From nano-aggregates to macroscopic fibers: an integrated framework reconciling phase separation and laminar flow theories in high-moisture extrusion

High-moisture extrusion (HME) has emerged as a key technology for converting plant proteins into meat analogs with fibrous textures resembling animal muscle. Despite growing research interest, the multi-scale mechanisms governing fiber structure formation during HME remain incompletely understood. Plant-protein HME is typically conducted at moisture contents of 50–75% and processing temperatures of 100–170 °C. Within this operating window, the effects of key processing parameters and equipment design factors on shear-field distribution, residence time, and heat transfer are examined. The fiber formation mechanism is then elucidated through a zone-by-zone analysis of protein structural evolution from feeding through melting and into the cooling die. Based on recent window-equipped in-situ SANS measurements and complementary dead-stop and ex-situ SAXS/SANS analyses, an integrated multi-scale framework is proposed. In this framework, fiber formation arises from three coupled processes. Thermal denaturation and nano-aggregation occur in the barrel. Spinodal decomposition driven by temperature gradients produces protein-rich and water-rich domains in the cooling die. Laminar flow subsequently induces the elongation and alignment of these phase-separated domains. This integrated Denaturation–Phase-Separation–Flow (DPSF) framework links nano-aggregate building blocks, spinodal decomposition and laminar flow within a single multi-scale continuum and identifies quantitative criteria for fiber formation in HME.