Microstructures of potato protein hydrogels and aerogels produced by thermal crosslinking and supercritical drying
Aerogels are highly porous structures created from hydrogels by solvent exchange and supercritical CO2 drying. They can be produced from proteins and used for encapsulation of active ingredients. In this work, the structural relationship between various protein based hydrogels and aerogels was investigated. We compare egg white protein (EWP) and patatin rich potato protein isolates (PPI) in terms of gel and aerogel production thereof. Thereby, aerogels were produced for the first time from patatin-rich potato protein isolates. Both protein sources can form strong hydrogels, but they differ in their molecular properties, especially regarding the presence of reactive groups available for covalent crosslinking. Therefore, we studied the rheological profiles of hydrogels of both protein groups and applied the ratio of elastic modulus at gel formation temperature (G'hot) and after cooling to ambient temperature (G'cool) as criterion for comparing gel formation capacities produced from structurally different proteins. The ratio G'cool/G'hot was also identified as an indicator of the relation of non-covalent bonds to covalent and/or hydrophobic interactions and for characterizing aerogel structural properties. PPI hydrogels were found to be dominated by hydrophobic interactions, while covalent disulfide bonds determine EWP hydrogel stabilization. Nevertheless, PPI and EWP hydrogel and aerogels showed similar stabilities. More than the protein source, the pH during hydrogel formation was found to decisively influence the protein interaction potential and gel/aerogel characteristics. Irrespective of the protein source low pH led to brittle aerogels with high specific surface areas and low G'cool/G'hot ratios. At a pH close to the isoelectric point (IEP) brittle gels with low specific surface areas and high G'cool/G'hot ratios were measured. At alkaline pH gels were most stable, the specific surface area and G'cool/G'hot ratios were intermediate. Overall, the results help to control the structural properties of gels and aerogels towards potential applications in food systems.