Rheological and structural properties of rice bran protein-flaxseed (Linum usitatissimum L.) gum complex coacervates

Complex coacervation of protein/polysaccharide mixtures has obtained considerable research in the encapsulation of bioactive materials. The rheological properties of the coacervates of rice bran protein/flaxseed gum (RBP/FG) at different pH values including 3.3, 4.0 and 5.3 and varying protein to polysaccharide ratios 3: 1, 6: 1 and 9: 1 were investigated using small amplitude oscillatory shear (SAOS). Moreover, the interrelationship between the zeta-potential, FTIR and viscoelastic properties of the samples was studied. The coacervates showed shear-thinning phenomenon due to the linear reduction of complex viscosity (eta*) by increasing frequency. Furthermore, the highest complex modulus (G*) and more compact coacervate structure were obtained at pH = 4.0, revealing less deformability and flow behavior. All the coacervates showed higher storage modulus (G') than loss modulus (G '') indicating the formation of highly interconnected gel-like structure and were fitted by the Power law model. The maximum fracture stress was obtained at pH 4.0 revealing the highest intermolecular interactions between RBP and FG. The absolute zeta-potential of the coacervate at pH 4.0 and R = 9: 1 was close to 0 and less than the other samples in which the highest fracturability and gel strength of the complex coacervate was achieved. It seems this property of the coacervate would be suitable for encapsulation of bioactive materials. The high aggregation of coacervates at pH 4.0 makes them more readily neutralized by protein binding. FTIR results showed that the spectrum of the coacervate was different from each individual biopolymer, related to their compatibility and intermolecular interactions between the functional groups of RBP and FG. Consequently, RBP/FG coacervates can be managed to some extent to achieve proper textural properties in dairy formula through manipulating pH and biopolymer mixing ratio.