Preparation and characterization of nanocellulose hydrogel and its application for preservation of fresh pork

Hydrogels have attracted much attention for fresh preservation in the field of food at present, because of their low toxicity, biocompatibility, and degradability. However, mechanical properties and fatigue resistance have limited the application in food preservation. In this study, the multifunctional nanocellulose/polyvinyl alcohol/sodium alginate (SCNFs/PVA/SA) composite hydrogels were prepared with excellent mechanical properties and fatigue resistance using low-temperature (-20 ℃) cyclic freezing-thawing. The raw materials were also taken as soy hull nanocellulose (SCNFs), polyvinyl alcohol (PVA), and sodium alginate (SA). The microstructure of the composite hydrogel was observed by scanning electron microscopy (SEM). The chemical structure and crystallinity were identified by Fourier transform infrared spectroscopy (FT-IR) and X-ray powder diffraction (XRD). The thermal stability was obtained by a thermogravimetric analyzer (TGA). The mechanical properties of the composite hydrogel were determined by a texture analyzer. Finally, the composite hydrogels were used for the preservation applications of fresh pork at 4 ℃. The SEM images showed that the regular and uniform three-dimensional network structure was formed inside the composite hydrogel, which was conducive to the mechanical stability of the composite hydrogel. The FT-IR results showed that the SCNFs, PVA, and SA with the high aspect ratio formed hydrogen and ester bonds in the composite hydrogel. More functional groups were exposed in the cyclic freezing-thawing process at -20 ℃, indicating the high cross-linking density between the monomers. The XRD results further proved that the crystallinity in the composite hydrogels was enhanced by the presence of hydrogen bonds and the addition of SCNFs. The TGA results showed that the SCNFs increased the thermal stability of the composite hydrogels. The reason was that the SCNFs strengthened the hydrogel matrix, or the interaction of internal hydrogen bonds enhanced the stability of the composite hydrogels. At the same time, the composite hydrogel shared low density, water content, and water solubility, but with excellent swelling degree. The uniform three-dimensional network was formed internally, where the SCNFs were cross-linked with PVA and SA by hydrogen and ester bonds. The -20 ℃ low-temperature cyclic freezing-thawing also caused the chain entanglement among monomers, indicating the high crosslinking density. The mechanical tests showed that the elongation at break and tensile strength of the composite hydrogel were 230% and 17 kPa, respectively. The compressive strength was 70 kPa under 70% stress, indicating great toughness (170 kJ/m³). Furthermore, the composite hydrogel still had excellent mechanical properties (tensile strength of 12 kPa) after five cycles of stretching under the 70% strain, indicating excellent fatigue resistance. The high tensile strength and fatigue resistance were attributed to the uniform three-dimensional network structure of the composite hydrogel after the addition of SCNFs. The pork preservation showed that the composite hydrogel maintained the stability of pork color to prevent the loss of internal moisture of pork in the rapid change of the internal pH value of pork. At the same time, the composite hydrogel was used to isolate the outside air, in order to inhibit the growth of bacteria inside pork and the oxidation of unsaturated fatty acids and protein inside pork. The composite hydrogel was also used to extend the shelf life of pork to 10 days. Therefore, the high tensile, toughness, and fatigue-resistant composite hydrogel can be expected for the food packaging, in order to extend the shelf life of frozen meat products. The composite hydrogel can also fully meet the needs of refrigerated meat products. A new type of preservation material can be served to ensure food quality and safety. In addition, the composite hydrogel can also be applied in the food preservation industry of packaging and refrigerated meat products, due to the low cost, simple preparation, and strong reproducibility.