Figure 3a, the woody cellulose fibers in nature are ribbon-like. Traditionally, the wood is subjected to a pulping process to obtain pure cellulose, which is then further processed into other materials for food, biomedical, environmental mediation, electronic devices, energy conversion, and other applications. In general, woody cellulose-based materials are easily available, renewable, biocompatible, and biodegradable, thus making them ideal raw materials for many advanced biomaterials. Woody cellulose fiber shows excellent physical properties and chemical stability and is a new material for a sustainable society and industrial ecological development [
32].
2.1.2. Plant
Cellulose is widely present in most plants, having an annual production of 10
11–10
12 tons through photosynthesis, similar to trees. It can be obtained from many sources, mainly cotton, wheat straw, sugarcane bagasse, ramie, hemp, flax, etc. [
33]. Plant fibers mainly comprise polymers such as cellulose, hemicellulose, lignin, wax, and pectin. Cellulose is the main component of plant fibers, composed of spirally entangled cellulose microfibrils bound together by an amorphous lignin matrix, for example, cotton cellulose, in
Figure 3b. Lignin helps the plants protect against biological attack and acts as a reinforcing agent to strengthen them against gravity and wind, and hemicellulose acts as an adhesive between cellulose and lignin [
34]. Plant species, climate, maturity, and soil conditions would affect the physical and chemical properties of plant fibers. Plant fibers are renewable, degradable, low cost, and widely available compared to resources such as oil, natural gas, and coal. Using plant fibers instead of nonrenewable resources is significant in alleviating the energy crisis and environmental pollution problems.
Figure 2.
Biosynthesis of (
a) wood cellulose, reprinted with permission from [
35], 2016, ACS; (
b) tunicate cellulose, reprinted with permission from [
36], 2007, Springer; and (
c) BC, reprinted with permission from [
37], 2019, Frontiers Media S.A.
Figure 2.
Biosynthesis of (
a) wood cellulose, reprinted with permission from [
35], 2016, ACS; (
b) tunicate cellulose, reprinted with permission from [
36], 2007, Springer; and (
c) BC, reprinted with permission from [
37], 2019, Frontiers Media S.A.
2.1.3. Tunicate
Tunicates are the only animals that can produce cellulose in nature. The tunicate cellulose is generated through a cellulose synthase complex (
Figure 2b), and the produced tunicate cellulose is characterized by a large diameter, high crystallinity, and great molecular weight (
Figure 3c). Tunicate cellulose is embedded in a protein matrix to form a leathery mantle that can protect the animals from physical damage and predators. It has been reported that there are over 2300 species of tunicates worldwide, and tunicate cellulose has become a new type of biopolymer for many different applications [
26].
2.1.4. Algae
Algae can also produce cellulose microfibers in their cell walls, such as green algae, red algae, and yellow-green algae [
38]. It has been found that cellulose synthesis occurs at the plasma membrane-bound cellulose synthase, except for some algae that produce cellulosic scales in the Golgi apparatus. There are also great differences in microfiber structure between different algae, probably due to different biosynthesis processes (
Figure 3d).
2.1.5. Bacteria
BC is a natural nanostructured polymeric material mainly produced by bacteria [
39]. Like tunicate, the bacteria produce cellulose via cellulose synthase complexes (
Figure 2c). BC is ribbon-like, with a width in the range of 0.01–0.10 μm, which is 2–3 orders of magnitude smaller than the diameter of plant cellulose (generally 10 μm), and the fiber length ranges from a few hundred nanometers to the micron level, and the fibers cross each other to form a mesh-like structure (
Figure 3e,f). BC differs from plant cellulose in that it is not a structural component of the cell wall and therefore does not contain impurities such as hemicellulose and lignin but is a product of microbial metabolism [
40,
41]. BC not only has the properties of plant cellulose, but also has other, more outstanding advantages, such as high purity, high degree of polymerization, great crystallinity, high hydrophilicity, high permeability and air permeability, excellent Young’s modulus, good biocompatibility, etc. [
42]. Under specific culture conditions, BC can be prepared by static fermentation, dynamic fermentation, and fermentation in special molds, showing different structural and performance characteristics.
