Tuning the rheological and tribological properties to simulate oral processing of novel high internal phase oleogel-in-water emulsions

The current study reported novel oil-in-water high internal phase emulsions (HIPEs) stabilized by oleogel and a protein. Effects of ethyl cellulose (EC, oleogelator) and oil fraction on the microstructures, stability, linear/non-linear rheological, and tribological properties of the emulsions were investigated. CLSM and Cryo-SEM observations indicated that the increase of oil and EC content contributed to a smaller size (from 13.3 to 8.17 μm) and tighter packing of the droplets. EC was distributed inside the oil droplets and tended to move towards the interface. Gelation of the oil phase allowed the emulsions with higher physical stability against centrifugation. Compared to medium internal phase emulsions (60 wt% oil), HIPEs (75 wt% oil) were more sensitive to shearing. Small amplitude oscillation shearing analysis revealed that the inclusion of EC led to stiffer (higher moduli), stronger (higher critical stress), and more brittle (lower critical strain) properties of the emulsions. Large amplitude oscillation shearing tests showed that the emulsions with higher oil content presented weaker strain overshoot behaviors, while HIPEs with EC showed strain thinning characteristics. Lissajous-Bowditch curves further revealed that HIPEs were more sensitive to large deformation. Emulsions without EC showed higher structural elasticity and were more resistant to large deformations, while emulsions with EC exhibited higher nonlinear responses. Tribology tests suggested that the addition of EC contributed to higher friction coefficients of emulsions in mixed and hydrodynamic lubrication regions. The information obtained would be helpful in understanding the oral behavior of concentrated emulsions and designing functional food.