Relative contributions of charge and surface coverage on pH-induced flocculation of protein-stabilized emulsions

To predict the stability of protein-stabilized emulsions against flocculation under different conditions (pH and concentration), a quantitative description of the effect of the relevant factors is essential. Typically, pH is considered to affect the protein charge (viz. zeta potential) and thereby the interactions between the emulsion droplets. In this study, it is shown that emulsion flocculation is not only determined by the interactions between the droplets (pH/charge), but also by the surface coverage (pH and protein concentration). Two distinct regimes of flocculation were identified. At zeta potentials of |9-27|mV, emulsions were stable against flocculation if the protein concentration was sufficiently high to fully cover the interface (C>Ccr). At a lower zeta potential (i.e. below a critical zeta potential ζcr of |9|mV), flocculation occurred even at high concentrations. In this regime, flocculation below ζcr was reversible at C>Ccr, while it was partly irreversible at Ccr, indicating a type of bridging flocculation at Ccr. This shows that emulsion flocculation can be estimated based on the relevant parameters (ζ and protein radius including the association behaviour of the protein).