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Acid-induced gelation of fractal aggregates with different sizes formed by preheating whey protein isolate solutions has been investigated in the protein concentration range 10–60 g/L as a function of the net protein charge density (α). Homogeneous gels could be formed in a range of α that depended on the heating time and temperature. Decrease of the negative charge density of the proteins increased the rate of gelation, but lowering α to less negative values than approximately −2.5 by adding HCl increased turbidity of the solutions and led to syneresis. Strong syneresis also occurred at more negative net charge densities (higher pH) if the heat treatment was too severe. The elastic modulus of homogeneous gels increased with increasing protein concentration. The heterogeneity of the gels decreased with increasing protein concentration. The temperature dependence of the gelation rate was characterized by an activation energy of 155 kJ/mol independent of the charge density, protein concentration and aggregate size. Increase of the temperature increased the gelation rate, but did not influence the elastic modulus of the gels. The present findings on acid-induced gelation are compared with gelation of the same WPI aggregates induced by adding CaCl2 reported elsewhere.

International audience; Gelation of aqueous solutions of casein micelles (MC) in mixtures with fractal aggregates of whey protein isolate (WPI) has been studied first by addition of different amounts of aggregates at a fixed concentration of micelles and then by substitution of micelles with aggregates at a fixed total protein concentration. Addition of small amounts of aggregates (1–5 g/L) to suspensions of micelles (20–60 g/L) at a fixed pH led to a strong decrease of the gelation temperature (Tg) and to an increase of the elastic modulus (G′) of the mixed gels. A minimum in Tg and a maximum in G′ as a function of the WPI fraction were obtained in mixtures with fixed total protein concentration and fixed pH and explained by the opposing influence of increasing WPI aggregate concentration and decreasing concentration of micelles. Increasing the total protein concentration at the same WPI fraction caused a decrease of Tg and increase of G′. Decreasing the pH at the same protein composition led to a decrease of Tg but caused an increase of G′. Tg decreased sharply if a small amount of CaCl2 was added, but if more than about 30 mM of calcium was added, Tg increased again. Gelation of mixtures did not depend significantly on the size of the WPI aggregates, but if fractal aggregates were replaced by microgels, weaker gels were produced and the temperature of gelation was higher. Potentiometric titration curves showed that protonation of the micelles and of the WPI aggregates in the mixtures was independent. It is concluded that MC and WPI aggregates co-aggregate during gelation and there is a transition between gelation at a critical temperature for pure MC suspensions and gelation controlled by an activation energy for pure WPI aggregate suspensions.

Potentiometric titration curves were obtained for aqueous solutions of native whey protein isolate (WPI) and β-lactoglobulin (β-lg) at different NaCl concentrations. The curves crossed at approximately the same pH, which is considered to be the isoionic point (IIP). Titration with NaCl confirmed that the pH was independent of the salt concentration up to 0.1 M at the IIP, which was 4.9 for WPI and 5.0 for β-lg. Fractal aggregates and microgels of different sizes were formed by heating protein solutions at different pH and different concentrations. The titration curves of the aggregates depended on the type of aggregates, but not on their size. The IIP increased by at most 0.2 pH units after aggregation. For a given pH larger than IIP, the charge density of the proteins (α) was reduced after denaturation and aggregation. The reduction was stronger for microgels than for fractal aggregates. Addition of NaCl or increasing the protein concentration mitigated the effect. Comparison between WPI and β-lg showed that the pH dependence of α was almost the same for pH > 5.0 both for native and aggregated proteins.

International audience; Gelation of whey protein fractal aggregates was induced by simultaneously adding salt (CaCl 2 or NaCl) and acid (HCl). Further, simultaneous addition of both specifically binding (Ca 2þ) and non-binding (Na þ) cations was studied at a fixed protein charge density. The effect of adding mixtures of ions on pH, gelation kinetics, elastic modulus and gel microstructure was investigated. For all studied systems the time of gelation (t g) had an Arrhenius dependence on temperature, characterised by an activation energy (E a). The value of E a depended on the type and concentration of added ions. The elastic modulus of gels was found to be independent of the ion composition, reaching 0.2e0.3 kPa at a protein concentration of 40 g L À1 and 1 kPa at 60 g L À1. At conditions of strong electrostatic repulsion between aggregates , addition of NaCl reduced t g. However, when the repulsions were weak, further addition of NaCl increased t g .

International audience

Solutions of the globular whey protein β-lactoglobulin (β-lg) were heated at different protein concentrations leading to the formation of polydisperse fractal aggregates with different average sizes. The structure of the solutions was analyzed with light scattering as a function of the protein concentration. The osmotic compressibility and the dynamic correlation length decreased with increasing concentration and became independent of the aggregate size in dense suspensions. The results obtained for different aggregate sizes could be superimposed after normalizing the concentration with the overlap concentration. Dense suspensions of fractal protein aggregates are strongly interpenetrated and can be visualized as an ensemble of fractal 'blobs'. The viscosity of the heated β-lg solutions increased extremely sharply above 80 g L(-1) and diverged at 98 g L(-1), mainly due to the sharply increasing aggregate size. At a fixed aggregate size, the viscosity increased initially exponentially with increasing concentration and then diverged. The increase was stronger when the aggregates were larger, but the dependence of the viscosity on the aggregate size was weaker than that of the osmotic compressibility and the dynamic correlation length. The concentration dependence of the viscosity of solutions of fractal β-lg aggregates is much stronger than that of homogeneous β-lg microgels. The behavior of fractal aggregates formed by whey protein isolates was similar.