Figure 3.
SEM images of cellulose obtained from different sources: (
a) softwood, reprinted with permission from [
43], 2012, RSC; (
b) cotton, reprinted with permission from [
44], 2013, Elsevier; (
c) tunicate, reprinted with permission from [
45], 2015, Elsevier; (
d) algae, reprinted with permission from [
46], 2015, Springer; BC, (
e) reprinted with permission from [
47], 2016, RSC, and (
f), reprinted with permission from [
48], 2022, Elsevier.
Figure 3.
SEM images of cellulose obtained from different sources: (
a) softwood, reprinted with permission from [
43], 2012, RSC; (
b) cotton, reprinted with permission from [
44], 2013, Elsevier; (
c) tunicate, reprinted with permission from [
45], 2015, Elsevier; (
d) algae, reprinted with permission from [
46], 2015, Springer; BC, (
e) reprinted with permission from [
47], 2016, RSC, and (
f), reprinted with permission from [
48], 2022, Elsevier.
2.2. Nanocellulose Obtained from Different Sources
As mentioned above, cellulose can be extracted from many different sources, and the cellulose properties are source-dependent. After further processing to nanocellulose, the differences are still present in the final products. In
Table 1, we summarized the typical geometrical characteristics and crystallinity index of nanocellulose originating from different cellulose sources. A more detailed discussion of the properties of various nanocellulose derived from different sources can be found in
Section 3.
5. Performance of Nanocellulose-Based Composites as Food Packaging Materials
Food packaging materials are an extremely important part of the food processing industry and have always been the research focus in the food field. As ideal food packaging materials, they should protect commodities, maintain food quality stability, increase commercial food value, promote sales, and facilitate storage and logistics [
152]. Non-biodegradable polymers derived from fossil fuels are the most used materials in food packaging. With increased focus on global environmental issues, the development of biodegradable polymer materials has gained great interest. In recent decades, nanocellulose has been predominantly employed to create biocomposites because of its green source, high specific surface area, high crystallinity, and nontoxic and biodegradable qualities. Many research works have proven that combining nanocellulose and other substances may provide beneficial functionalities, such as barrier characteristics, mechanical properties, antibacterial properties, etc. [
153]. The application and advantages of cellulose nanocomposites in the food packaging are summarized in
Table 5.
5.1. Barrier and Mechanical Properties
For ideal food packaging materials, barrier and mechanical properties are critical to realistic applications. On the one hand, good barrier properties could protect the food from gas and moisture and slow the oxidation reaction and spoilage; on the other hand, the excellent mechanical properties would avoid physical and chemical damage during transportation and sales.
Shi et al. successfully developed cellulose-based food wrapping paper with high barrier and antibacterial properties by constantly depositing multilayer films on the surface of the paper using chitosan (CS) and carboxymethylated nanocellulose. The obtained multilayer coating not only enhanced the paper’s resistance to grease, oil, water, air, and water vapor, but also improved the paper’s mechanical strength. The modified wrapping paper exhibited no visible cytotoxicity and had an antibacterial rate of 95.8% against E. coli and 98.9% against Staphylococcus aureus [
154].
CNC and garlic extract (GE) from garlic peel were blended with chitosan to prepare biocomposite films. UV barrier, thermal and mechanical properties, biodegradability, and antibacterial activities were tested on the films. CNC enhanced tensile strength, Young’s modulus, and elongation, compared to chitosan films, but decreased film transparency. On the other hand, the combination of CNC and GE slightly lowered the mechanical properties. The inclusion of CNC reduced the transparency of the film marginally, whereas the addition of GE dramatically increased the UV barrier properties. The integration of CNC and GE did not influence the thermal stability of the chitosan films. The chitosan composite films’ degradability rate was greater than that of neat chitosan films. The antibacterial characteristics of films were investigated against
E. coli,
Streptomyces griseorubens,
Streptomyces alboviridis, and
Staphylococcus aureus, which found that GE in composite films significantly inhibited bacterial growth. Due to the improved physical properties and better antibacterial activity, chitosan films containing both CNC and GE from garlic peel showed potential as active food packaging materials [
155].
