Abstract
The use of biomacromolecules as the principal ingredients for the development of biodegradable films and as environmentally friendly materials has increased since these biomacromolecules offer various benefits such as biodegradability, nontoxicity and biocompatibility. However, the weak structural integrity and poor barrier properties of biobased materials continue to challenge the process of converting the raw materials into useful products. Therefore, there is a growing interest in identifying alternative modification or processing techniques to improve the handling of biomacromolecules. Hence, the goal of this research project was to understand the effect of pulsed electric fields (PEF) on the modification of interactions, properties, and microstructure of biomacromolecules.
The first part of the research investigated PEF-induced modification of colloidal properties and microstructure of single and composite biomacromolecule systems. This applied the characterisation of single biomacromolecules of zein (ZN), chitosan (CS) and poly(vinyl alcohol) (PVOH) using the hydrodynamic properties, thermal stability, crystallinity and molecular properties using Fourier transform infrared spectroscopy (FTIR). The dispersions were treated using a range of specific energy (Q) 50-70, 130-150, 295-320, 595-620 kJ/kg delivered at a constant electric field strength (E) (3.1-3.5 kV/cm), frequency (f) (50 Hz) and pulse width (32 μs). These studies showed that an increase in specific energy modified the hydrodynamic properties of biomacromolecules, including the particle size and zeta-potential. This finding was related to the higher colloidal stability of ZN and CS dispersions at Q >150 kJ/kg, but PVOH was modified at higher energy (595-620 kJ/kg). FTIR results showed a decrease in the ordered helices in ZN and CS, which was likely due to the protein unfolding and dissociation of large polysaccharide units, respectively. The molecular structure of PVOH showed enhanced intramolecular interaction on the assembly of the matrices, perhaps due to the crosslinking interactions during the assembly process. Results from differential scanning calorimetry (DSC) assay showed that the melting enthalpy of the three biomacromolecules increased with the increase in specific energy, which was also reflected in the improvement in the overall thermal stability of ZN and CS as obtained from thermogravimetric analysis (TGA). A similar trend was followed in the crystallinity, where ZN and CS showed improved crystallinity when increasing the specific energy with PVOH showing increased amorphous behaviour, which was attributable to the effect of PEF on the saturation of the PVOH molecular chains. On the overall, subjecting single biomacromolecules to PEF processing resulted in disaggregation of large molecules into smaller units and subsequent reassembly during the solvent removal process with the formed matrices showing stabilisation through hydrogen bonding and electrostatic forces.
The second part of this research investigated PEF induced modification of the structure and functional properties of biodegradable films using a composite dispersion of ZN-CS-PVOH (ZCP). PEF was applied at either specific energy of 60-400 kJ/kg (E =3.4 kV/cm, f= 50 Hz), pulse frequency 50-220 Hz (Q=400 kJ/kg, E=3.4 kV/cm) or electric field strength of 0.8-3.4 kV/cm (Q=610 kJ/kg, f=50 Hz). The viscosity, particle size and zeta-potential of the composite dispersions increased with increasing PEF intensity (Q>150 kJ/kg, E≥1.6 kV/cm and f≥50 Hz), which was confirmed using FTIR as the establishment of new interactions including glycosylation of zein and numerous bioconjugation reactions. Apart from improvements in the melting stability and thermal profile of the cast films, the tensile strength increased, and the water vapour permeability decreased with increasing PEF intensities. Further, the elongation of the films improved on application of medium intensities of Q, E and f, but decreased at extremely high intensity. The microstructural morphology observed using SEM showed the formation of a regular structure on varying the electric field strength, with extremely high intensity resulting in an irregular morphology. Process optimisation for high integrity films of better stability in the wet environment showed that low specific energy (<100 kJ/kg) and high frequency (≥100 Hz) were desirable for better stability in water, but high energy and low frequency were desirable for better tensile strength. Under PEF, it was possible to obtain matrices with different physicochemical and surface properties, demonstrating for the first time, the mechanism of PEF involvement in the self-assembly of composite biomacromolecules. Further studies involving in vitro digestion of the treated films showed that the degradation profile and the yield of amino groups were not affected by PEF treatment, which did not also influence the cytotoxicity of the degradation products against Hepa-1c1c7 cell line.
Overall, this study has demonstrated the potential of using PEF as a tool for the modification of biomacromolecules. As the industrial application of PEF in the food and related industries is gaining momentum, the findings of this study will serve future process designs for PEF treatment of bio-based materials where secondary biomacromolecules may be incorporated for network development.