Molecular structure and fluid properties of native and enzymatically cross-linked caseins after spray drying

• Multiscale analysis correlates cross-linking with nano-structure of casein micelle. • Cross-linking impacts the shape of the powders not the total rehydration time. • Cross-linking decreases size of the micelle and avoids the formation of aggregates. • Cross-linking significantly improves viscosity even after spray-drying. Milk casein powder (MPC) is used in several food applications for its functional properties and to increase the protein content of the final products. Microbial transglutaminase (mTGase) is an enzyme that catalyzes the formation of intra-molecular cross-linking of the casein micelle (CM), which increases the micelle stabilization against dissociation caused by acid, urea, high pressure, and thermal treatments. However, almost all these properties have been measured directly after the enzymatic treatment and very few after a typical semi-industrial spray-drying and re-hydration process. This study aimed to determine the effect of spray-drying on casein micelles previously treated with mTGase. Four micellar casein retentates with different cross-linking degrees (ΔDP = 0, 36, 48, 58 %) have been produced and then submitted to the spray-drying operation. The macroscopic characteristics of the cross-linked MCP in a solid state and the molecular characteristics of the CM, after rehydration in water, were evaluated. Mild mTGase treatment led to slight slower kinects of rehydration, but the highest cross-linking rates led to a faster kinects of rehydration. After rehydration, cross-linked MCP dispersions depicted smaller viscosity values than the non-cross-linked MCP. Nano scale analysis indicates that mTGase has contributed to a decrease in size and increase sphericity of the caseins micelles. SAXS results showed that cross-linking does not induce an extensive modification of the CM at the small scale. The outcomes of this research suggest that mTGase treatment maybe an interesting strategy for industrial applications.