Adsorption of oat proteins to air–water interface in relation to their colloidal state

• Oat proteins effectively lowered the surface tension at air–water interface.
• Surface tension was lower at neutral pH compared to that of at alkaline pH.
• Interfacial layer at neutral pH contained large protein aggregates/clusters.
• Transglutaminase improved dispersion stability against dissociation/re-association.
• Transglutaminase caused increased negative charge and higher surface tension.

The colloidal size and charge of oat protein isolate (OPI) at pH 7.2 and 9.0 with/without transglutaminase (TG) treatment were investigated and related to its surface activity at air–water interface. OPI was prepared from defatted oat flour. It was dispersed in water (pH 7.2), or in buffer (pH 9.0) and the soluble fraction (supernatant after centrifugation at 10 000 × g) was used for particle size, ζ-potential and dynamic surface tension measurements. Dispersions were found to be electrostatically stable (ζ-potential > −35 mV) with average particle sizes of ∼70 nm at pH 7.2 and ∼30 nm at pH 9.0. When diluted at pH 7.2, dissociation and re-association of the particles occurred resulting in increased polydispersity and increased size, while at pH 9.0 particle size and ζ-potential were unchanged after dilution. Dynamic surface tension measurements revealed slower adsorption dynamics and higher final surface tension values at pH 9.0 than at pH 7.2. TG-treatment of OPI dispersion resulted in formation of inter-molecular covalent linkages and led to decreased average particle size and increased stability against dilution in OPI dispersions at neutral pH. TG-treated proteins showed increased negative charge at pH 7.2 and resulted in higher surface tension values compared to the untreated samples. The topography images of adsorbed or spread layers of OPI at a/w interface revealed that oat globulins were effectively adsorbed and existed as monomeric particles or formed aggregates at the interface depending on pH.

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