Understanding the effect of pH on structure formation in high moisture extrudates produced from soy protein-dietary fiber blends

The effect of pH on the structure and texture of high moisture extrudates prepared from blends of soy protein isolate (SPI) and a soy dietary fiber-enriched fraction (SDF), achieving different protein-DF ratios, was studied. Samples hydrated to 50 % moisture at pH 3.0, 5.0, and 7.0 underwent rheological analysis following a temperature profile (40–140–60 °C) relevant for high moisture extrusion (HME). At pH 7.0, lower complex viscosity (|η∗|) at 140 °C and steeper |η∗| increase during cooling indicated greater protein-protein interaction disruption at high temperature followed by enhanced intermolecular interaction and/or cross-linking during cooling than at pH 3.0 and 5.0. The use of a lower protein-DF ratio resulted in elevated |η∗| during heating and lower |η∗| after cooling at all pH values, the latter due to DFs interfering with protein network formation. Extrudates produced at pH 7.0 using a Thermo Fisher Process 11 extruder exhibited distinct anisotropic structure and pronounced water syneresis, whereas extrudates produced at low pH had mostly isotropic structure. This was attributed to the lower viscosity and thus greater deformability at the end of the barrel and to more pronounced disulfide bond formation upon cooling in samples at neutral pH, compared to at low pH. The use of a lower protein-DF ratio at pH 7.0 transformed extrudate structures from layered to fibrous and reduced disulfide bond formation. This study demonstrated that pH and protein-DF ratios significantly affect bulk viscosity and disulfide bond formation during extrusion, thereby determining the structure and texture of the resulting extrudates.