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Cellulose-Based Hydrogel Films for Food Packaging

  • Tabli Ghosh
  • Vimal KatiyarEmail author
Living reference work entry
Part of the Polymers and Polymeric Composites: A Reference Series book series (POPOC)

Abstract

The use of fossil-based plastic in food packaging has increased the plastic-based waste, carbon footprint, and global warming, which has led to the development of alternatives such as hydrogels for biodegradable stringent food packaging industries. Hydrogels consist of biopolymers having three dimensional networks can trap a large quantity of water and formulation of cellulose-based hydrogels have laid high impact for food packaging application with improved biodegradability, biocompatibility, mechanical properties, plasticizing effect, etc. Cellulose hydrogels can be imparted as thin layers onto the polymers to improve its wettability, appearance, degradability, and resistance towards environmental agents. Cellulose-based hydrogels are mainly formulated from cellulose, bacterial cellulose, and its derivatives. Further, use of cellulose and its derivatives with gelatin, low-methoxyl pectin, polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA), polyethylene glycol (PEG), protein, etc., provide a better property for packaging food products. Various bioactive compounds such as silver nanoparticles and other antioxidants, antifungal agents can be embedded onto hydrogel films to improve its properties. Use of cellulose hydrogel as packaging material mainly depends on its hydrophilicity, swelling property, molecular weight, stability, physical, mechanical and chemical properties. Cellulose hydrogels generally consist of various chemistry of hydrogels such as physical cross-linking, chemical cross-linking, interpenetrating hydrogels, which find significant importance in biodegradable food packaging. Dry hydrogels from biopolymers can be used individually or in conjugate with others. However, use of individual polymers for making hydrogel creates problems in hydration which enhance water-polymer interactions than polymer-polymer interactions. In contrast, blending and composites of polymers help in enhancing interactions between polymer-polymer matrices than water-polymer matrices. The tailored properties of blends or composites of hydrogel can be formed through electrostatic interactions between opposite charges, formation of cross-links through covalent bond, formation of physical networks, and interpenetrating polymer networks.

Keywords

Cellulose Cellulose derivatives Hydrogel Biodegradability Food packaging 

1 Introduction

Food packaging plays a crucial role in day-to-day life of human beings as being used in preserving food products; protecting them from external environmental factors such as unfavorable gaseous conditions, microorganisms, insects; and promoting them for consumerization [42]. Further, packaging provides easy transportation of food stuffs by acting as a safe guard against deterioration caused during transportation. The most commonly used food packaging materials include paper, aluminum foil, paper board, plastic films, metal container, glass and their recycled form, edible films, coatings, and hydrogel [4, 27, 28, 37, 42, 54, 67, 69, 70]. Among various available packaging materials, polymer-based packaging materials provide some advantageous properties over others such as light weight, easy handling, water impermeability, transparent, soft, heat sealability, and others. The conventionally utilized polymers for food packaging include polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), poly vinyl acetate (PVAc), ethylene vinyl alcohol (EVOH), and others. The use of these non-biodegradable fossil-based polymers create environment pollution, which increases the carbon footprint in the world thereby diminishing petroleum-based resources (nonrenewable resources) and creating many technical problems during recycling or carbonizing [36]. In this regard, for fulfilling the demand of consumer without harming the environment, biopolymers (bioplastics and biodegradable polymers) are explored for its various applications [73]. The developed biodegradable polymers include polylactic acid (PLA), polyglycolic acid (PGA), polyhydroxyalkanoates (PHA), polyethylene succinate (PES), polycaprolactone (PCL), polypropylene adipate (PPAd), polybutylene succinate (PBS), polybutylene succinate-co-adipate (PBSA), and others, which are chemically or microbial derived polymers from various sources [5, 31, 34, 38, 58, 59, 62, 66, 71, 73]. Moreover, agro-based biopolymers including cellulose, lignocellulose, hemicellulose, chitosan, proteins, lipids, and their derivatives along with composites, and their hydrogels can be utilized for making biodegradable packaging materials [11, 12, 16, 56]. In concern with the properties of biopolymers in comparison to conventional polymers, biopolymers should deliver all required properties for preserving and protecting the stored food materials. Due to the poor properties of biopolymers in comparison with biomass-based polymers, properties and quality of biopolymers are tailored by various techniques such as blending of biopolymers with conventional polymers, development of polymer biocomposites, combination of biopolymers which is made possible by improving polymer interaction, intermolecular forces, cross-linking within the molecules, etc. [33, 61, 68].

Hydrogel-based polymers are defined as a class of polymeric materials having three dimensional network structure and being puffed up with water providing exclusive physical properties [16, 50]. The uniqueness for capturing ample amount of water is observed due to their hydrophilic nature, which helps in creating a wide application in the field of food packaging. However, the hydrogel-based food packaging has a great potential to deliver a new opportunity for developing sustainable and green packaging [22] along with the property of improving the freshness of fruits and vegetables [51]. Though utilization of all biopolymers-based hydrogel in food packaging has not been reported and used, but its beneficial properties such as biocompatibility, nontoxicity, and biodegradability make this class a noticeable entity in the field of food packaging [50]. In addition, hydrogels for food packaging can be prepared individually or with other biopolymers and conventional polymers to design an ideal type of food packaging. Figure 1 shows some of the biopolymers intended for use as a hydrogel-based packaging materials. Further, use of hydrogel provides a new prospect over other packaging forms due to their improved water capturing and mechanical properties along with acting as a proper food packaging material with higher efficacy. Hydrogels are primarily casted to films and dried for tailoring its property [35].
Fig. 1

