- The overall objective of this dissertation study was to develop and validate the performance of cellulose nanomaterials (CNs) incorporated fruit coatings for improving the storability of postharvest pears and bananas under various storage conditions. Four specific studies were conducted to fulfill the research objectives as stated below.
First, cellulose nanocrystal (CNC, 0, 0.1% and 0.2%, w/w) reinforced chitosan (CH, 2% w/w) coatings were evaluated for delaying ripening and quality deterioration of postharvest D’Anjou pears (Pyrus communis) during ambient (20 ± 2 °C and 30 ± 2% RH) and cold storage (-1.1 °C and 90% RH), respectively. The concentration of incorporated CNC affected coating performance. The 0.1% CNC reinforced CH coating (CH-CNC) decreased chlorophyll degradation of pear peels, and reduced weight and firmness loss as well as change of total soluble solid content of pear flesh during 3-weeks of ambient storage. However, 0.2% CNC incorporated CH coating induced surface speckling and pithy brown though it provided similar control on weight and firmness loss compared to 0.1% CNC reinforced CH coating.
In the second study, CNC Pickering emulsion incorporated CH coating (CH-PCNC, 0.1% CNC/3% oleic acid/2% CH, w/w) was developed to provide better stability under high relative humidity (RH) cold storage (90% RH and 1.7 °C) for Bartlett pears. For evaluating coating performance, CH-PCNC films derived from coating formulations and stored under 43% and 83% RH were subjected to FTIR analysis. Results showed no large absorption band for –OH stretching, whereas that of previously developed CH-CNC films stored at 83% RH exhibited an elevated peak in comparison with film conditioned at 43% RH. These results indicated that CH-PCNC held more stable matrix than CH-CNC under high RH conditions. Water vapor permeability (WVP) of CH-PCNC film was four times lower than that of CH-CNC film, indicating more hydrophobic and stable coating against high RH conditions. Moreover, CH-PCNC coatings delayed pear ripening and reduced senescent scalding incidence of pears during 3-months of cold storage.
In the third study, cellulose nanofiber (CNF) based emulsion coating (0.3% CNF/1% OA/1% sucrose ester fatty acid, w/w) was studied to improve the storability of postharvest bananas (Musa acuminate) under ambient conditions (20 ± 2 °C and 50 ± 5% RH). Properties of coating suspensions and derived films were investigated to understand the mechanism of improved hydrophobicity, wettability and surface adhesion onto fruit surfaces. The developed emulsion coatings had low contact angle and surface tension, as well as high spread coefficient onto fruit surfaces, indicating good adhesion with banana skin. Coated bananas showed hindered ethylene biosynthesis pathway and reduced ethylene and CO2 production, as well as reduced fruit skin chlorophyll degradation, weight and firmness loss, thus enhancing the marketability of fruit.
Finally, CNFs derived from different raw materials and production methods were evaluated and their performance by incorporating to previously developed CNF emulsion coatings on bananas were validated. A fine CNF without mineral incorporation had lower particle size, effective diameter and better dispersion, presenting better compatibility with the emulsion coating system. When incorporating into CNF based emulsion coating, it exhibited more uniform coating coverage without introducing visible white substances on the fruit peels and less peel spots at the ripening stage together with similar impact on overall appearance, chlorophyll degradation and firmness reduction. This study indicated that different CNFs with varied intrinsic properties could impact coating performance of CNF based emulsion coatings.
The results from this dissertation study demonstrated that CNs could be used as a coating forming matrix or carrier of emulsion droplets, reinforcing agent by incorporating to other biopolymers, and/or Pickering emulsifier to improve gas and moisture barrier as well as adhesion properties due to their superior intrinsic properties and compatibility with other polymers. The CNs incorporated coating improved the storability of postharvest pears and bananas under different storage conditions by reducing hydrophilicity of the biocomposite coating materials, providing stronger barrier to water and gas exchange through a modified atmosphere within fruit, and improving wettability and adhesion between coating suspension and fruit skins. Developed coatings could have great potential for reducing food waste of postharvest fruit.