The effect of nitrogen injection on the structure and textural properties of casein-based extrudates

This study introduces a novel micro-foaming extrusion process for the production of innovative cheeses by injecting nitrogen (N₂) (0, 0.05, 0.10, and 0.15% w/w) into a hot plasticized casein melt sheared at three different screw speeds (250, 300 and 350 RPM) during high-moisture extrusion. Structural and mechanical properties of the extrudates were assessed using X-ray microcomputed tomography (μCT), low-field nuclear magnetic resonance (LF-NMR), and tensile and cutting force tests. Macroscopic and μCT images revealed small and larger coalesced pores in extrudates with 0.05% w/w N₂ and ≥0.10% w/w N₂, respectively. The porosity ranged from 8.5 to 43.3% and increased with N₂ injection. LF-NMR revealed that water molecules of less unbound water (T_2.2) increased (from 19.9 ± 0.2 to 21.3 ± 0.3 ms) with increasing N₂ and screw speed (from 250 to 350 RPM), confirming the more open porous protein structure in micro-foamed extrudates. Tensile tests showed that micro-foamed extrudates exhibited lower tensile strength (1.3 ± 0 to 0.6±0 N) and extensibility (36.9 ± 2.5 to 10.9 ± 1.9 mm) compared to non-foamed extrudates (2.9 ± 0.2 to 2.1 ± 0.2 N, 38.3 ± 1.3 to 20.1 ± 1.4 mm) in both perpendicular and parallel directions. The cutting force test revealed a reduced hardness (8.7 ± 0.7 to 3.4 ± 0.3 N) once N₂ was injected into the protein network structure. Anisotropic index ranged between 0.97 ± 0.0 to 2.52 ± 0.28, exhibiting mechanical properties like other protein-based extrudates. This novel extrusion micro-foaming process creates new processing routes for developing and controlling the texture of extruded cheeses and high-protein products.