Biopolymers intended for use as a hydrogel based polymeric material. (Note: Substitution of R groups will provide derivatives of cellulose molecules as detailed: methylcellulose (R: H, CH3), ethylcellulose (R: H, CH2CH3), carboxymethylcellulose (R: H, CH2COONa), hydroxypropylcellulose (R: H, [CH2CH(CH3)O]nH), hydroxyethylmethylcellulose (R: H, CH3, [CH2CH2O]nH))

Moreover, in consideration to contact of food materials to packages, food packaging can be classified into three categories, viz. primary packaging, secondary packaging, and tertiary packaging (Fig. 2) [13]. Primary packaging remains in contact with products, which can directly be purchased by the consumers for consuming purpose; secondary packages are used to carry the primary packages such as boxes, cartons; and tertiary food packaging which contain the large amount of goods for transportation purpose, and also carry the secondary packages. Under all the classes, secondary and tertiary food packaging can be recycled and reused, but primary food packages gets contaminated, which make them unacceptable for reuse and recycle. Noticeably, hydrogel-based packaging materials act as primary packaging materials for food products, which stay in contact with the food products.
Fig. 2

Classes of packages based on its contact with food material

Remarkably, over past few days, focus has been shifted to hydrogel-based food packaging due to their water trapping properties [75]. However, shortcomings on the usage of these hydrogels lie within their swelling and de-swelling property. Considering these limitations, there is a need of more concentrated research to evaluate the hydrogel as food packaging material. Development of polysaccharide-based hydrogels is gaining attention for food packaging. Polysaccharide is a class of carbohydrate mainly consisting of a number of monosaccharide units. Polysaccharide-based biopolymers involve starch, cellulose, chitosan, gums, chitin, and others having an enormous way to develop hydrogel-based food packaging [16, 28, 57, 63]. These kind of biopolymers are generally extracted from renewable resources such as plant materials, fruit waste, shells, waste of insects, extract of plants, microbes, etc. The film forming capability of these biopolymers provide an opportunity towards their use as a food packaging material. Moreover, the strategical techniques need to be formulated to overcome the existing shortcomings such as hydrophilicity (having a tendency to absorb water) due to more polymer-water interaction. Polysaccharide-based polymers have immense prospective for industrially viable approach in the field of stringent food packaging and beverage industry.

Cellulose is one of the superior, well utilized, and widely available polysaccharide-based biopolymers being used to formulate hydrogel for its enormous beneficial properties for acting as a food packaging material [10, 41]. The biopolymer has many beneficial properties such as biodegradability, biocompatibility, nontoxicity, easy availability, which makes it one of the focused and significant biopolymers in human life [32, 45]. The acid-alkali treatment of biomass involves soda pulping and bleaching which yields cellulose and further can be shaped in various forms to be utilized as hydrogels such as powders, films, particles, composites, achieving diverse application in various fields of life [7, 14]. The beneficial properties of Cellulose-based hydrogels provide prospects to replace fossil-based materials in the field of stringent food packaging and food-based industry as stabilizer, additives, thickeners, etc.

Based on the above discussion, this chapter mainly accounts the development and use of cellulose based hydrogels with improved properties which are suitable for food packaging materials. In the beginning, a brief introduction towards available biopolymers for making hydrogel which are intended for utilizing in food packaging materials are demonstrated. In addition, details about cellulose-based hydrogel with other polymers for improved properties has been outlined and different techniques required to characterize the hydrogel for acting as a better food packaging material are detailed. Finally, a detail case study on available cellulose-based hydrogels as food packaging materials and their biodegradation study has been outlined.

2 Biopolymer-Based Hydrogels for Food Packaging Applications

As discussed, various natural biopolymers provide suitable properties for developing dry hydrogel with tailored properties for acting as food packaging materials. Further, this dry hydrogel can be reinforced with bioactive compounds, health beneficial cells, and protectable drugs for improved properties. The bioactive compounds are well available commercially and can be easily extracted from food-based sources such as (1) spices including ginger, garlic, cardamom; (2) fruits and vegetables including apple peel, sohiong, carrot, guava, papaya, orange, etc., and many other sources. Moreover, bioactive compounds include phenolic, flavonoids, anthocyanin, carotenoids, vitamin C, minerals, and others. Incorporation of active compounds to the films can improve its property in terms of antioxidant, antimicrobial, antibacterial, antihypertensive, anticholestremia, antifungal, etc. [28].

2.1 Cellulose-Based Hydrogels

Nowadays, cellulose, its derivatives, and various forms such as cellulose nanocrystals (CNC), cellulose whiskers, and cellulose fibers are utilized to fabricate hydrogel for various applications. This specific biopolymer can be uniformly applied on the polymer surfaces as thin sheets of deposit to tailor the packaging properties. The available cellulose derivatives as represented in Fig. 1 include methyl cellulose (MC), carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC), and hydroxypropylmethyl cellulose (HPMC) [60]. The available derivatives can be combined with other polymeric materials such as gelatin, low-methoxyl pectin, polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), polyethylene glycol (PEG), protein, etc. [16, 22], to provide superior properties for packaging food products.