Translucent films were made from faba bean protein isolate (FBP) using the solution casting method, reinforced with varied CNC content (1, 3, 5, and 7 wt%) prepared by acid hydrolysis of pinecones, while glycerol was used as a plasticizer. The FTIR and SEM data confirmed that intramolecular interactions between CNC and proteins could induce a more compact and uniform film. These interactions had a favorable impact on mechanical strength, as seen by greater tensile strength and Young’s modulus compared to control films, though much stiffer films were expected as the CNC content increased. The addition of CNC increased the thermal stability of the FBP films by raising the typical onset degradation temperature. Furthermore, the linkages formed between CNC and proteins reduced the water affinity of the films, resulting in a decrease in moisture content and water solubility as well as an increase in water contact angle, resulting in more hydrophobic films as the CNC content in the matrix increased [
163].
CNC modified by TEMPO oxidation (TM-CNC) was used to improve the performance of canola protein-based films. Varied weight ratios of modified (TM-CNC) and unmodified nanocrystalline cellulose (U-CNC) were tested. 19.61% of initial -OH groups were transformed to -COOH groups by TEMPO oxidation. The addition of U-NCC and TM-NCC boosted tensile strength substantially, with the greatest value of 8.36 MPa for 5% TM-NCC, compared to 3.43 ± 0.66 MPa for control films. In contrast to the control, both U-NCC and TM-NCC improved the water barrier and thermal characteristics of the films [
164].
Cellulose nanofibrils-CNF with less than 1% of lignin and lignocellulose nanofibrils-LCNF with 16% of lignin were blended in various ratios to prepare composite films. The inclusion of LCNF in the formulations increased the films’ antioxidant and UV-blocking characteristics, as well as their mechanical and barrier properties. The addition of 25% LCNF to CNF films improved mechanical properties (94% increase in tensile stress and 414% increase in tensile strain at break) while lowering the water vapor transmission rate by 16% and oxygen transmission rate by 53%. The presence of nanocelluloses with varied chemical compositions and morphologies have contributed to the improved performance. Moreover, the presence of lignin in LCNF helped to increase interfacial adhesion between CNF and avoid the formation of accessible pathways for gas molecules [
165].
5.2. Antibacterial Property
Food rich in nutrients and water is prone to microbial spoilage, thus leading to great economic loss. In order to prevent that, food packaging materials with satisfactory antibacterial properties are always necessary.
Thongsrikhem et al. used cinnamaldehyde as a crosslinking agent and an antibacterial ingredient to make a gelatin-BC nanocomposite membrane (GCB). Heat treatment at 120 °C for 3 h increased the reaction of the amine group with the aldehyde group of cinnamaldehyde via Schiff base and Michael addition, lowering the GCB film’s water solubility. The addition of BC to gelatin increased the composite film’s tensile strength and decreased its water vapor permeability. The GCB film was nontoxic to L929 cells and possessed significant antibacterial action against
E. coli and
S. aureus [
156].
He et al. developed a coating material containing CMC and CNC with immobilized AgNPs (CNC@AgNPs) in varying proportions. Compared to uncoated paper, CMC/CNC@AgNPs showed better tensile strength, lower water vapor and air permeability, and greater antibacterial activity against
E. coli and
S. aureus. Furthermore, due to the immobilization effect of AgNPs on CNC, the release rate of AgNPs from the coated paper was greatly decreased. When strawberries were packaged using CMC/CNC@AgNPs-coated paper in ambient conditions, strawberries preserved better shape than unpackaged strawberries, and the shelf life was extended to seven days [
157].
