Sequential modulation of pH and ionic strength in phase separated whey protein isolate – Pectin dispersions: Effect on structural organization

• Shearing of mixtures of whey protein isolate (WPI) and pectin yielded a dispersion of WPI droplets in a pectin phase.
• At intermediate pH, core–shell structures were formed from such a dispersion.
• At low pH, the dispersion transitioned to compact WPI – pectin aggregates.
• Addition of salt at high pH led to clustering of WPI droplets.
• Lowering of pH in salt-containing WPI dispersions led to formation of a more extended shell.

The effect of pH change, salt addition or sequences of pH change followed by salt addition or vice versa on the structure of a whey protein isolate – pectin dispersion obtained from a phase separated system after mild homogenization was studied. Solutions of whey protein isolate and pectin were combined at pH 6.1, allowed to phase separate and then mildly homogenized to form a dispersion consisting of whey protein isolate droplets in a surrounding pectin phase. The dispersion was then subjected to a pH change (6.1 → 5.2 → 3.2), to an addition of NaCl (0–200 mM) or to a combination thereof. The systems morphology, size, appearance, and rheology were then characterized by optical and confocal microscopy, static light scattering, turbidity measurements, visual inspections and steady shear rheometry to gain insights into structural rearrangements. Results indicated profound alterations in the structure of dispersion both when pH was changed and when NaCl was added. Formation of core–shell structures from the whey protein isolate droplets were observed when pH was decreased while clusters of droplets were formed when NaCl was added. The type of sequence (pH then salt or salt then pH) had no influence on the morphology of the structures created. Results were discussed in the context of changes in molecular interactions between the two biopolymers involved in transitions from segregative to aggregative phase separation processes. The gained insights may be of importance to food manufacturers intended on creating new structures from mixtures of proteins and carbohydrates.

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