Whey protein isolate and flaxseed (Linum usitatissimum L.) gum electrostatic coacervates: Turbidity and rheology

Coacervates were prepared with flaxseed (Linum usitatissimum L) gum (FG) and whey protein isolate (WPI) solutions in water. Structural transitions during coacervate formation were monitored by turbidimetry and zeta potentiometry. Biopolymer mixing ratio R (1:4-15:1, w/w) and pH (6.0-1.4) effects on pH-dependent phase transitions (pH, pH(phi 1), and pH(phi 2)) were examined. Where R = 1:1 (w/w), pH, pH(phi 1), and pH(phi 2) were observed at pH 5.4, pH 5.0, and pH 1.8, respectively. The highest optical density (OD600 = 0.617 +/- 0.009) occurred at pH 3.4, the pH(max). As R increased from 1:4 to 15:1, pH(phi 1) and pH(phi 2) also increased (pH(phi 1) 4.2 to 5.2 and pH(phi 2) 1.8 to 2.2). Accordingly, pH(max) shifted from 3.0 to 4.8 while pH(c) was independent of R. The shift of pH(max) was consistent with isoelectric point (IEP) of the WPI-FG mixture determined by electrophoretic mobility measurements. The maximum WPI-FG coacervate formation occurred at R = 2:1 (w/w) and pH(max) = 3.8 with OD600 = 0.783 +/- 0.018. Dynamic shear viscosity and viscoelasticity of WPI-FG coacervates were determined at a range of pH and R. WPI-FG coacervates exhibited shear-thinning behavior and gel-like properties. The highest dynamic viscosity and viscoelasticity of WPI-FG coacervates were observed at pH 3.8 with R = 2:1 (w/w). Electroneutrality of WPI-FG mixture favored coacervate formation with a more compact structure and improved rheological properties. Light microscopy observations revealed that uncharged WPI-FG mixtures formed coacervates that were more compact than charged coacervates and exhibited improved rheological properties. (C) 2016 Elsevier Ltd. All rights reserved.