From gels to high-moisture extrudates: pH and 11S:7S globulin ratio govern multiscale structure formation

Soy protein isolates (SPIs) and concentrates (SPCs), which primarily consist of glycinin (11S globulin) and βjls-end-space/-conglycinin (7S globulin), are the dominant ingredients in the production of plant-based meat products using high-moisture extrusion (HME). Yet the links between their gelation and structure formation under HME are not fully understood. In this work, we employ small-angle scattering (SAS) techniques, complemented by rheology, on heat-set gels to elucidate the multiscale aggregation pathways of fractionated 11S and 7S globulins, and relate these to the extrudate structure. Heat-set gels revealed a three-level structural hierarchy: compact protein units that persist after heating; aggregates whose mid-scale size and morphology vary with pH; and networks characterised by clustering strength. Near the isoelectric point (pI), both globulin-enriched fractions formed particulate gels. Under these conditions, 7S globulin-enriched gels were stiffer because their smaller aggregates created a higher density of contacts per unit volume, resulting in a higher clustering strength. Away from the pI, fibrillar aggregation dominated, and 11S globulin-enriched gels became stiffer as a result of forming thicker and more tightly connected strands. At pH 7, blending experiments revealed that even small additions of 7S globulin weaken the gels; however, under HME, the same additions enhance anisotropy. Specifically, 7S globulin promotes deformability and alignment, while 11S globulin contributes to network strengthening through covalent crosslinks. Together, these results establish parallels between heat-induced gelation and extrusion, highlighting the complementary roles of 11S and 7S globulins in structuring high-moisture extrudates of soy proteins.