Rheological Behavior of Food Emulsions Mixed with Saliva: Effect of Oil Content, Salivary Protein Content, and Saliva Type

In this paper, we studied the effect of saliva on the rheological properties of β-lactoglobulin- and lysozyme-stabilized emulsions, prepared at pH = 6.7 in relation to variation of emulsions- and saliva-related parameters. The effect of oil–volume fraction (2.5% w/w to 10% w/w), salivary protein concentration (0.1 to 0.8 mg ml⁻¹), and the use of both stimulated and unstimulated saliva was investigated. Viscosity and storage modulus were measured before (η _emul and G′_emul, respectively) and after addition of saliva (η _mix and G′_mix). To better estimate the changes due to saliva-induced flocculation of the emulsions, the ratios η _mix/η _emul, G′_mix/G′_emul were calculated. In addition, tan δ (=the ratio of the loss and storage moduli) was investigated to evaluate the viscoelastic behavior of the emulsion/saliva mixtures. Increasing the oil–volume fraction and salivary protein concentration resulted in an increase in η _mix/η _emul and G′_mix/G′_emul, while a decrease in tan δ of the emulsion/saliva mixtures is occurring. When compared with unstimulated saliva, mixing β-lactoglobulin-stabilized emulsions with stimulated saliva led to a reduction in η _mix/η _emul and G′_mix/G′_emul, and an augment of tan δ at all measured deformations. In case of lysozyme-stabilized emulsions, the use of stimulated saliva increased G′_mix/G′_emul for γ < 3 when compared to unstimulated saliva. The effect of stimulated saliva on the η _mix/η _emul and tan δ in this mixture is similar to that of unstimulated saliva. These results indicate that the influence of stimulated saliva on the rheological parameters of emulsion/saliva mixtures largely depends on the type of emulsions. To conclude, our findings demonstrate that the rheological behavior of emulsions upon mixing with saliva is greatly affected by both saliva and emulsion properties.