Though use of unmodified form of cellulose in various fields have been extensively studied throughout the world, but presence of functional groups (hydroxyl groups) in it acquires a new prospect for tailoring the properties of hydrogels through the formation of ionic, covalent, complex cross-linking networks, multiple component-based networks (hybrid cross-links), and composite-based networks (blends self-assembling networks) [10]. Further, cellulose is sensitive towards pH, temperature, ionic, electro responses, which in turn imparts a great impact on its usage. On this basis, cellulose-based hydrogels can be developed through reversible gel formations such as polyelectrolyte complexes, interpolymer matrix, hydrophobic associations, and hydrogen bonding as represented in Fig. 3 and cellulose tends to be a perfect material for preparing better water containing materials through various cross-linking networks. Cellulose and its derivatives mixed with other polymeric materials impart improved polymer-polymer interaction over water-polymer interactions to be used in food packaging and industrial applications. In this way, physical cross-linking between cellulose molecules is the primary reason for forming cellulose hydrogels. But, the difficulty in developing cellulose hydrogel is the selection of the appropriate solvent for making the solution as cellulose is not easy to dissolve because of its hydrogen bonding throughout the structural lay out. Although, some solvents such as ionic liquids (ILs), alkali aqueous system, urea (thio urea) aqueous system, N-methylmorpholine-N-oxide (NMMO), LiCl/dimethylacetamide (DMAc) system are lately used to dissolve cellulose for making hydrogels [10]. Moreover, a newer prospect has been made through the use of bacterial cellulose for developing cellulose hydrogel, taking the advantage of specific bacterial species. In addition, various design parameters may affect the property of physical and chemical hydrogel and formulation of cellulose with other materials that help in improving its properties such as better mechanical properties, biodegradability, biocompatibility, nontoxicity, plasticizing effect, etc. [63].
Fig. 3

Mechanism of formation of cellulose-based hydrogels intended for food packaging through (a) Hydrophobic interactions; (b) Polyion formation; (c) Cross-linking within molecules; and (d) Hydrogen bonding within the molecules

2.2 Starch-Based Hydrogels

Starch is a class of available renewable polysaccharide, consisting of number of α-glucose unit and can be extracted from cereals, tubers, corns, root, waxy potatoes, and others [18, 26, 43, 44, 46]. Further, amylose and amylopectin are the two unit of starch molecule, where amylose contains α (1 → 4) glucose unit and amylopectin consists of α (1 → 4) glucose unit having a branch unit of α (1 → 6) glucose unit [55]. Starch has a potential to act as a film former, which is mainly imparted by amylose molecule. Further, starch-based films are biocompatible, nontoxic, flexible, and oxygen impermeable, which make them a great candidate for acting as a film former in the field of food packaging [16]. On the contrary, this specific biopolymer lacks in efficient mechanical properties, which can be tailored by adding other biopolymers or synthetic polymers without compromising its biodegradability. In addition, modifications in terms of chemical and surface or blending with other polymers or plasticizing materials makes them appropriate for acting as film former materials. However, starch-based hydrogel has not been used yet for using as food packaging. Though the development of starch-based hydrogel with other synthetic polymers intended to put an enormous impact in the field of food packaging industry.

2.3 Agar-Based Hydrogels

Agar is a type of polysaccharide, generally obtained from agarose and agaropectin, where agarose contains upto 70% of the mixture [20]. The component agarose consists of units of agarobiose and agaropectin, which is carrying D-galactose and L-galactose unit, respectively. This component is mainly obtained from algae having a jelly-like texture. Agar is generally used as an alternative for gelatin, as a thickener, and a clarifying agent. Moreover, agar can be well explored to be used as a hydrogel in combination with active materials and other biopolymers, which has a great impact in the field of food packaging industry [28, 61]. The increasing urge for fresh food products with improved shelf life has inspired the researchers to derive new techniques to develop food packaging material. In this regard, hydrogel made up of agar with reinforced silver nanoparticle has a proficiency in extending the shelf life of fior di latte cheese, where fior di latte is a kind of fresh and semi-soft cheese which is generally produced in the style of Italian mozzarella, having elastic texture and pale yellow color and the silver nanoparticles performed as the active agent under in vitro condition [8]. The food products should keep proper color, odor, consistency, sensory, and all properties during the storage period. Further, silver nanoparticels are a remarkable candidate for formulation of active packaging, having limiting dose of silver ions of 0.05 mg of Ag/kg in food products [28].

2.4 Chitosan-Based Hydrogels

Chitosan is recognized as the widely available polysaccharide unit having property of biodegradability, biocompatibility, and nontoxicity and can be well utilized for developing hydrogels [30, 64, 72]. In addition, film formation is extensively acceptable by consumers because of antimicrobial, antihypertensive, antibacterial property of chitosan. This biopolymer can be extracted by deacetylation of chitin, and is available from crabs, shells, etc. [74]. The extracted chitosan can be well tailored in various forms to be utilized in various spheres of human life. Chitosan with immense properties provide an option to alter the use of fossil-based materials, which in turn reduces polymer-based waste.

The unmodified form of chitosan provides wide application, but presence of a number of functional groups in chitosan such as primary amine, primary and secondary hydroxyl group make a breakthrough for tailoring its property according to the need of forming ionic, covalent, complex cross-linking, multiple component-based networks (hybrid cross-links), composite-based networks (blends self-assembling networks) [76]. Chitosan is sensitive towards pH, temperature, ionic, electro responses, which impart a great impact on its usage. Further, chitosan-based hydrogels can be developed through reversible gel formations such as polyelectrolyte complexes, interpolymer matrix, hydrophobic associations, etc. In this way, chitosan is a perfect material for preparing better water-containing materials through various cross-linking networks. Chitosan can be mixed with other polymers with improved polymer-polymer interaction than water-polymer interactions to be used in food packaging and industrial applications. Further, in tissue engineering, chitosan-based hydrogels due to their biocompatibility material provide a wide array for use in the preparation of bone and scaffolds materials. Various design parameters may affect the property of physical and chemical hydrogel and formulation of chitosan with other materials that help in improving its properties in terms of better mechanical properties, biodegradability, biocompatibility, nontoxicity, plasticizing effect, etc. However, chitosan-based hydrogel in the field of food packaging applications are still underutilized, but they promise a wide potential for developing hydrogels in specified field.

