Changes of fibrous structure of high-moisture extruded pea protein isolate for three cooling die geometries

High-moisture extrusion has emerged as a promising technology for producing plant-based meat products with meat-like fibrous structure, wherein the cooling die is a key component. This study investigated the impact of three die dimensions (circular, square, and flat) on system response parameters, protein conformation changes, and the textural quality of pea protein isolate (PPI) extrudates. Results showed that the flat die with larger outlet dimensions, demonstrated reduced specific mechanical energy (669.99 kJ·kg-1) due to higher die pressure (4.34 MPa) and mass flow rate (299.40 g·min-1). Extrudates from the flat die exhibited a slender, tightly arranged fibrous structure, with enhanced mechanical properties and a maximum tensile resistant force of 0.30 kg. Moisture distribution (including bound and non-flowing water) was also significantly affected by the die wall. For the protein structural changes, the α-helix content in circular and square die extrudates was lower than that in flat die extrudates, with significant variations in secondary structure observed between the center and edge regions. However, no significant changes were found for the protein molecular weight. These findings highlight the importance of die wall area and distance from the wall in controlling extrudate quality, particularly in texture adjustments. This study provides a reference for optimizing fibrous structures by designing the die dimension.