Thermo-responsive droplet-droplet interactions in high internal phase emulsions: A strategy for adipose tissue substitution

Adipose tissue shows specific temperature-dependent rheological behaviour. It remains a challenge to find suitable plant-based composites that replicate this behaviour. Here, we hypothesised that introducing a secondary network in a high internal phase emulsion (HIPE) could impart reversible, temperature-dependent behaviour to the emulsion, mimicking the properties of animal adipose tissue. Oil droplets in protein-stabilised O/W emulsions were crosslinked through hydrogen bridge-based interactions (tannic acid) or charge-based interactions (calcium ions), to introduce a secondary network. Rheological methods and texture analysis were used to investigate the thermo-dependent response of the HIPEs. The addition of calcium did not yield temperature-dependent behaviour. However, HIPEs crosslinked with tannic acid (TA) formed a secondary network replicating relevant small- and large-deformation rheological properties of bovine adipose tissue. TA significantly increased the stiffness and hardness of the HIPE and induced a softening and hardening pattern with temperature, similar to that of bovine adipose tissue. The stiffness of the HIPEs decreased upon heating by 2 orders of magnitude, yet, importantly, remained overall gel-like. During cooling, stiffness increased back to the original value. The crosslinked emulsions remained stable throughout the temperature cycling. Replacing the liquid oil with a solid fat in the HIPE resulted in a more brittle emulsion gel, but also in emulsions that were unstable during temperature cycling due to the protrusion of fat crystals, the formation of a fat crystal network and subsequent coalescence of the HIPEs. Overall, the use of a hydrogen bridge-based secondary network in HIPEs seems promising to allow the formation of emulsion gels with rheological properties similar to those of adipose tissue. In particular, this approach allows the use of liquid oil to replicate some of the relevant rheological properties of adipose tissue.