2.5 Protein-Based Hydrogels

Proteins are being classified as one of the group of macromolecules generally consisting of long chains of amino acids linked together through peptide bonds [65]. Protein contains mainly amino and carboxyl groups; however, they also contain sulfur and phenyl group [65]. Proteins can be classified according to the molecule present in it such as simple protein consisting of amino acids only, conjugated protein consisting of non-peptide compounds and derived protein which is formulated through the action of heat, chemical actions, and enzymes. The examples of protein include elastin, collagen, hemoglobin, etc. [19]. Biopolymers of protein can be developed through condensation reaction, metal catalysis, enzyme action, optimized environmental conditions, etc. In the past few decades, the utilized protein sources for producing biopolymers include casein, whey protein, whey protein isolates, soy protein, gluten, gliadin, pea protein, egg protein, etc. Besides this, various other kinds of vegetable and animal proteins are extensively utilized for the development of biopolymers. The development of hydrogel primarily depends on its structure, hydrophobic nature, types of bond involved, etc. Further, polysaccharide-based hydrogels with plasticizers are used for protein release. Considerably, protein biopolymers with unique properties can be cross-linked with other natural resources to be utilized in the field of food packaging application. Development of polyion-complex hydrogel films from gelatin and pectin component provide tailored properties with better mechanical and water resistance properties [16].

Combinations of biopolymers for forming hydrogels can be classified according to the selected polymers, bonds formations, methods of synthesis, as represented in Fig. 1. Cellulose-based hydrogels can be considered as a homopolymer, if only one kind of cellulose units has been taken for hydrogel development. Accordingly, they can be named as co-polymers, when the involvement of different polymer groups exists. Further, the formation of bond between molecules varies and can be reversible and irreversible depending upon the interaction type. If the cellulose molecules with selected molecules are linked through forming covalent bonds (chemical hydrogels), the resultant hydrogel will be irreversible and less prone to water absorption in various environment conditions. Synthesis of cellulose-based hydrogels through chemical hydrogel is preferable to store food products, where less water should penetrate through the packaging materials and can get a stable swelling stage depending on the interactions. In adverse, physical hydrogels are considered as reversible and unstable, which is less preferable to cellulose-based hydrogels due to weak bonding between polymer molecules such as hydrogen bonding, electrostatic interactions, ionic interactions, hydrophobic interactions, etc. Further, this type of hydrogel provides less resistance towards changing environment, which will be a harmful factor towards storage of food products within this hydrogel. Additionally, the factors such as nature of side groups of other polymeric materials, chemical, mechanical, and physical response towards relative humidity and temperatures should be considered before preparing cellulose-based hydrogel for the purpose of food packaging. Perishable food products are very prone to environmental conditions such as temperature, gaseous conditions inside the package which in turn depends on the oxygen transfer rate and water vapor transfer rate of the packaging materials, water uptake capacity, etc.

Further, the food products according to their shelf life can be classified as (1) perishable food products; (2) semi-perishable food products, and (3) shelf-stable food products. Among these, perishable food products have very less shelf life (3–4 days) and get spoiled easily in unfavorable environmental conditions. Example of perishable food products include milk and milk products, meat and meat products, fish products, poultry products, which should be kept in favorable environmental conditions for improved shelf life. The semi-perishable food has better shelf life than perishable food products and get spoiled only if handled carelessly and do not require refrigeration. This category of food products includes potatoes, onions, salamis, pastries, etc. On the other hand, shelf-stable food products get spoiled over a long storage time and required no such storage conditions to improve shelf life, as this kind of products are generally pretreated earlier in such a way that possess longer shelf life such as rice flour, wheat flour, canned foods (dipped in sugar or salt solution), dried food products (dry fruits, nuts, pasta, etc.) etc. As mentioned, these products are prepared through the application of various unit operations such as drying, canning, etc. In concern with these categories of food products, cellulose-based hydrogels for food packaging application can be prepared with tailored properties according to the need. Moreover, chemical modifications of cellulose could provide a better way to improve the packaging property through grafting, cross-linking, blending, curing of molecules within the cellulose and other selected biopolymer molecules. Further, grafting can be initiated through initiator such as dicumyl peroxide, irradiation techniques, ionic grafting, etc.

3 Properties of Cellulose-Based Hydrogel for Food Packaging Application

The properties of hydrogels for food packaging application mainly depend on the formation of electrostatic interactions, covalent bonds through cross-linkers, network formation through interpenetrating polymer networks. Chemical composition of nonionic-based hydrogels is a principle factor to be considered for determining swelling. The parameters of hydrogels can be well-studied through various techniques such as film thickness, hygroscopicity, color, and transparency. The swelling properties of ionic-based hydrogels are mainly dependent on the pH of the components. Some of the considerable properties of hydrogel films are detailed below which include swelling mechanism, chemical properties, wettability, barrier properties, etc.

