Are Oat and Barley Protein Concentrates Always More Surface Active than Raw Extracts?

The study compares the equilibrium surface tension (extrapolated to t → ∞ after 30 min measurements), surface compression (dilatational), rheology parameters (storage and loss moduli, E′ and E″), as well as foamability and foam stability for the raw and processed oat and barley seed extracts. The extracts were obtained at pH "typical" (8.5) and elevated (12.0). The extracts obtained at higher pH showed lower surface tension values (down to 42 mN/m) and ∼5 times higher extraction yield, with the remaining surface activity parameters comparable or slightly worsened. Surprisingly, the protein concentration by isoelectric precipitation did not prove advantageous over the raw extracts in terms of surface activity. On the other hand, acidic hydrolysis (3 h boiling at pH 2.0) enhanced the surface activity of both the oat and barley extracts, especially those obtained at lower pH. Even more importantly, the hydrolysates showed at least 0.5% solubility (expressed as the dry mass of the supernatant phase) even at pH in the proximity of the isoelectric point (IEP) of the starting proteins. In contrast, for the unmodified raw extracts and concentrates, the solubility dropped to practically zero around IEP. Thus, extraction at pH 12.0 combined with subsequent acidic hydrolysis at pH 2.0 (omitting the protein purification step) seems to be the most efficient way to obtain surface-active oat and barley extracts with a high yield.