Polymers 2023 , 15 , 1393
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while most importantly the packaging must look aesthetically pleasing. Table 2 lists some of the categories investigated when studying the properties of packaging materials [81]. One primary concern is the structural aspects of the packaging material including tensile and tear, strength, bending, compression, puncture and folding parameters that need to withstand different loading conditions during stacking, transfer and transportation. The strength of natural fibre-reinforced composites relates to two factors: (a) the stiffness and strength of the natural fibres and (b) compatibility between the fibres and the matrix. Strength and stiffness depend on the arrangement of cellulosic fibrils in the microfibrils present in the fibres, while the mechanical properties of natural fibres are contingent on the part of the tree or plant from which the fibre has been collected. Crystalline and amorphous fibre characteristics differ between parts of the tree and between trees. Previous studies suggested that fibres with fewer amorphous contents such as hemicellulose, lignin and pectin possess better mechanical properties [82,83]. The concentration of natural fibres incorporated into composites also impacts the mechanical properties. An increase in fibre content increases mechanical properties up to an optimal concentration beyond which the mechanical strength starts to show a decrease trend [84]. Tao et al. [85] studied the effect of jute and ramie fibre loading in PLA com- posites and found that 30% natural fibre content in PLA provided the optimal mechanical properties. Another factor affecting the properties of natural fibre-reinforced composites is compatibility, where interaction between the fibres and the matrix plays an important role in uniformly distributing the applied load into the matrix. Kamarudin et al. [86] reported that PLA/kenaf composites showed excellent mechanical strength at up to 40% fibre loading due to good fibre–matrix interfacial interaction. Beyond this critical fibre loading value, poor filler matrix compatibility resulted in earlier fracture of the composite. Several surface modification techniques (both physical and chemical) have been studied to improve the compatibility of natural fibres and matrix materials. Composites prepared from natural fibres show promise applied as food packaging materials. However, the role of food packaging materials is not limited to protecting products from physical and mechanical damage during distribution [87]. Food packaging must also control the transfer of water vapour, oxygen and/or carbon dioxide, which impact rates of oxidation, microbial development and physiological reactions of food degradation. Plastics are commonly permeable to small volatiles such as gases (O 2 , CO 2 ), water vapour, organic vapours and liquids [88,89] and water absorption barrier properties are essential basic requirements when packing food. The moisture barrier property is important in food packaging as this preserves the texture in both dry and moist food and controls microbial growth of aerobic spoilage. Important parameters responsible for the control water vapour permeability (WVP) are fibre content and size, fibre/matrix adhesion and crystallinity and plasticisation of the matrix [90–92]. The dispersion of fibres in the matrix can evoke impermeability due to the tortuosity effect. However, the WVP of natural fibre-reinforced composites significantly increases due to the hygroscopic nature of the fibres and poor dispersion in the matrix. By contrast, hydrophilic polysaccharide matrices have low WVP properties. Sirvio et al. [93] observed that incorporation of up to 50 wt% of cellulose microfibrils in alginate films decreased the WVP due to an increase in tortuosity. However, the aggregation and percolation of small natural fibres in polymer matrices can result from poor fibre/matrix adhesion, leading to voids in the polymers which encourage the transport of water molecules [94]. Other ways to improve the WVP of natural fibres include coating with PLA [95]. Das et al. [96] observed enhanced vapour permeability and absorption capacity of 15.9% and 48.1%, respectively, in shredded betel nut composite sheets compared with cardboard sheets. Food must also be protected from oxygen and carbon dioxide, which cause many degradation reactions. High levels of CO 2 in chilies limit the Krebs cycle, whereas low levels of O 2 decrease the activation of cytochrome oxidase, polyphenol oxidase, glycolic acid oxidase and ascorbic acid oxidase [97]. Migration into food is another parameter to be considered when choosing materials for food packaging applications [98]. Toxicological substances such as pesticide residues such
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