3.1 Swelling Property

The swelling property of dry hydrogels lies within its hygroscopic nature, which provides limitation to its use as food packaging materials. Swelling of matrix will start after the dried hydrogel starts to captivate water molecules from environment. As a result, the process of combining water to the polar ends begins and swelling of the dried hydrogels starts, which is considered to be a disadvantage of using hydrogel as food packaging materials. Physical state of water differs within the hydrogel due to swelling, which can be well studied by differential scanning calorimetry [23]. In polysaccharide-based hydrogels, the molecules of water categorized on the basis of non-freezing, freezing, free, and bound water and phase transition phenomena. Glass transition state of water, crystallization, and melting property of water present in hydrogel directly effects the water physical state within the hydrogel [75]. Additionally, due to the driving forces between hydrogel networks, the layers will absorb more water towards many dilutions, where the absorbed water within this region fills all the large pores spaces, network spaces, and middle of macropores. Thus, the absorbed water may degrade the hydrogel films by disintegrating it into various pieces, where the degree of disintegration truly depends on the chemical property and composition of the films. The covalent bond due to chemical interactions can be a barrier to this process of swelling water within the pores and centers and networks. As a solution to this problem, combination of biopolymers can be utilized to develop hydrogels for proper food packaging materials with tailored properties, where polymer-polymer interactions could be the dominating factors over water-polymers interactions. The principle reason behind this improved polymer-polymer interaction between the molecules is the various structures and chemical composition molecules, which can link with themselves by forming bonds (such as covalent bonding), resulting in neglecting water and polymer interactions within the molecules. The amount of moisture retained (Wr) and equilibrium swelling ratio (eqsw) can be calculated using stated Eqs. 1 and 2 [17, 49].
$$ {\mathrm{W}}_{\mathrm{r}}=\frac{{\mathrm{W}}_{\mathrm{s}}-{\mathrm{W}}_{\mathrm{d}}}{{\mathrm{W}}_{\mathrm{s}}}\times 100 $$
(1)
$$ {\mathrm{eq}}_{\mathrm{s}\mathrm{w}}=\frac{{\mathrm{W}}_{\mathrm{s}}}{{\mathrm{W}}_{\mathrm{d}}} $$
(2)
where Ws and Wd represent weight of swollen hydrogel and dry hydrogels, respectively.

3.2 Chemical Properties

The chemical properties of the selected biopolymers are very crucial to be considered for having a remarkable role in the field of biodegradable hydrogel for acting as a food packaging material. Depending on the chemical properties, the selected polymers will link through chemical- and physical-based hydrogel. Further, if more than one biopolymer is present than more polymer-polymer interactions will be dominating one over polymer-water interactions. The study of chemical compositions can be well studied by Fourier transform infrared spectroscopy, attenuated total reflectance in case of films [39, 40, 63], and NMR study can provide in depth study on interactions of polymer in hydrogel [9]. Further, by studying the functional group available in the individual and mixed polymer compositions, grafting mechanism and cross-linking mechanism along with information about the formed functional groups will be known. Polymer grafting is a mechanism of linking two polymers, where the functional group of one polymer will attach to the backbone of other polymer forming a new functional group, which will change the functionality of the formed groups. As discussed, grafting can be of various kinds as follows (1) free radical grafting, where an initiator will start reaction by generating free radical molecules; (2) grafting through living polymerization, where living polymers themselves have the ability to react for a long time ignoring the degree of reaction termination is negligible; (3) ionic grafting which proposed ionic line (Cationic and anionic mechanism) for grafting; (4) grafting by irradiation technique, where polymer irradiation approach are followed to generate free radicals, and (5) photochemical grafting, where grafting of polymer molecules occur due to the presence of chromophore in polymer molecules, which absorbs energy and shifted to higher energy orbital and others.

3.3 Wettability

The wettability of the hydrogel films can be measured by using contact angle analyzer, where the microdrops of liquid are poured onto the films. The resultant contact angle is captured by a digital camera which further determines the surface property and energy. The wettability of materials mainly depends on the hydrophilic and hydrophobic property of the materials, which in turn depends on the surface attraction of the films towards selected liquids determining surface tension. The higher attraction of specimen towards the selected liquid defines the hydrophilicity of the specimen, where the specimen is noted as hydrophilic material possessing low contact angle. On the other hand, hydrophobic material possesses no attraction towards liquids and have very high contact angle. The contact angle or wettability of selected specimens is measured using the Eq. 3, where ɵ represents the contact angle of the polymeric material.
$$ {\upgamma}_{\mathrm{sv}}={\upgamma}_{\mathrm{sl}}+{\upgamma}_{\mathrm{lv}}\ \mathrm{cos}\uptheta $$
(3)
where ɣsv, ɣsl, and ɣlv are the surface tension of solid vapor, solid-liquid, and liquid-vapor interfaces, respectively.

