Mechanism and optimization of NaCl removal from soy sauce residue using low pressure superheated steam drying: Role of condensation and moisture migration.

Soy sauce residue (SSR), a high-salt byproduct of condiment production, poses environmental and valorization challenges due to its NaCl content. The high salinity hinders its direct utilization in applications such as animal feed or fertilizer. This study introduces low-pressure superheated steam drying (LPSSD) for simultaneous dehydration and desalination. NaCl removal was driven by condensation-induced dissolution and steam-flow convection, with moisture as the key carrier. LPSSD achieved a 34.8% NaCl reduction, significantly surpassing vacuum drying (p < 0.001). Condensation and multi-stage condensation further enhanced desalination by rebuilding concentration gradients, while increased steam-material contact area improved efficiency. A robust kinetic model linked NaCl content to moisture ratio (R2 > 0.92), revealing a three-stage process. Stratified experiments confirmed upward salt migration during condensation and convective transport thereafter. Compared with washing-based desalination, LPSSD can reduce wastewater generation associated with process water use and may provide an eco-friendlier alternative for SSR valorization.