An antibacterial composite film based on sodium alginate (SA)/CNF containing peanut red skin extract (PSE) was created by crosslinking with Ca
2+. The results showed that the SA/CNF/Ca
2+/PSE (SCCP) film had high mechanical strength, great water resistance, and outstanding UV barrier properties. The films had a higher radical scavenging activity in the ABTS assay than the DPPH assay, especially in the presence of 10% ethanol; the maximal ABTS scavenging activity was 99.28%. When the film was applied to pack fruits, the weight loss of fruits was lower for the SCCP film than for the control group. Furthermore, both gram-negative and gram-positive bacteria were successfully prohibited [
158].
5.3. Intelligent Packaging
Compared to traditional packaging materials, intelligent packaging has attracted great interest in recent years, and the research mainly focuses on environmentally sensitive materials to indicate the food quality changes.
Moradi et al. developed a unique intelligent colorimetric marker by employing anthocyanins from black carrots and BC nanofibers to check the freshness of rainbow trout and carp slices. Anthocyanin-BC composite film could detect pH change in the packaging materials as storage time increases, and the indicating label changed color correspondingly (
Figure 8a). By comparing the label color to the standard color, consumers may visually determine the freshness of fish. The indicator is simple to make, inexpensive, ecologically acceptable materials, and easy to use, thus showing great potential [
136].
Shi et al. prepared an intelligent pH-sensitive membrane by combining cyanidin-3-glucoside (C3G) and BC, tested as a tilapia freshness indicator. The results revealed that when BC’s crystallinity increased, the mechanical characteristics of C3G films increased dramatically. BC-C3G films were enhanced in terms of crystallinity and transmittance. Naked eyes can clearly see the color changes of BC-C3G films throughout the freshness monitoring process (
Figure 8b), and it has a dependable color response (ΔE) and high sensitivity to TVB-N and TAC changes [
166].
Figure 8.
Application of nanocellulose indicator for food freshness monitoring: (
a) fish, reprinted with permission from [
136], 2019, Elsevier; (
b) tilapia fillets, reprinted with permission from [
167], 2022, Elsevier; (
c) chicken breast, reprinted with permission from [
135], 2020, Elsevier.
Figure 8.
Application of nanocellulose indicator for food freshness monitoring: (
a) fish, reprinted with permission from [
136], 2019, Elsevier; (
b) tilapia fillets, reprinted with permission from [
167], 2022, Elsevier; (
c) chicken breast, reprinted with permission from [
135], 2020, Elsevier.
As a colorimetric freshness indicator for detecting the freshness of chicken breast, a sugarcane bagasse nanocellulose-based hydrogel was produced. In this process, the nanocellulose was cross-linked by Zn
2+ to obtain a robust self-standing hydrogel. The pH-responsive dyes (bromothymol blue/methyl red) were incorporated into the hydrogel, which changed color depending on the freshness of the chicken sample. Since CO
2 levels rose with chicken deterioration due to microorganisms’ growth, the indicator hydrogel’s optical color changed from green to red on the third day (
Figure 8c), indicating that the bacterial counts (CFU/g) had exceeded the acceptable limit for human intake. This innovative colorimetric freshness indicator produced with a nanocellulose hydrogel responds quickly to chicken deterioration and is intended to make bagasse nanocellulose more useful as a value-added material in smart packaging [
135].
5.4. Preservation
The shelf life of food is important since the food products need a few days or even several months until they are delivered to the customers. In order to preserve the food better and extend the shelf life of the products, some kinds of effective packaging are vital.