3.4 Color Determinations

High consumerization of any packaging material mainly depends on the customer acceptance and the customer will firstly observe the color of packages, whether the stored food items are visible or not from outside. So in this sense, the color of food packaging materials plays a critical role in the field of global marketing of the polymers. The advantage of transparent packaging materials is easily convenient to the customer for observing the conditions of food products whether it is safe or on the verge of spoilage. For this reason, the color parameters (L*, a* and b*) and color kinetics of packaging material are studied to know about the effect of environmental conditions such as temperature, gaseous conditions, on packaging materials. The color parameters are generally measured by Hunter color lab, portable colorimeter, spectrophotometer, and others. Each color parameter has some significance over other parameter. In addition to L, a*, b* values, other parameters such as hue, chroma have some specified norms in the color measurement phenomena [3]. Theoretically, the factor L* describes about the variation of darkness (L* = 0 indicates perfect black) to lightness (L* = 100 indicates perfect white); a* specifies degree of redness (+a) and greenness (−a); and b specifies the degree of yellowness (+b) to blueness (−b). From L, a*, b* value hue angle (H°), chroma (C*) and total color differences (ΔE) can be determined using the detailed Eqs. 4, 5, and 6, where hue angle specifies the most dominating color and chroma specifies about the saturation of coloring material present. Further, hue angle of 0°, 90°, 120°, and 240° specifies the coloring effects of red, yellow, green, and blue, respectively. So in this regard, color determination of hydrogels provides the information regarding the consumerization of hydrogels as packaging material.
$$ \mathrm{Hue}\ \mathrm{angle}={\tan}^{-1}\left(\mathrm{b}/\mathrm{a}\right) $$
(4)
$$ \mathrm{Chroma}=\surd \left({\mathrm{a}}^2+{\mathrm{b}}^2\right) $$
(5)
$$ \Delta E=\sqrt{{\left(\Delta L\right)}^2+{\left(\Delta a\ast \right)}^2+\left(\Delta b\right)2} $$
(6)

3.5 Water Vapor Uptake Ratio, Water Vapor Adsorption Kinetics and Water Vapor Adsorption Isotherm

The water barrier properties of hydrogel can be understood by understanding the water vapor permeability (WVP), water vapor uptake ratio, water vapor adsorption kinetics, and water vapor adsorption kinetics [61].The barrier properties of hydrogel films play a crucial role for acting as a food packaging material. The water vapor transmission rate can be measured using gravimetric method following ASTM E-96-95 [61]. According to reported study, WVP can be determined under 50% RH at 25 °C using following Eq. 7.
$$ \mathrm{WVP}=\frac{\mathrm{WVTR}\times \mathrm{y}}{\Delta \mathrm{p}} $$
(7)
where y, Δp, and WVTR represent the mean film thickness, partial water vapor pressure difference through two sides of the hydrogel films, and water vapor transmission rate.
The water vapor uptake ratio (WVUR) is another critical parameter to be considered for proper storage of food products as they are greatly influenced by the presence of water vapor, which can be determined by using Eq. 8. Further, the presence of inadequate amount of water vapor may create detrimental effect to the food products. The water vapor absorption study can be efficiently carried out by using saturated solution of KNO3 under constant temperature of 25 °C for 24 h.
$$ \mathrm{WVUR}=\frac{{\mathrm{W}}_{\mathrm{a}}-{\mathrm{W}}_{\mathrm{b}}}{{\mathrm{W}}_{\mathrm{b}}}\times 100 $$
(8)
where Wa, Wb denote weight of hydrogel films after and before dipping, respectively.

Moreover, a method for determining water vapor adsorption of hydrogel films include dispersion of specific amount of hydrogel film in saturated KNO3 solution. KNO3 solution should be kept in Fido airtight glass jar having a silicon gasket and clamp lid, which is generally utilized to obtain constant relative humidity. In this study, water vapor absorption kinetics can be studied by a plot between weight increment against storage time, where equilibrium water content and rate constant of water vapor adsorption are acquired.

In the year 2013, Rhim has followed static gravimetric process to obtain water vapor adsorption isotherms of hydrogel-based films [61]. In the study, hydrogel-based film samples are kept at different saturated salt solutions at static water activity, and equilibrium moisture content was taken after 20 days. Further, use of GAB model (Eq. 9) is sufficient to fit the data of hydrogel film samples. Moreover, parameters of GAB model can be well obtained by Marquardt-Levenberg algorithm utilizing Solver function of Excel®.
$$ \mathrm{W}=\frac{{\mathrm{W}}_{\mathrm{O}}\mathrm{Ck}\ {\mathrm{a}}_{\mathrm{w}}}{\left(1-\mathrm{k}{\mathrm{a}}_{\mathrm{w}}\right)\, \left(1-\mathrm{k}{\mathrm{a}}_{\mathrm{w}}+\mathrm{Ck}{\mathrm{a}}_{\mathrm{w}}\right)} $$
(9)
where k is a parameter equivalent properties of multilayer molecules in regards with bulk liquid, C is the Guggenheim constant, aw water activity, w, w0 denote the equilibrium moisture content at aw in dry weight basis and moisture content of mono-molecular layer on the internal surface of hydrogel films.

3.6 Mechanical Properties

The mechanical properties of the food packaging material are important parameter to be considered for having a great application in the field of packaging materials. The mechanical properties of films mainly depend on the cross-linking and intermolecular bonds between polymer molecules which mainly include tensile strength, elongation at break, elasticity of modulus, and can be measured following the ASTM standard D882–88 using Universal Testing Machine.

3.7 Antibacterial and Antimicrobial Activity

Antimicrobial and antibacterial activity of hydrogel relates to the presence of active compounds in hydrogels, which cause no harmful effect to the food constituents present inside the package. Further, both the activities can be tested against microbes.

3.8 Packaging Test

The usefulness of packaging materials is determined by storing an amount of perishable food products inside packaging materials. The storage of food products should be done at predetermined storage conditions, which will enhance the product quality. Low temperature and low gas concentrations are preferable for long-term storage of fruits and vegetable products. With increase in temperature, the molecular randomness increases which results in increasing respiration rate of food products. Moreover, anti-fogging effect of packaging materials is another factor concerned with improved shelf life of food products.

