Colloidal state and emulsifying properties of protein isolates from kilned and non-kilned oats

There is a strong need for basic insights into the colloidal state of oat proteins as function of environmental conditions, and the relationship with functional properties. Relevant in this regard is the commonly applied industrial oat groat heat treatment ('kilning') which is here hypothesized to affect the colloidal and functional properties of oat protein isolates (OPIs). Here, we prepared OPIs from non-heat-treated (NHT) and industrially heat-treated (IHT, kilned) oats and investigated colloidal and emulsifying properties of OPI_NHT and OPI_IHT dispersions, as a function of protein concentration (0.5-2.0% w_protein/v) and pH (7.0-9.0). OPI_NHT and OPI_IHT dispersions at pH 7.0 were more prone to aggregation and had lower colloidal stability than at pH 9.0, attributable to increased electrostatic repulsion at higher pH. Notably, initially increasing dispersion pH to 9.0 prior to lowering the pH to 7.0 improved the dispersion colloidal stability at neutral pH, potentially due to irreversible protein structural changes. Overall, OPI_IHT dispersions were more colloidally stable than OPI_NHT dispersions, presumably due to kilning-induced protein structural changes resulting in pronouncedly aggregated proteins in IHT oats not being recovered in the OPI itself. Oil-in-water emulsions prepared from OPI_IHT dispersions had higher apparent stability than those prepared from OPI_NHT dispersions. Moreover, emulsion stability improved (i) at alkaline conditions, where protein aggregation was reduced, and (ii) upon prior protein aggregate removal by centrifugation. These observations suggest a negative influence of oat protein aggregates on emulsion apparent stability; however, the underlying mechanisms remain unclear and should be studied further. These findings offer critical insights into how oat kilning influences the protein colloidal and emulsifying properties.