Tuning of rheological behavior of soybean lipophilic protein-stabilized emulsions

Soy lipophilic protein (LP) is a protein fraction mainly composed of a phospholipid-protein complex isolated from defatted soy meal. This work investigated effects of seasoning materials, i.e., NaCl, sucrose, and acetic acid, on rheological properties of LP-stabilized O/W emulsions. LP dispersions in water were homogenized with soybean oil to create LP-stabilized emulsions of which added oil was 50 wt%. LP emulsions immediately flocculated but resisted coalescence at least for 8 weeks. An addition of NaCl and sucrose separately decreased emulsion viscosity probably due to salting-in and hydration effects, respectively. LP emulsions initially in a liquid form at pH 7.2 became semisolid at pH 5.7 by stepwise acidification, and thereafter returned to liquid by further acidification below pH 4.2. The drastic change was attributed to insufficient inter-droplet electrostatic repulsion within the pH range (pH 5.7–4.2) according to the zeta-potential analysis. The storage modulus (G′) of the solidified emulsions was higher than the loss modulus (G″) and not strongly dependent on frequency, indicating internal network structure. Cryo-SEM clarified that acidification neither caused more inhomogeneous distribution nor promoted coalescence of oil droplets, revealing that the acid-induced solidification resulted from enhanced non-covalent interactions between contacted and packed oil droplets. The solidified emulsion was stable against coalescence for at least 8 weeks under a commercial shelf-life condition. These findings show that LP can create stable emulsions highly tunable regarding rheological behavior, thereby emphasizing potential of LP for application to various food emulsion products varying in texture from liquid to semisolid.