Influence of protein and mineral composition on the formation of whey protein heat-induced microgels

The aim of the present study was to investigate the effect of the initial protein and mineral composition on the formation of whey protein microgels at a protein concentration of 4 wt%. As a model system, both demineralized α-lactalbumin (α-la) and β-lactoglobulin (β-lg) were mixed at 20/80, 50/50 and 80/20 α-la/β-lg ratios and heated (85 °C, 15 min) in the pH range 5.7–6.2. Pure dispersions of α-la were not forming microgels but rather precipitates (12%) and a majority of soluble aggregates (83%). Upon mixing with β-lg, microgels were only obtained for the 20/80 ratio at pH 5.7 or for pure β-lg at pH 5.8. Interestingly, the latter ratio was close to the natural composition occurring in most of the commercial whey protein isolates (WPIs). For higher ratios, particles were more or less aggregated and not spherical. As for α-la alone, most of the denatured proteins formed soluble aggregates. These results might be explained by the fact that α-la reduced the hydrophobic attractive forces involved in microgel formation, shifting the equilibrium towards the soluble aggregates pathway. This assumption was confirmed by the very low surface hydrophobicity of the soluble aggregates determined by ANS binding experiments. The conversion yield from native protein into microgels was about 51% for the 20/80 α-la/β-lg model mixture, whereas it was higher (70–85%) for commercial WPIs under similar experimental conditions. To test the possible effect of the initial mineral composition of the commercial WPIs, these products were demineralized and their ability to form microgels was tested again. After demineralisation, WPIs behaved very similarly to the 20/80 α-la/β-lg model system, the yield of conversion into microgels dropping to about 65% at pH 5.7. It was therefore concluded that in addition to the α-la/β-lg mixing ratio that was controlling hydrophobic interactions between denatured proteins that was accounting for about 50% of the microgel conversion yield, the initial mineral composition of WPIs contributed to another 20–35% yield as it was modulating the repulsive forces between the denatured proteins, promoting aggregation.

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