Effect of heat treatment on capillary suspensions prepared from different protein sources

In capillary suspensions, small bridges of a secondary liquid lead to strong attractive forces between the particles, thereby enhancing network formation. We studied the rheological behavior of whey, potato and pea protein aggregate networks in oil — referred to as protein oleogels — using water as bridging liquid. Aggregates varied in size and hydrophobicity. Changes in gel strength following water addition resulted from an increased degree of clustering of the protein aggregates upon bridge formation. Additional particle rearrangements into denser networks upon heating further increased gel strength by up to a factor of 60. Such network densification was confirmed by simulations, in which a temperature increase was shown to lead to a reduction in particle distance related to changes in bridge geometry. A heat treatment is therefore an effective approach to further enhance the gel strength of biopolymeric capillary suspensions. When the water was evaporated, this network-contracting effect appeared even stronger and increased with the amount of evaporated water. Due to the high brittleness combined with strong particle clustering of these gels, their networks disintegrated into larger agglomerates upon mixing. Therefore, a second water addition did not lead to restoration of the network strength to comparable values as after the first water addition. Largest effects of heating were obtained for potato protein oleogels with aggregates of small size and initially weak gels. The ability to modify network structure by heating provides plenty of opportunities for the food, cosmetic and pharmaceutical industries to design biopolymer-based soft materials with functionality-enhancing rheological properties.