The project systematically studied how hydrogel structure impacted its interaction with particulate matter. Crosslinked biopolymer hydrogels either as free-standing films or as particles (10s to 100s microns) were developed that had high abilities to bind MP. MP trapping rates as high as 100% were achieved upon using crosslinked gelatine hydrogel particles and ~ 70% when using crosslinked gelatine hydrogel films. The difference in MP binding per gram of hydrogel is most likely due to the higher surface area of particles compared to films. Model spherical MPs with a wide range of sizes (20 nm-90 μm) and chemistries (with carboxylate, sulphate, polystyrene, poly(methyl methacrylate) or amine surface groups) were successfully trapped. Systematic studies demonstrated that MP adsorption rates were independent of NaCl concentration up to 58 g/L. They also showed that MP adsorption rates depended on pH with maximum adsorption rates around pH 6. The adsorption process was proved to be reversible, allowing biopolymer hydrogel films or particles to be regenerated for reuse. The biopolymer hydrogels were shown to be biodegradable in aquatic environments.
As a side-stream of the project, the gelation of the biopolymers used during the project was investigated using space-resolved dynamic light scattering (DLS). This technique can probe motion in transparent materials over time and space, provided that the materials scatter the light. The principle is the following: a laser beam goes through the sample and a camera collects images of the illuminated sample volume at a given angle. The so-obtained images are a collection of bright and dark ‘grains’, called ‘speckles’. The intensity of each speckle varies with time. If the sample is fluid-like, the dynamics in the sample are fast, and the speckle intensity fluctuates fast. If the sample is a solid like a gel, the dynamics in the sample are very slow, and the speckle intensity hardly changes with time. Hence, while it is hardly possible to see when a biopolymer solution has gelled without moving the sample around to check whether it is liquid or solid, space-resolved DLS easily ‘sees’ gelling. It indeed corresponds to a significant slowdown of speckle motion. Hydrogels prepared with one of the biopolymers used in the project were seen to expel water after gelling. Such a phenomenon is called syneresis. Using several purposely-designed space-resolved DLS tools, gelling and syneresis could be visualised. It was shown that space-resolved DLS could probe gel shrinkage during syneresis. It was also demonstrated that syneresis could be significantly hindered when the gels were in contact with surfaces they were adhering to. These results were presented at the International Symposium on Food Rheology and Structure (Wageningen, the Netherlands) in June 2023 and at the International Congress on Rheology (Athens, Greece) in July-August 2023, where a best poster prize was awarded. A research article is being written.
A review article about biopolymer hydrogel particle dispersion for food applications was also written during the project. It allowed promoting the experience and knowledge gained during the first year of the project about hydrogel particle fabrication and characterisation.
