Aggregation and gelation properties of preheated whey protein and pectin mixtures at pH 1.0–4.0

• Preheating whey protein (WP) changed the properties of complexing with pectin at pH 1.5–4.0.
• Aggregation of preheated WP at pH 1.5–4.0 was enhanced by a larger amount of pectin.
• Pectin induced gelation of preheated WP at pH 1.5 to 3.0 that varied in microstructures.
• Electrostatic attraction between the two polyelectrolytes was important to gelation.
• Calcium ions strengthened the gel network of mixtures with preheated WP and pectin.

The objective of this work was to understand aggregation and gelation properties of mixtures with preheated whey protein (hWP) and pectin at 1:0–5:1 mass ratios quickly acidified to pH 1.0–4.0 with HCl under agitation. Aggregation properties were studied by measuring turbidity and zeta-potential of hWP–pectin mixtures at pH 1.0–4.0. Gelation properties were characterized for visual inspection of gel formation in vials and small strain oscillation tests. Temperature and time sweeps, also with addition of 5 mM N-ethylmaleimide or 10% urea, were used in rheological tests to understand molecular forces causing gelation. The quickly-acidified gels had less homogenous microstructures and lower strengths than those using slow acidification enabled with glucono-delta-lactone and without agitation. hWP, pectin, and their mixtures had different gelation properties at pH 1.0–4.0. At the studied conditions, pectin neutralized positive charges of hWP, induced gelation at pH 1.5–3.0, and changed properties of hWP gels. hWP gelled at pH 3.5–4.0, while the hWP–pectin mixtures did not. Gelation of hWP alone and its mixture with pectin involved different molecular interactions. Both physical forces (hydrophobic attraction and hydrogen bonds) and disulfide bonds were important to the gelation of hWP at pH 3.5–4.0, while physical forces dominated the gelation of hWP–pectin mixtures, with electrostatic attraction between the two polyelectrolytes playing a significant role. The findings suggest the gelation properties of hWP–pectin mixtures at pH 1.0–4.0 are dependent on the biopolymer ratio and composition.

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