INFORM September 2024

16 • inform September 2024, Vol. 35 (8)

Examples of capillary suspensions made with increasing amounts of water. Source: Koos, E., JCOCIS , 19, 6, 2014.

these challenges, and produce functional solid fat alternatives? The solution may lie in food particles. STRUCTURING OILS WITH PARTICLES Food particles come in all shapes and sizes, such as proteins, sweeteners, and starches. But most particles cannot gel oil on their own since they lack the ability to aggregate into strong networks. Even chocolate, which is more than 60 percent par ticles of sugar, cocoa, and milk powder, flows like a viscous fluid when the solid fat is melted. Capillary suspensions form when particles are suspended in a solvent and a secondary immiscible fluid forms the bridges between particles, creating a network that gives the oil its solid-like properties. Hence, by adding water to food particles in oil, we can give the particles and the oil structure.

This is conceptually similar to the way sandcastles hold their shape. Dry sand flows as a powder, but if the sand is wet it can be molded because the water forms a bridges between the grains of sand. Capillary suspensions were first reported in Science (2011) by Erin Koos and Norbert Willenbacher (https:// doi.org/10.1126/science.1199243). They suspended 11 percent (by volume) calcium carbonate particles in an oily organic sol vent and water at 0.1 to 0.5 weight percent. Koos and other authors went on to catalogue capillary suspensions made with variables like different particle shapes, majority and secondary fluids, particle roughness, mixing conditions, and order of addi tion for different material applications from ceramics to foods. By adding a small amount of water to particles in oil, the water forms bridges between the particles allowing them to form a capillary network. For polar particles, water will have limited interaction with the oil and wet the surface of the polar particles, producing concave bridges and forming a network. For non-polar particles in oil, neither the particles nor the oil prefers to associate with the water the bridges. The result ing bridges are convex and the water droplets are shielded from contact with the oil phase by the particles. In both cases the water causes the particles to cluster and connect throughout the oil to form the capillary network, preventing sedimentation. Capillary suspensions can form with particle fractions as low as 11 percent and become firmer with increasing parti cle fraction as more bridges are formed. The versatility of this gelation method allows researchers to modulate a network’s strength by altering the particle fraction and the particle polar ity. The capillary network’s strength and morphology changes with bridging fluid’s properties and concentration. For parti cles in oil bridged by water, adding more water causes multiple particles to be connected by a singular meniscus. Compared to clusters made of the same number of particles but discrete bridges, the continuous meniscus produces stronger bridges and firmer gels. Although, adding too much secondary fluid will cause the particles to be swept into spherical agglomer ates which weaken network strength. Capillary networks can also recover their structure after a sufficient rest period. The networks nearly return to their original gel strength which may prove useful in applications where both structure and workability are required (https:// doi.org/10.1007/s00397-017-1040-1). For comparison, wax oleogels form the best networks after recrystallization where

For water-bridged particles in oil:

Polar particles

Pendular bridges

Particle clusters

Network formation

Non-polar particles

Capillary bridges

Polar particles

Non-polar particles

Examples of particle clusters that aggregate to form capillary net works and corresponding micrographs. The water is shown in white around the spherical particles, on a (black) oil backdrop to highlight the particle networks.