This work focused on the Lallemantia royleana seed mucilage (LRSM) effect on the rheological, physicochemical, and microstructural attributes, and the in vivo oral breakdown of cold-set emulsion-filled gel (EFG) based on the whey protein isolate (WPI). Emulsions containing 0.1 and 0.5% LRSM displayed the largest and smallest droplet sizes (3.38 vs. 1.26 ) and polydispersity index (0.60 vs. 0.23), respectively. Also, the 0.1–0.5% LRSM-WPI EFGs compared to the control had greater elastic modulus in the linear viscoelastic (LVE) region (7006.1–7167.8 vs. 4438.3 Pa), flow-point stress (484.6–540.1 vs. 327.8 Pa), network strength (7227.8–7451.2 vs. 5380.1 ), network extension (10.91–11.44 vs. 10.05), lower LVE limit (1.74–2.26 vs. 2.53 %), same loss tangent (0.17), similar frequency dependence of elastic modulus (0.087–0.091 vs. 0.095), and inhomogeneous microstructures. Hardness, springiness, and chewiness at 0.1% and adhesiveness and cohesiveness at 0.5% LRSM-WPI EFGs were the highest. Raising the LRSM from 0% to 0.5% improved the water-holding capacity (75.49%–87.68%), swelling ratio (0.20%–2.59%), and decreased syneresis (11.08%–9.13%). The 0.5% LRSM-WPI EFG compared to the control needed a higher chew duration (16.22 vs. 11.80 s) and chew cycle (24.76 vs. 18.31), and its bolus had a smaller particle size (0.94 vs. 1.65 mm).

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