The interplay between soy proteins and dietary fiber in determining the structure and texture of high moisture extrudates

The (joint) contribution of soy proteins and dietary fibers (DFs) in determining the structure and texture of high moisture extrudates remains largely unclear. Blends of soy protein isolate (SPI) and a soy DF-enriched fraction (SDF) were prepared in different ratios, and their bulk rheology and water mobility distribution when hydrated to 60% moisture were related to the structure, texture, and protein extractability of high moisture extrudates prepared at the same moisture level. Increasing the DF proportion in hydrated SPI-SDF blends resulted in higher storage and loss moduli in linear region. In the non-linear region, however, higher DF levels resulted in more viscous behavior. The mobility of weakly interacting water protons decreased in samples with more DF. High moisture extrudates made from SPI had a layered microstructure consisting of V-shaped lamellae. Extrudates prepared from a blend containing 25% DF had more fibrous appearance with thin, branched protein-rich lamellae. Extrudates containing 52% DF exhibited a macrostructure without the presence of distinct layers or fibers and lacked a continuous protein network in their microstructure. The contribution of disulfide bonds to sustaining the protein network increased in extrudates containing more DF. Extrudates containing more DFs had lower cutting strengths and anisotropic indices. Strong negative correlations were found between the phase angle (obtained at 95 °C) in the non-linear region of the hydrated blends (indicating the relative importance of the viscous response) and the cutting strength of the extrudates in both longitudinal (R = -0.759) and perpendicular (R = -0.883) directions to the flow. In conclusion, the ratio of soy protein to DF had a significant impact on the characteristics of high moisture extrudates, which were associated with alterations in rheological properties, water mobility distribution and protein extractability.