3.1. Oxygen barrier properties
Lipid oxidation is responsible for most of the deterioration of foodstuffs (Lu et al. 2018). Also, the presence of oxygen in the food environment increases the growth rate of bacteria and unwanted harmful reactions that accelerate the spoilage of food. Hence, besides protecting food from physical damage and contact with dust and other pollutants, food packaging should protect food from deterioration reactions by preventing the oxygen transfer between the interior part of the packaging and surrounding atmosphere (Arrieta et al. 2017). In terms of oxygen barrier properties, based on a rough classification suggested by Wang et al. (2018), polymeric packaging materials can be classified into five categories including poor, low, medium, high, and very high. The most common non-biodegradable (PP, PE, and PS) and biodegradable (PLA and PHA) polymers have poor oxygen barrier properties that hardly meet the requirements for packaging foodstuffs like fresh meat, cheese, and instant coffee. Although some biodegradable polymers like PVA have high oxygen barrier properties (due to their hydrogen bonding) in dry conditions, their high water absorption capacity leads to a significant drop of oxygen barrier properties in moist conditions, which restricts their use for most food applications (Wang et al. 2018). High oxygen barrier cellulose nanomaterials (Fig. 1) have been used to make composite films using impermeable commercial polymers like PVC and PVA, and have also been used as coatings for low oxygen barrier films (Fig. 2) (Hubbe et al. 2017; Vilarinho et al. 2018; Wang et al. 2018). However, when used as a thin layer, the high hydrophilicity of cellulose nanomaterials restricts their use to dry foods and dry conditions. Due to the abundance of hydroxyl groups on the surface of cellulose nanomaterials, their homogeneous dispersion in the polymer matrix, which is crucial for effective oxygen barrier action, is a challenge. Low compatibility with packaging materials results in low interfacial adhesion between polymer and filler and creates cavities for the diffusion of oxygen molecules. Also, the adsorption of water results in the loss of the oxygen barrier ability of cellulose fibers. Hence, for taking full advantage of these fibers as fillers by improving the oxygen barrier properties of polymers, the following issues must be addressed: i) their high tendency to agglomerate (due to their surface hydroxyl groups), ii) low compatibility with commercial packaging materials, and iii) high hydrophilicity and water absorption capacity (Dufresne 2017; Kargarzadeh et al. 2018; Liu et al. 2019; Mohit & Arul Mozhi Selvan 2018).
3.2. Water vapor permeability and water uptake
The transfer of water vapor between food and the surrounding environment plays a crucial role in the shelf life of food products. In the case of fresh foods like vegetables and meat, it is important to avoid dehydration of the product, whereas dry foods like bread must be protected from the humidity of the air (Wang et al. 2018). Water vapor permeability (WVP) refers to the amount of water that transfers across the film per unit of area and time. As the thickness of the film and the water vapor pressure difference across the film are important factors in the amount of transmitted water, the water vapor flux must be normalized for these two variables. Hence, WVP can be expressed as
where WVTR is water vapor transmission rate (g/m2·day), x is the thickness of the film (m) and ΔP is the differential vapor pressure of water across the film (Pa).
The improvement of barrier properties associated with the use of cellulose fibers as a filler is ascribed to the decrease of the diffusion of gases by creating a more tortuous path across the film (Ferrer et al. 2017; Hubbe et al. 2017; Wang et al. 2018). The use of cellulosic fibers in various matrices from water-soluble polymers like PVA (Pereira et al. 2014) to hydrophobic commercial polymers like PLA (Espino-Pérez et al. 2018) has been reported to decrease the WVP. However, the water sensitivity of films, which refers to the resistance of the film against water vapor, preserving its structure and avoiding swelling, can be affected by the presence of cellulose fillers. For most of the commercial hydrophobic polymers, water uptake can be a reliable test to evaluate the resistance of films against water. In terms of food freshness and shelf life, water vapor permeability is an important characteristic, especially for the packaging of wet food products like meat and vegetables (Azeredo et al. 2017). Although cellulose is not a water-soluble polymer and most of its hydroxyl groups are involved in intra- and inter-molecular interactions in the crystalline regions, the surface hydroxyl groups, and those groups in the amorphous region are able to hold a large amount of water. As the volume of cellulose or plant fibers increases as water is absorbed, the shape of the produced composites may change. Also, evaporation of adsorbed water at high temperatures or dry weather leads to the shrinkage of the reinforcing material and detachment between the filler and host polymer, resulting in a loss of mechanical strength. Furthermore, adsorbed water molecules can loosen the inter-molecular interactions between cellulose chains by disassociating the hydrogen bonding among them. Hence, the adsorption of water in a humid environment results in the plasticization of composites containing cellulose fibers and leads to a significant increase in oxygen permeability (Wang et al. 2020).
Thus, to preserve the oxygen and water vapor barrier properties as well as the dimensional stability and mechanical strength of food packaging, especially in a humid environment, and to avoid increasing the weight of the composite, modification of cellulose fibers should be carried out. Improvements in the interfacial interaction between the host polymer and fiber and decreased hydrophilicity of the incorporated fibers can enhance the barrier properties (Espino-Pérez et al. 2018) and decrease the swelling and water uptake (Chuayplod & Aht-Ong 2018). Non-bonding modification of fibers with a hydrophobic material to shield the fiber, or reaction of surface hydroxyl groups with a coupling agent to reduce the accessible hydroxyl groups for adsorbing moisture, are approaches to overcome this problem. If a chemical bond between the filler and polymer can be formed through a coupling agent, the oxygen and water vapor barrier and mechanical properties of the prepared composite can be improved in both dry and wet conditions (Mishra et al. 2001). The results of modifications carried out to reduce the water uptake are shown in Table 1.
3.3. Mechanical strength
Tensile properties are the most studied aspect of composite films prepared using cellulose fibers. The high strength and stiffness of cellulose fibers and their high aspect ratio make them good candidates for reinforcing purposes. Besides, lightweightedness, transparency, and biodegradability are advantages of cellulose fibers in comparison with their traditional counterparts like glass and carbon fibers. Food packaging must have enough mechanical strength to resist handling damages and protect food items from damage during distribution. Hence, mechanical strength is one of the most important aspects of food packaging materials (Yu et al. 2014). Recently the application of cellulose fibers for improving the mechanical properties of food packaging materials like PLA (Shojaeiarani et al. 2019), and other petroleum-based thermoset polymers like PE (Ferreira et al. 2019) and PP (Kahavita et al. 2019) has gained a lot of attention. Despite the many advantages of cellulosic fibers as a reinforcing filler, they suffer from some serious drawbacks including low compatibility with commercial polymers, low dispersibility in commonly used non-polar matrices like PLA, and high moisture adsorption, which leads to the loss of mechanical properties with time. The low compatibility of cellulose nanoparticles with most of the commercial food packaging polymers results in low interfacial adhesion between the filler and the polymer and diminishes the stress transfer from the polymer to the cellulose filler, weakening the mechanical strength of the prepared composites. The detachment of the cellulose fillers and polymer matrices increases the accessible surface hydroxyl groups for the adsorption of moisture and leads to the loss of mechanical strength and barrier properties, which may in turn affect food quality. Thus, physical and chemical modifications that improve the compatibility of the cellulose filler and polymer matrix enhance the mechanical and barrier properties of the packaging materials and increase the shelf life of foods (Mohit & Arul Mozhi Selvan 2018; Vilarinho et al. 2018; Zhang et al. 2020). A comparison of the effects of these modifications in terms of tensile strength, Young’s modulus, elongation at break, and tensile strain are shown in Table 2.