Building upon the methodology that we developed as part of the PARTIFACE project, we successfully expanded the particle production method to other lignin types and at a larger, pilot scale production. The results were consistent; while different lignin types yielded different particle sizes, it was evident that the hemicellulose coating improved the particles’ dispersibility in water and in acidic environments. We collaborated with Lignopure (Germany) to upscale the production of the hybrid particles, in which we not only obtained particles with similar morphology and composition as the laboratory scale production, but we also developed a buffer-free method that allowed us to recycle the GGM-rich effluent water for several additional cycles of particle synthesis, improving the resource efficiency of the process.
Additionally, we further characterized the physicochemical properties of the particles. We found that the presence of hemicellulose generally increased the surface roughness and hydrophilicity of the lignin particles. However, the presence of laccase, which was added to create covalent bonding between the hemicellulose molecules and the lignin surface, made the surface more hydrophobic instead. As emulsifiers, nonetheless, the particles largely followed the emulsifying properties of the original lignin types rather than their altered surface chemistry, showing very similar emulsifying power at different pH values compared to the unmodified particles. Results from small angle neutron scattering, neutron reflectometry, and atomic force microscopy indicated that the particles disintegrated at the oil-water interface, the degree of which is affected by the type of lignin used in the particles. All in all, these results pointed out that the lignin type is the determinant when designing the emulsifying properties of the hybrid lignin-hemicellulose particles.
We have also studied the behaviour of the hybrid particles in conditions relevant to the development of consumer products, particularly in the personal care, food, and pharmaceutical industries. For food applications, we chose emulsions as a case study, particularly in relation to the hybrid particles stabilize emulsions and its contents. Emulsions made with vegetable oils, such as rapeseed oil and medium-chain triglycerides, showed a remarkable physical stability over 42 days of storage. The hybrid particles performed similarly to unmodified particles, although laccase-treated particles aggregated the emulsion droplets to a higher degree. In vitro digestion simulation of the particles based on the INFOGEST protocol indicated that the hemicellulose coating stabilized the particles from aggregation even during the harsh pH and ionic strength changes of the digestion fluids. We also found that the hybrid particles prevented the degradation of vitamin A in emulsions just as well as the uncoated particles, with hardwood lignin performing better than softwood lignin, retaining more than 80% of the original vitamin A concentration after 42 days of storage. Lignopure has also conducted direct application tests on personal care products, where they demonstrated improved re-dispersion capacity and maintained small particle size, improving the functionality and appearance. Nevertheless, cell cytotoxicity study using Caco-2 cells indicated a concentration dependence, where cell viability remained higher than 75% up to 0.4 mg/mL of the lignin particles, reaching below 50% at 5 mg/mL and above. This suggests that while the particles are safe at lower concentrations, applications requiring more concentrated particles may need further studies to confirm its safety.