CNF composite films containing glucose-derived carbon dots (GCD) and N-functionalized GCD (NGCD) were prepared by Ezati et al. The results showed that GCD and NGCD could effectively block the ultraviolet radiation and increase the water vapor permeability of the membrane without affecting the mechanical properties. At the same time, the composite films were resistant to oxidation, with a 99% ABTS and 80–85% DPPH free radical scavenging rate. When these films were applied to citrus or strawberry, they could effectively inhibit the growth of fungi and prolong their shelf life by 2–10 days. In addition, they reduced the weight loss caused by transportation and storage, thus ensuring the freshness of food and improving the economic benefits [
166].
A new technique, coaxial 3D printing, was used to create cellulose nanofibers (CNF)-based labels with dual functionalities of fruit freshness preservation and visual monitoring. The shell of fibers was created with CNF-based ink containing blueberry anthocyanin, which had excellent formability and printing fidelity, as well as sensitive visual pH-responsiveness for freshness monitoring. Chitosan containing 1-methylcyclopropene (1-MCP) was injected into hollow microchannels of fibers, where 1-MCP was trapped by the electrostatic effect of chitosan and CNF. The 3D printed labels extended the shelf life of litchis by 6 days while also sensibly indicating variations in freshness, as proven by Headspace-Gas Chromatography-Ion Mobility Spectrometry [
168].
6. Industrialization of Nanocellulose Production Worldwide
In recent years, the production of nanocellulose from biomass resources has become a hot topic, and intensive investigations have been carried out worldwide. Under this driving force, nanocellulose production has gradually developed from lab-scale to pilot- or even industrial-scale. In
Table 6, we summarized the representative companies and institutes producing nanocellulose and pushing the products into markets. It can be seen that the pilot and industrial production lines of nanocellulose are currently mainly located in developed countries, such as the United States, Canada, Japan, Sweden, Finland, etc. As a representative enterprise in the preparation of CNC, CellForce in Canada developed a pilot production line based on sulfuric acid hydrolysis to prepare CNC in 2012, with a production capacity of 1 ton per day. In 2012, the U.S. Forest Service started up the first nanocellulose production plant in the United States in Wisconsin, mainly based on the sulfuric acid hydrolysis method to prepare CNC (10 kg/d) and the grinding method to prepare CNF (1000 kg/d) [
169]. In 2014, the University of Maine built up a CNF pilot production line based on the mechanical refining method, with a production capacity of 1 ton per day. In April 2015, American Process realized the commercial production of nanocellulose based on the AVAP method with a production capacity of 1 ton per day, and the nanocellulose products show controllable morphology and surface hydrophilicity and hydrophobicity. It produced lignin-containing nanocellulose, which can realize the reinforcement and filling of plastics [
170]. Founded in 2016, Cellulose Lab is a Canadian company providing many different types of nanocellulose products, including CNC, CNF, and BC, and it has become one of the top suppliers of cellulose nanomaterials in the world [
171]. Founded in 2015, FineCell in Sweden has invented an oxalic acid-based technology to prepare a nanocellulose product in dry powder, making it easier to be used by mixing with plastics [
172]. In 2019, Ocean TuniCell AS in Norway successfully produced tunicate nanocellulose on a large-scale based on microfluidization combined with different pretreatment technologies, such as TEMPO-mediated oxidation, carboxymethylation, and enzymatic treatment. Right now, it has commercialized this unique animal nanocellulose by developing them in to TUNICELL 3D-bioinks [
173].
