Self-assembly of oat proteins into various colloidal states as function of the NaCl concentration and pH

Abstract

Oats are increasingly used in a range of liquid and semi-solid foods. Insights into the behavior of native oat proteins in aqueous media are crucially important to understand their functionality, but are sorely lacking. Here, for the first time, the impact of varying NaCl concentration and pH on the colloidal state of native oat proteins was systematically studied. Non heat-treated oat groats were used as raw material to produce oat protein isolates (OPI). Upon addition of varying concentrations of NaCl to a native OPI solution, oat proteins self-assembled into different colloidal states. Most notably, at <50 mM NaCl, oat proteins formed protein aggregates with a fractal dimension of 1.8, while at 50–300 mM NaCl, oat proteins microphase-separated to form concentrated protein droplets of ≥1 μm. To allow assessing the impact of pH systematically, titration experiments were performed to establish the relation between pH and protein charge density (α). Upon changing the charge density of a native OPI solution, protein microphase separation was found to occur in the region α = −155 (pH 8.2) to α = −118 (pH 7.1). At lower charge densities (and thus pH values), this phenomenon was no longer observed, but protein aggregation occurred, and pronouncedly so at the protein iso-ionic point (α = 0, pH 5.0). In conclusion, oat proteins self-assemble into different types of colloidal structures at varying NaCl concentration and pH. This could have important implications for the properties of oat proteins in a variety of foods and should be investigated further.