Role of pulse protein physical modifications in achieving stable viscoelastic nanoemulsion gels

Abstract: Plant proteins have been widely used; however, their functionality needs to be improved to better suit their utilization in developing nanoemulsion gels. In this study, thermal (75ºC for 30min), high-pressure homogenization (15,000psi for 6 cycles) and their combined effect were used to modify the faba bean protein isolate. The modified protein solutions containing 12.5wt% proteins were used to prepare 30wt% O/W nanoemulsions using high-pressure homogenization. The stability and gelation behaviour of the nanoemulsions were characterized by droplet size, charge, rheology (small and large deformation), and freeze-thaw stability. The effect of protein modification was observed in the droplet size, as the combination of heat and high-pressure homogenization further decreased nanoemulsion droplet size from 600 to 300nm, thus improving the gel strength. In large deformation tests, thermal modification of protein increased true fracture strain from 46.5 (untreated) to 55.3%, indicating improved elasticity. Environmental stresses had significant effects on the properties of the nanoemulsion gels. Heat treatment (90^oC, 30min) and salt addition (0-3wt% NaCl) to the nanoemulsions led to droplet aggregation and an increase in the gel strength. All nanoemulsions gels showed a higher G′ than G″, indicating strong gelation due to the close packing of nano-scale droplets coated with a strong interfacial pre-treated protein layer. In the large deformation test, heat treatment of the nanoemulsion gels reduced the true fracture strain from 55.3 (unheated) to 39.7%, indicating structure breakdown. In contrast, an increased salt concentration decreased the true fracture strain. All gels were self-supporting and did not show any destabilization during four weeks of storage. The nanoemulsions were also freeze/thaw stable, irrespective of protein modification, except for the unsalted nanoemulsions, which showed some oiling-off. Therefore, stable, strong and elastic nanoemulsion gels can be achieved by physically modifying the pulse proteins before emulsification and by selectively applying environmental stresses to the nanoemulsions.