Formation and functionality of whey protein isolate–(kappa-, iota-, and lambda-type) carrageenan electrostatic complexes

Abstract

The formation of electrostatic complexes between whey protein isolate (WPI) and (κ-, ι-, λ-type) carrageenan (CG) was investigated by turbidimetric measurements as a function of pH (1.5–7.0), biopolymer weight-mixing ratio (1:1–75:1 WPI:CG) and NaCl addition (0–500 mM) to better elucidate underlying mechanisms of interaction. Emulsion stabilizing effects of formed complexes was also studied to assess their potential as emulsifiers. Complex formation followed two pH-dependent structure-forming events associated with the formation of soluble (pH c) and insoluble (pH ϕ1) complexes. For both the WPI–κ-CG and WPI–ι-CG mixtures, pH c and pH ϕ1 occurred at pH 5.5 and 5.3, respectively, whereas in the WPI–λ-CG mixture values were slightly higher (pH c = 5.7; pH ϕ1 = 5.5). In all mixtures, maximum turbidity was found to occur near pH 4.5, before declining at lower pHs. Biopolymer mixing ratios corresponding to maximum OD was found to occur at the 12:1 ratio for both the WPI–κ-CG and WPI–λ-CG mixtures, and 20:1 ratio for WPI–ι-CG mixture. The addition of NaCl disrupted complexation within WPI–κ-CG mixtures as levels were raised, whereas when ι-CG and λ-CG was present, complexation was enhanced up to a critical Na + concentration before declining. Adsorption of CG chains to the small WPI–WPI aggregates during complexation was proposed to be related to both the linear charge density and conformation of the CG molecules involved. Emulsion stability in the mixed systems (12:1 mixing ratio), regardless of the CG type (κ, ι, λ), was significantly higher than individual WPI solutions indicating enhanced ability to stabilize the oil-in-water interface.