Over the last decades, the industrialization of nanocellulose showed the greatest progress in Japan. Especially after the development of TEMPO oxidation method by Professor Akira Isogai from the University of Tokyo, many Japanese companies have put a lot of effort into commercializing this functionalized nanocellulose [
174]. For example, Nippon Paper has begun to build production lines based on this method, with a designed capacity of 500 tons CNF/year [
175]. In January 2017, Oji Paper announced that they have started producing CNF with high viscosity and thixotropy based on phosphoric acid pretreatment, followed by mechanical treatment, with a production capacity of 40 tons of CNF per year. Regarding the application of nanocellulose, Oji Paper and Mitsubishi Chemical jointly launched a commercialized nanocellulose (CNF) film in 2013, which can be used to manufacture large-scale displays and solar cells. Oji Paper has also taken advantage of the dense characteristics of CNF and cooperated with Nikko Chemicals to develop its application in cosmetics. Mitsubishi Pencil used the thixotropic properties of CNF as a tackifier for ink, and successfully developed a gel ink ballpoint pen with good thixotropy. Its ink viscosity during writing was reduced by about 50% compared with traditional products, and this product was successfully released in the Japanese domestic market in May 2017 [
176]. In addition, nanocellulose also shows great potential in reinforced composite materials. For example, Professor Hiroyuki Kono from Kyoto University has developed a method to prepare a nanocellulose-reinforced resin, and its purpose is to use nanocellulose to reinforce resin materials and use them in automobiles. Since nanocellulose is a light-weight material, its addition could reduce the weight of the car and thus reduce fuel consumption, further promoting the emission reduction of carbon dioxide [
177].
The industrialization of nanocellulose also started quite early in Nordic countries such as Sweden and Finland. For example, INNVENTIA is dedicated to promoting the commercialization of CNF and introduced the nanocellulose into mobile phones in February 2011. A pilot-scale production line with a production capacity of 100 kg per day CNF was also manufactured by INNVENTIA [
178]. StoraEnso in Imatra and UPM in Lappeenranta are two major Finnish companies who are engaged in the research of microfibrillated cellulose (MFC). In addition, VTT and Aalto University have developed continuous film preparation based on CNF-based plastic materials for food packaging [
179].
Although the abovementioned companies have successfully industrialized the nanocellulose production to some extent, these production lines are still mainly based on the sulfuric acid hydrolysis method, TEMPO oxidation method, and mechanical method, and there are still many persistent issues, such as the difficult recovery of inorganic acid, large water consumption, expensive catalyst, or high energy consumption. Recent studies proposed that the AVAP process is sustainable and the chemicals can be recovered; however, due to the use of sulfur dioxide, the entire system needs very high airtightness [
180]. Therefore, green, efficient, and sustainable methods for preparing nanocellulose on an industrial scale still require further efforts by researchers.
7. Conclusions and Outlook
With the global shortage of petrochemical resources, climate warming, and environmental pollution, people pay more and more attention to how to reduce energy consumption, reasonably allocate nonrenewable resources, expand the use of renewable resources, and take the “green” road in line with the “concept of ecological civilization in the new era”. Nanocellulose and its derived nanocomposite have become a research hotspot in food packaging because of its many excellent properties, including high strength, large specific surface area, excellent barrier property, and good biocompatibility, safety, nontoxicity, and degradability. In food packaging materials, nanocellulose-based nanocomposites can be used as fresh-keeping and antibacterial packaging materials, smart packaging materials, and high-barrier packaging materials, showing the high application potential of nanocellulose-based composites. Therefore, as a kind of renewable and environmentally friendly packaging material, nanocellulose-based composites improve the safety and quality of food and are one of the important directions to realize the development of an “environment-friendly industry” in the food industry in the future.