4 Application of Cellulose–Based Hydrogel in the Field of Food Packaging

Polyvinylpyrrolidone (PVP) and carboxymethyl cellulose (CMC)-based hydrogel for food packaging PVP is a synthetic, water soluble polymer unit consisting of N-vinylpyrrolidone (Fig. 4) having a wide application in the field of medical, food packaging, food industry as emulsifiers, thickeners, food stabilizer, and as membrane material, which is further used in making daily used products such as toothpastes, hair gels, hair sprays, shampoos, etc. [6, 15, 21, 24, 25]. Remarkably, in food engineering and technology, PVP is greatly used for preserving food flavors, retains food quality, and acts as refining agent in beer and wine industry and others [3, 52, 77]. Further, the hydrophilic nature of PVP is discovered in the year of 1939, so having water solubility and providing an ability towards film forming property can be used for making dry and wet films with tunable mechanical properties.
Fig. 4

Chemical structure of PVP

CMC is a cellulose derivative which is biodegradable, hydrophilic, and is used in wide variety of application including food packaging, biomedical, textile industry, and others. CMC is a kind of ether derivative of cellulose, where H atoms of hydroxyl groups of cellulose are substituted by carboxymethyl unit [8]. It is predicted that the world market of using CMC may spread up to 892 million pounds by 2017. The utilization of CMC in the field of food packaging with other available biopolymers such as gelatin, chitosan, lipids, glycerol, starch are widely observed for making edible food packaging in the form of films and coatings [1, 47, 48, 53]. In addition, the mixing of other biopolymers with CMC aims to improve the properties in terms of improving its physical property such as mechanical properties, thermal stability, barrier properties (oxygen transmission rate and water vapor transmission rate), which helps in improving shelf life of perishable food products. Considerably, for improving barrier properties of films, both solubility and diffusivity should be improved with the effect of added material to reduce permeability of oxygen and water vapor.

The combined use of PVP and CMC in developing hydrogel has an important role in the field of food and beverage industry. In this regard, PVP-CMC-based hydrogel films has many significant properties essential for packaging application including flexibility, transparency, water-retaining capability, and breathable film, which retain freshness of the food products for long duration. Combined effect of different factors such as temperature range, relative humidity, and storage period may significantly affect the properties of PVP-CMC-based hydrogel, which are considered crucial factors for food packaging. A study on developing PVP-CMC-based hydrogels using solution casting method were reported, where individual polymer solutions were prepared in the controlled environment of moist heat exploration [63]. The development of PVP-CMC-based hydrogel is carried by following steps: (1) Preparation of individual polymer solution, (2) treatment of the polymer solution at 120 °C under 15 lbs. pressure for a period of 20 min, and (3) finally, solution casting method is followed to obtain the targeted PVP-CMC-based hydrogel. Interestingly, agar, glycerine, and poly ethylene glycol were added to the mixture for formulating the hydrogel, which provide PVP-CMC-hydrogel with a thickness of 0.09–0.1 mm. The functional groups in the hydrogel can be detected using FTIR spectroscopy in line with attenuated total reflectance (wavelength range: 4000–400 cm−1). Further, for acting as a food packaging materials, combination of materials should improve the mechanical properties, under dry atmosphere, where the PVP-CMC-based hydrogel attain 45.6 ± 1.5, 45.5 ± 1.9, and 42.2 ± 4.3 MPa of tensile strength at 25 °C, 35 °C, and 45 °C, respectively, in comparison to CMC which has 43.7 ± 3.3, 42.1 ± 7.3, 45.7 ± 3.9 MPa of tensile strength at 25 °C, 35 °C, and 45 °C, respectively. So the combination of materials has been found to improve the mechanical properties. Further, E-modulus also improved in comparison to CMC when PVP were mixed to prepare the hydrogel. The E-modulus of PVP-CMC was 2183 ± 183, 2213 ± 83, 1957 ± 55 MPa at 25 °C, 35 °C, and 45 °C, respectively. On the other hand, CMC has 1722 ± 158, 1816 ± 287, and 1977 ± 380 MPa of E-modulus at 25 °C, 35 °C, and 45 °C, respectively. The development of PVP-CMC-based hydrogel has been explored for stringent food packaging, providing better mechanical and hydrothermal effects in comparison to neat CMC hydrogels. The addition of PVP in CMC provide an inhibitory action to the plasticizing effect of moisture till 50% RH, and this combination of hydrogel provide higher stability under hydrothermal treatment. The swelling and de-swelling effect of PVP, CMC, PVP-CMC hydrogel-based films provide a clear image about the cross-linking, ionic strength, physical cross-linking within the polymer molecules. At pH 6.5, the water absorption capability of CMC and PVP-CMC remain similar, whereas PVP shows a different kind of behavior by absorbing less amount of water in comparison to CMC and PVP-CMC. The water absorbing capability of the hydrogels remains maximum in the first 15 min, which is 12.05 ± 0.33 g/g, 4.20 ± 0.16, and 10.82 ± 0.50 g/g for CMC, PVP, and PVP-CMC hydrogels, respectively. Further, with increase in time the water absorption capacity uniformly increased and reached an equilibrium state. Both CMC and PVP-CMC hydrogel films provide good water holding ability upto 120 min. The water holding capability of PVP, CMC, and PVP-CMC at around 30 min are 76.47 ± 0.97, 84.99 ± 0.38, and 83.84 ± 1.0%, respectively. The storage modulus of PVP-CMC hydrogels is found to improve in comparison to CMC hydrogel. In adverse, with increase in %RH, the storage modulus decreases for both the hydrogels, i.e., CMC and CMC-PVP. The storage modulus of CMC is 2940, 2580, 1060, 90, 40 MPa at %RH of 0, 10, 30, 50, and 70, respectively. For PVP-CMC, the storage modulus is 3260, 2990, 2020, 180, 70 MPa at 0, 10, 30, 50, and 70%RH, respectively. It is reported that, the PVP-CMC hydrogel provide lower creep compliance than CMC during creep and higher strain recovery. Further, formation of physical cross-linking between PVP-CMC hydrogel can be obtained with increase in temperature and the differentiation between the surface morphology, structure, and topography, internal structure of hydrogels can be well studied by using SEM and AFM. So in conclusion, blending of biopolymers with synthetic polymers with enhanced polymer-polymer interactions than water-polymer interaction could provide better materials for food packaging. The tailored properties of food packaging materials as hydrogels can be obtained through irreversible bond formations through covalent chemical bonding, intramolecular and intermolecular bonds, physical bonding through distinct polymer networks.

