Biopolymer/ZnO nanocomposites

undergo biotransformation reactions to absorbable products. As microorganisms consume the degraded polymer, carbon dioxide, water, and biomass are produced and returned to nature through the biocycle.

Fig. 1.16. General mechanism of degradation of biodegradable polymer materials

1.8 Literature review on antimicrobial bionanocomposites for food packaging

(O2 and CO2) properties. Most importantly, nanocomposite containing 5 wt% ZnO showed excellent antimicrobial properties against E. coli (99.99% reduction in cell growth after 24 h).

Kim et al. (2019) have synthesized PLA/ZnO nanocomposites by solvent casting method and studied their physical and antibacterial properties (Kim et al., 2019). They have found that adding 3 wt% of ZnO into PLA completely inhibits the growth of E. coli. Therefore, these bionanocomposites could be active food packaging materials with excellent antibacterial properties and UV barrier properties. Shankar et al. (2018) have synthesized cubical-shaped ZnO (50100 nm) and incorporated them in PLA to fabricate PLA/ZnO nanocomposites (Shankar, Wang, et al., 2018). The nanocomposite film exhibited excellent UV-light barrier properties. Also, the nanocomposites possessed improved tensile strength (37.5%), and WVP was reduced by 30.5%. Moreover, the nanocomposite showed excellent antibacterial activity against E. coli and L. mono-cytogenes. Further, the storage test revealed that the minced fish paste packed with PLA/ZnO film exhibited strong antibacterial activity. Tajdari et al. (2020) have fabricated PLA- based nanocomposite by incorporating two different morphologies of ZnO (cylindrical and spherical) and their functional properties, such as optical, mechanical, hydrolytic degradation and antibacterial properties (against E. coli and L. monocytogenes) were investigated (Tajdari et al., 2020). It was observed that the dispersibility of ZnO in a polymer matrix depends on ZnO morphology, and it plays a significant role in enhancing the physiochemical properties of nanocomposites. The cylindrical ZnO showed better mechanical properties (20%) as compared to spherical ZnO. On the other hand, in the case of optical properties and degradation, spherical ZnO showed better results (60%). Saral Sarojini et al. (2019) fabricated Mahua oil-based polyurethane (PU)/chitosan (CS) incorporated with ZnO nanoparticles (30 nm) (K. et al., 2019).

Adding 5 wt% of ZnO into PU/CS exhibited improved tensile strength, barrier properties and hydrophobicity of the film. Moreover, nanocomposite possesses good antimicrobial properties and extends the shelf life of carrot pieces up to 9 days. Ejaz et al. (2018) have synthesized bovine skin gelatin (BSG) based nanocomposite films incorporated with clove essential oil (CEO) and

ZnO nanorods (< 100 nm) (Ejaz et al., 2018). Then the nanocomposite films were used for shrimp packaging. The results showed that nanocomposite possesses improved mechanical, oxygen and UV-barrier properties. The storage test in the refrigerator exhibited that BSG/CEO/ZnO nanorod nanocomposite films with 50% CEO (w/w of protein) showed the highest antibacterial properties against Salmonella Typhimurium and L. monocytogenes. The results clearly indicate the suitability of the nanocomposite films for active food packaging applications. Mohammadi et al. (2019) have developed bionanocomposite films by using carboxymethyl cellulose (CMC), ZnO nanoparticles and different percentages of okra mucilage (OM) to extend the shelf life of chicken breast meat stored at 4°C (Mohammadi et al., 2019). The antimicrobial activity of nanocomposite was studied using total viable counts, S. aureus counts, and Lactic Acid Bacteria counts of the chicken breast during the storage period. The results showed that CMC/OM 50%/ZnO gives the best inhibitory results against microorganism growth and exhibited the lowest chemical changes and highest sensory score in the chicken breast during storage. Youssef et al. (2016) have developed a novel nanocomposite film consisting of chitosan, carboxymethyl cellulose (CMC) and ZnO nanoparticles by solvent casting method (Youssef et al., 2016). The nanocomposite film with different concentrations of ZnO (2, 4 and 8 wt%) was studied for physiochemical properties (mechanical, thermal etc.). Moreover, they also investigated the influence of nanocomposite film packaging on the shell life of white soft cheese. The nanocomposite film showed enhanced thermal, mechanical properties and good antibacterial activity against S. aureus, P. aeruginosa, and E. coli and antifungal activity (Candida albicans). Also, the storage test (7°C for 30 days) revealed the improved shelf life of white soft cheese packed in nanocomposite films. Noshirvani et al. (2017) also prepared carboxymethyl cellulose/chitosan/oleic acid (CMC-CH-OL) based nanocomposite containing ZnO nanoparticles (<25 nm) to extend the shelf life of sliced wheat bread (Noshirvani et al., 2017). The presence of ZnO nanoparticles and oleic acid significantly decreased the water vapor transmission rate (WVTR) of nanocomposite film. The results revealed that the nanocomposite film showed excellent antimicrobial properties and could extend the shelf

life of sliced wheat bread from 335 days. Moreover, the staling rate of bread was studied by Differential Scanning Calorimetry (DSC) analysis. The results revealed that the nanocomposite film delayed the staling rate of sliced bread, indicating the suitability of nanocomposite film to be used in sliced bread packaging. Petchwattana et al. (2016) have prepared poly(butylene succinate) and ZnO nanoparticles (10 nm) nanocomposite films in the blown-film extruder for food packaging applications (Petchwattana et al., 2016). The antimicrobial activity revealed that 6 wt%

of ZnO nanoparticles could inhibit S. aureus and E. coli with an inhibition zone of 1.25 and 1.31 cm, respectively. The tensile test showed that incorporating ZnO nanoparticles into poly(butylene succinate) film increases the tensile strength while decreasing the elongation at break. Kumar et al. (2019) have synthesized agar-based nanocomposite films containing hexagonal-shaped ZnO nanoparticles (average size: 24 nm) prepared from Mimusops elengi fruit extract (S. Kumar et al., 2019). The nanocomposite film was used as packaging material for the shelf life extension of green grapes. Incorporating ZnO nanoparticles into the nanocomposite film increases the thermal stability, whereas the tensile strength decreases. The storage test of green grapes packaged with nanocomposite films containing 2% (w/w) and 4% (w/w) of ZnO nanoparticles showed a fresh appearance up to 14 to 21 days, respectively, in ambient conditions. Therefore, agar/ZnO nanocomposites film showed promising ability to be used in active food packaging.