This review describes the inter-relation of cellulose chemical structure and its source, and their different physicochemical properties are discussed. Though cellulose extracted from plants is the most investigated, the cellulose purified from bacteria and animals with unique structural characteristics has raised increasing interest. To date, enzymatic hydrolysis, TEMPO-oxidation, and carboxymethylation are widely utilized pretreatments to help defibrillation of cellulose during processing into nanocellulose, which not only helps reduce the energy consumption but also provides extra functional groups for the final products. Acid hydrolysis, including both mineral and organic acids, could remove amorphous regions, resulting in cellulose nanocrystal (CNC), though the highly corrosive conditions and the low yield of CNC are persistent issues. Through mechanical treatments, such as refining, homogenization, microfluidization, sonification, ball milling, and the aqueous counter collision (ACC) method, cellulose nanofibrils (CNF) could be produced, but the extremely high energy input prohibits the commercialization of these techniques. Bacterial cellulose (BC), a unique bacteria-derived nanocellulose, has recently gained numerous research interests. Its higher aspect ratio and larger diameter make it a promising material for food packaging applications. In order to facilitate the application of nanocellulose in food packaging, it has always been processed to different forms of materials, such as film, gel, coating membrane, and emulsions, by various fabrication technologies, including solution casting, Layer-by-Layer (LBL) assembly, extrusion, coating, gel-forming, spray drying, electrostatic spinning, adsorption, etc. Thanks to the nontoxicity, good biodegradability and biocompatibility, high aspect ratio, low thermal expansion coefficient, excellent mechanical strength, and unique optical properties, nanocellulose-based food packaging materials have been widely applied to pack fruits, meat products, instant foods, dairy products, and beverages. Since nanocellulose and the functional fillers incorporated into the cellulose-based nanocomposites impart the materials’ excellent barrier and mechanical properties, antibacterial activity, and stimuli-responsive performance, they have greatly improved the quality stability and shelf life of foods.
So as to provide sufficient nanocellulose products for green food packaging, many companies in Europe, Africa, and Asia have been pushing the lab-scale production of nanocellulose for a pilot- or even industrial-scale production. However, there are still several persistent issues that should be overcome in the near future to realize the successful commercialization of nanocellulose-based composites as food packaging materials.
Expansion of the cellulose sources to other biomass besides the traditional raw materials, such as tunicate and BC, for higher quality nanocellulose to develop many advanced applications. Until now, the nanocellulose on the market is dominantly produced from wood and other plant-based sources. Though several works of research focused on the nanocellulose preparation from tunicate and BC and demonstrated their better performance than the woody nanocellulose, further investigation on the preparation–characterization–performance correlation of this specific nanocellulose is necessary.
Development of facial, cost-effective, efficient, and environment-friendly nanocellulose extraction method. Though several novel extraction methods have been developed, sulfuric acid hydrolysis and mechanical refining are still the most widely used ones. However, the harsh acid hydrolysis, high water consumption, huge amount of polluted wastewater, intense energy consumption, and low yield greatly prohibited industrially feasible nanocellulose production. Therefore, more efforts should be put into developing new nanocellulose preparation methods, such as organic acid-based methods, which already showed potential to be a green approach to preparing functionalized nanocellulose.
Development of new cellulose nanocomposite fabrication approaches. In the lab, the solution casting method is still widely used for research purposes, which is unsuitable for industrial production. In the pilot scale, extrusion is used, though it is not a perfect method since nanocellulose is always dispersed in water, which negatively affects the extrusion performance. Therefore, developing a scalable strategy to prepare nanocellulose-based composites for food packaging materials is vital.
Improvement of the performance of nanocellulose-based composites as packaging materials. As discussed above, the ideal food packaging materials require UV-proof, gas and vapor barrier properties, excellent mechanical force, and good hydrophobicity. Especially for the last one, new strategies need to be developed to alter the hygroscopic nature of nanocellulose and enhance the wet strength, thus making its applications more practical in daily life. For example, esterification as a pretreatment or coating with natural wax seems suitable to fulfil this purpose.
Development of nanocellulose-based intelligent packaging materials. Currently, achieving the cellulose nanocomposites’ responsive properties is mainly realized by incorporating various organic and inorganic fillers. However, the release and migration of functional fillers and their potential health risks have not been comprehensively evaluated. Future studies should not only focus on the safety issue of the nanocellulose itself but also on the functional fillers used.
Design of the food-specific, nanocellulose-based packaging materials. Though many research works generally focused on the preparation and properties of packaging materials, they paid little attention to the interaction between the materials and the food, and even ignored various aspects which would influence the application of the materials. For example, the influence of the environmental conditions on the quality change of both food products and the packaging materials should be investigated to prove the feasibility and suitability of the packaging materials for the specific food.