Impact of pH and temperature on whey protein-proanthocyanidin interactions and foaming properties

Polyphenols form complexes with proteins that may improve whey protein foaming properties by altering their interfacial properties. This study aimed to identify the influence of partial protein denaturation (via pH shifts and pre-heating to 80 °C) on the complexation between whey protein isolate (WPI) and sorghum proanthocyanidins (PA) and the foaming properties of the resulting complexes. Increased turbidity and quenched fluorescence indicated that complexes were formed between PA and WPI or heated WPI (HWPI) at pH 3, 5, and 7. WPI bound most of the PA, but heat and pH treatments affected the extent of interaction. SDS-PAGE results indicated that there were mainly non-covalent interactions between WPI and PA, though some covalent interactions did occur. Dynamic light scattering results showed that the particle size increased for all the dispersions after adding PA, except for WPI at pH 7 diameters of 10–100 nm. Formation of these condensed, smaller particles is supported by atomic force microscopy images. Additionally, heat treatment resulted in a more compact structure of WPI and the WPI-PA complexes. Surface hydrophobicity of WPI decreased after adding PA, whereas surface tension decreased for WPI with PA at pH 5 but remained unchanged for other samples. With the addition of PA, the foamability of WPI at pH 5 was increased, but the foam stability was only increased for WPI at pH 7. Heat treatment increased the foaming properties of WPI at pH 7 but adding PA did not affect the foaming properties of HWPI. Overall, the structure and foaming properties of WPI-PA complexes are affected by pH and pre-heat treatment, indicating processing parameters can be controlled to produce complexes with desired functionality.