Using celluloses to reinforce the optimized alginate film in wet state: Effect of cellulose types and cooking treatment.

Alginate (Alg) as co-extruded casing is of interest to the meat industry as replacers for natural sausage casing. However, these studies on the mechanical reinforcement of Alg-based film are still limited in the wet state (e.g. co-extrusion process). In this work, Alg-D with the highest viscosity-average molecular weight (1.12 × 105) was selected from four types of alginates based on the results of the viscosity of Alg solutions and film strength. Next, three celluloses (cellulose nanocrystals (CNC), cellulose nanofibers (CNF) and microfibrillated fiber (MFC)) were added to the Alg-D matrix at different concentrations. SEM showed that the cross section of the Alg-based films became more compact and uniform when the size of celluloses decreased. The tensile test revealed that the strength (TS) of Alg-based films exhibited an initial increase followed by a subsequent drop as the cellulose content rose. The best mechanical strengthening effect was the Alg-CNC film (1.16 MPa), which increased by 93.33 % compared with that of pure Alg. Cooking treatment could further enhance this trend. The opacity increased gradually with the increase of cellulose content, while these films were still transparent enough for food packaging. These findings would have potential applications in food packaging, especially co-extruded sausage casings.

Using celluloses in different geometries to reinforce collagen-based composites: Effect of cellulose concentration.

Cellulose is frequently used to strengthen biocomposite films, but few literature systematically deliberates the effects of concentration of celluloses in different geometries on the reinforcement of these composites. Here we prepared three types of celluloses, including rod-like cellulose nanocrystalline (CNC), long-chain cellulose nanofiber (CNF) and microscopic cellulosic fines (CF). The effect of concentration of the three celluloses was examined on the barrier properties to water and light, thermostability, microstructure, and mechanical properties of collagen (COL) films. The addition of celluloses increased the watertightness and thermostability of composite films. Besides, FTIR showed a increased hydrogen bonding for COL/CNF and COL/CNC composite films, but decrease for COL/CF composites. As the concentration of CF and CNF increased, the strength of composites improved. The TS for COL/CNF (124 MPa) and COL/CF composites (113 MPa) were largely increased, compared with that of collagen ones (90 MPa). Considering the factors of crystallinity, hydrogen bonding, and interfacial tortuosity, COL/CNF composites possessed better mechanical behaviors than that of COL/CF and COL/CNC composites. Furthermore, Halpin-Kardos and Ouali models well predicted the modulus of COL/CNF composites when CNF was below and above percolation threshold (2.7 wt%), respectively.

Tunable mechanical behavior of collagen-based films: A comparison of celluloses in different geometries.

Collagen (Col) films were reinforced by celluloses in different geometries: microcrystalline cellulose (MCC), cellulosic fines (CF), cellulose nanofiber (CNF) and cellulose nanocrystals (CNC). The reinforcement mechanisms were investigated by the elastoplasticity and fracture appearance. Compared with the fracture stress of collagen film (67.5 MPa), the Col-CNF films effectively borne the stress (95.8 MPa) by intercrystalline fracture, ascribing the abundant hydrogen bonding and mechanical locking between cellulose and collagen. The toughness of Col-CF films was increased by the interfibrillar slippage of CF and pull-off of CF within the matrix, improving the strain-to-break from 8.37% to 12.13%. The films added with MCC and CNC weaken the mechanical behavior, due to the defects and lack of mechanical locking. Besides, the effects of celluloses' geometries on the thickness, density, water-tightness, thermal stability, crystallinity and FTIR of films were also investigated. These provide the evidence that the geometries of fillers diversely improve the behaviors of collagen film offering strategies for the film with adjustable mechanical properties.

A top-down approach to improve collagen film's performance: The comparisons of macro, micro and nano sized fibers.

The severe reduction of mechanical strength of collagen once it is extracted or dissociated from animal tissues and no additional crosslinking approaches are conducted, impede its application in biodegradable and edible food packaging. Here, for the first time, high pressure homogenization (HPH) was used to prepare diverse sized fibers and the related fibers-composed films' performance were investigated. These fibers have a diversity of effects on film performance. The films prepared with smaller sized fibers had a more uniform and denser structure. The mechanical and the water barrier properties of the films improved significantly as the fiber size decreased. No obvious change in FTIR and thermal properties suggests that the improved film performance is mainly attributed to the physical entanglement and non-covalent bonds. Given the forementioned benefits of the films, control of fiber size can be a potential industrial approach for producing collagenous materials in edible food packaging.

Metal ions increase mechanical strength and barrier properties of collagen-sodium polyacrylate composite films.

From the previous experiment, it was confirmed that the incorporation of 0.3 wt% sodium polyacrylate (PAAS) into collagen (Co) fibers can improve the mechanical properties and thermal stability of the composite films. In this study, Ca2+, Fe3+ and Ag+ ranging 0.001-0.004 mol/g were used to improve the properties of Co-PAAS blend films based on the rationale of their potential electrostatic interaction with these biopolymers. As expected, Zeta-potential film-forming solutions was decreased to some extent with the addition of metal ions. SEM images presented that the surface of the composites became coarser and internal structure became more stratified as metal ion contents increased. Tensile strength was increased by the addition of these ions with a varied optimal concentration: Ca2+ (0.003 mol/g), Fe3+ (0.002 mol/g) and Ag+ (0.001 mol/g). Water vapor permeability (WVP), solubility and light transmission value of films while causing film thickness no obvious change. In addition, the differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA) results indicated that the metal ions improved the thermal stability of the composite film. Therefore, Ca2+, Fe3+ and Ag+ with an appropriate addition amount can be used as a potential alternative to reinforce collagenous composite materials.