5 Biodegradability of Cellulose–Based Hydrogels for Food Packaging Application

Biodegradability of bio-based hydrogels provides an ideal breakthrough for the design and progress of stringent food packaging material, where they possibly make an effort towards the environment friendly packaging material [2, 29]. Involvement of microorganism to the biodegradation process of films is considered as a primary degradation approach. Under controlled atmosphere, the biodegradability of biopolymer-based hydrogels are promising, where change in the property of films with storage period provide the mechanism of degradability. Considerably, the most prominent parameters affecting the biodegradation process of films are chemical functional property, wettability (hydrophobic or hydrophilic), swelling and de-swelling nature, molecular weight, morphology, etc. [63]. In addition, deviation of films under observation for biodegradability can be well studied by observing the functional group degradation, wettability, surface morphology, mechanical properties, molecular weight, thermal stability, etc. Evaluation of CO2 is another significant factor for observing the degradation process. In this regard, biodegradability, biocompatibility, and nontoxicity nature of cellulose-based hydrogel is an essential property that needs to be considered for using as a food packaging material. In addition to biodegradability of packaging material, the hydrogels should provide protective and preservative action to the food substances. Biodegradation study of hydrogels can be done using Czapek-Dox liquid medium having constituents of glucose, dipotassium phosphate, magnesium sulfate, sodium nitrate, ferrous sulfate, and soil extract, which are used as inoculums for composting and the biodegradation of CMC, PVP, agar, glycerin, and PEG can be easily done by aqueous solution [63]. The stability of chemical properties of hydrogels within the period of biodegradation study can be studied through using FTIR. The immense alternation of chemical composition makes changes in the nature of peaks, which defines the molecular interactions within the biopolymers. Further, the inoculated microorganism in liquid medium increases with time, which will put a positive impact to the biodegradation method by metabolizing products. The surface morphology provides an evidence supporting the biodegradation study through deposition of microorganism or insoluble products produced by microorganisms. PVP-CMC-based hydrogel provides durable storage modulus defining elastic properties within 2 weeks of biodegradation, which directs about carrying elastic properties of the hydrogel within 2 weeks of storage [63]. Noticeably, storage modulus of PVP-CMC hydrogel decreases after 6 weeks of biodegradation study, and after 6 weeks of storage, the elastic property decreases prominently tending towards extending viscosity. PVP-CMC hydrogels provides better elastic property than viscous property. Weight loss is a considerable parameter for defining the biodegradation study of hydrogels. The % weight loss of PVP-CMC hydrogel obtains up to 10% and 38% for 2 and 8 weeks, respectively. The change in mechanical properties of dry hydrogel of PVP: CMC in the ratio of 20:80 in terms of E modulus during the period of biodegradation are ~ 1423.33, 1322.77, 1303.76, 1394.41, 1502.37 MPa for 0, 1, 3,5,7 weeks, respectively [63]. In addition, the tensile strength of specified hydrogel is ~20.93, 26.56, 25.56, 26.67, 30.62 MPa for 0, 1, 3, 5,7 weeks, respectively. In this regards, the hydrogel based on PVP, CMC, PEG, glycerin, and agar are considered as nontoxic and biodegradable films providing immense property to provide fresh fruits and vegetables an apposite storage for improved shelf life [63]. Similar to the PVP-, CMC-, PEG-, glycerin-, and agar-based hydrogel, other cellulose-based hydrogels can be formulated based on biopolymers cellulose derivatives, gelatin, starch, chitosan, protein isolates, pectin, and synthetic polymers poly (vinyl alcohol) with improved properties for acting as biodegradable hydrogel for food packaging applications.

6 Conclusion

Cellulose is extracted from bio-based renewable resources, which is widely available in nature. Cellulose-based hydrogels are biocompatible, eco-friendly, nontoxic material having immense capability to keep food products fresh and lively. This chapter highlighted the recently formulated cellulose-based hydrogels as a substitute to food packaging material. Moreover, combination of other polymer materials to the cellulose is found to impart tailoring properties to the hydrogel for acting as a food packaging material. The cellulose-based hydrogel have the ability to store fresh fruits and vegetables which release moisture as hydrogel has the ability to trap moisture. So considering this, an enormous research must be carried out to develop hydrogels based on the cellulose and its derivative for providing an alternative to the food packaging application.

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Copyright information

© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Department of Chemical EngineeringIndian Institute of Technology GuwahatiGuwahatiIndia

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