Emulsion-templated liquid oil structuring with soy protein and soy protein: κ-carrageenan complexes

In this research, we report on the use of unmodified proteins and protein:polysaccharide complexes to prepare oleogels through an emulsion-templated approach. The approach consists of the production of highly concentrated oil-in-water emulsions (phi(oil) = 0.6), followed by the evaporation of the water phase which results in the trapping of the oil in a physical network of biopolymers (protein or protein:polysaccharide complexes). As a model system, soy protein isolate (SPI) was chosen as protein and kappa-carrageenan (kappa-CG) as charged polysaccharide. The characterization of the colloidal aspects of the complex formation revealed that factors such as protein:polysaccharide ratio, pH and the ionic strength strongly influenced the zeta-potential, microscopic appearance and stability of the complexes. Complexes formed at an optimized SPI:kappa-CG ratio (15:1) were used for stabilizing concentrated emulsions (phi(oil) = 0.6) and resulted in a better long-term stability of emulsions compared to un-complexed SPI as confirmed from light scattering and microstructure studies. Rheological studies, protein load measurements and cryo-SEM imaging further revealed that the enhanced stability was due to interfacial accumulation of the complexes rather than the increase in the bulk viscosity. On removal of water from these emulsions, a solid-like matrix of dried oil was obtained which could be further sheared into oleogels. The rheological properties of the oleogels determined through oscillatory measurements revealed a good gel strength (G' > 40 000 Pa) and unique microstructural properties (liquid oil structured with a network of water-soluble protein). Interestingly, contrary to emulsion stability, uncomplexed SPI resulted in firmer oleogels compared to SPI:kappa-CG complexes. This research proves hydrophilic proteins can be used as structuring agents for hydrophobic liquids, such as liquid oil. (C) 2016 Elsevier Ltd. All rights reserved.