Inducing anisotropy in emulsion-filled hydrogels by unidirectional freezing

Creating fibrous structures in hydrogels is important for developing plant-based alternatives to animal products, such as meat or fish. Freeze texturization is a technique that allows the generation of fibrous structures in materials by unidirectional growth of ice crystals, which initiates the irreversible formation of fibres along the direction of freezing, with a scale ranging from micrometres to millimetres. This study aimed to systematically study the structure formation of hydrogels when undergoing freeze texturization, the effect of hydrogel microstructure, and the presence of filler particles (liquid: linseed oil droplets; solid: palm fat droplets) on structural and rheological properties of hydrogels after freeze texturization. Potato protein gels were prepared at pH 6 and 7 with protein concentrations ranging from 10 to 14% and analysed for mechanical properties, including Young's modulus E y , fracture strain ε f , fracture stress σ f , water holding capacity and mechanical anisotropy. Expectedly, we obtained particulate gels at pH 6 and fine-stranded gels at pH 7. Particulate gels exhibited lower stiffness and higher brittleness, while fine-stranded gels showed higher modulus values. Unidirectional freezing resulted in significant changes in gel properties and a dependency on the initial gel structure. Particulate gels were found to increase in fibre size with protein concentration, with higher concentrations showing larger, more coarsely structured networks. Fine-stranded gels exhibited finer fibre structures after freeze texturization compared to particulate gels. With freeze texturization, an increase in anisotropy and a decrease in ε f and σ f were shown for all gel types. At pH 6, gels showed an E y , increasing from 35 kPa to 110 kPa with increasing protein concentration, which decreased after freezing. Gels at pH 7 had a higher modulus (40–120 kPa) and freezing resulted, similarly, in a decrease in modulus and a higher mechanical anisotropy factor. The inclusion of liquid and solid fillers increased gel stiffness, firmness and fibre dimensions. Overall, modifying gel properties and incorporating fillers allows for control of the structure and mechanics of hydrogels. This research enhances our understanding of hydrogel behaviour and guides the creation of novel plant-based foods. • Unidirectional freezing delivers fibrous gel structure. • PH allows to control fibre dimensions. • Inclusion of particles increases fibre thickness.