Casein network formation at oil–water interfaces is reduced by β-casein and increased by Ca2+

Mixtures of bovine caseins can serve as a benchmark for understanding the functionality of microbial-based recombinant caseins at oil–water interfaces. In this work we show that, in the presence of Ca${}^{2+}$, the individual casein fractions form viscoelastic networks at the oil–water interface with comparable stiffness. In the absence of Ca${}^{2+}$, ${\alpha }_{s2}$- and $\beta$-casein interfacial network formation was strongly inhibited over the full deformation regime. For ${\alpha }_{s1}$-casein, the network stiffness was increased in the absence of Ca${}^{2+}$ at small deformations ($<$15%), but at large deformations ($>$50%) it was completely disrupted, to a similar stiffness as ${\alpha }_{s2}$- and $\beta$-casein. The interfacial structure formed by $\kappa$-casein was largely unaffected by Ca${}^{2+}$ due to limited phosphorylation. We hypothesize that the differences between calcium-sensitive caseins lie in the conformation they assume at the interface. Both ${\alpha }_{s2}$- and $\beta$-casein adsorb in a train-tail conformation with a tail extending into the aqueous bulk phase, whereas ${\alpha }_{s1}$-casein adsorbs in a loop-train conformation, with a loop that extends less into the bulk phase. The tail-train configuration is hypothesized to increase the inter-molecular Ca${}^{2+}$ bridging thereby increasing the interfacial stiffness of ${\alpha }_{s2}$- and $\beta$-casein.

Blending the casein fractions revealed a strong negative effect of $\beta$-casein on the interfacial modulus, which was more pronounced at a higher concentration. The presence of Ca${}^{2+}$ remained important for interfacial network formation of a casein blend. Without Ca${}^{2+}$, the interfacial network was less stiff, more viscous, and behaved like a 2d polymer solution.

With this work we showed that casein interfacial network formation at oil–water interfaces is mediated by Ca${}^{2+}$ bridging. Blending the different casein fractions decreased the interfacial viscoelastic properties through the presence of $\beta$-casein. These results indicate that future work on recombinant caseins should focus on single genetic variants, since a blend of variants will likely decrease interfacial functionality.