Transition cavity structure and SPI-WG ratios mediated the porous fibrous structural morphology changes of high-moisture extrudates

The cooling die is a critical region for forming fibrous structures during high-moisture extrusion, comprising transition and a cooling-forming section. Extant researches predominantly concentrated on the cooling-forming section, with a paucity of information on the transition section. Accordingly, in this study, the cavity structure of the transition section was improved to regulate the structural state of protein texturization within the cooling die. The cooling die was equipped with both compression and enlarged cavity structures for high-moisture extrusion (HME). The effects of different cavity structures on extrusion process response parameters, extrudate colour, textural properties, apparent characteristics of porous fibrous structures (expansion ratio, density and porosity), macro- and micro-structure, internal 3D morphology, water characteristics, and protein aggregation were investigated at varying soy protein isolate (SPI): wheat gluten (WG) blend ratios (10,0, 7,3, 5,5, 3:7 and 0:10). It was revealed that as WG in the raw material formulation increased: the torque, die pressure, melt temperature, and specific mechanical energy (SME) decreased; the output of extrudates increased; extrudate colour darkened; hardness and chewiness decreased; fibrous degree increased; and the water-holding capacity of the extrudates declined. Under suitable raw material ratios, the compressed cavity produced extrudates with darker colour, denser structure, higher hardness, chewiness, and water-holding capacity. Conversely, the enlarged cavity produced extrudates with lighter colour, loose and expanded structure, soft and elastic texture, poor water-holding capacity, and a porous fibrous structure. This study is the first to demonstrate that the transition cavity structure, a previously overlooked geometric factor, acts as a key processing determinant that works synergistically with the SPI-WG ratio to dictate the textural structure of the final product. These findings provide a theoretical and practical basis for improving the structural design of plant-based meat analogues through adjustments to the raw material ratios and the implementation of specific cavity structures.