The influence of milk fat content on the extrusion of rennet casein emulsion gels

The fat content in cheese is important for quality characteristics, like taste, texture, melting properties, and overall appearance. Understanding the role of fat during extrusion of casein emulsion gels will support cheesemakers in developing new products with diverse textures and functionalities. Four rennet casein emulsion gels with varying fat content (1–18% w/w) and a commercial rennet casein ingredient (Cagliata, 26.6% w/w fat) were extruded to investigate the role of milk fat on extrusion process and properties of the extrudate cheese. The extrusion process was assessed via specific mechanical energy (SME), and the extrudates were characterized using dynamic oscillatory rheology, texture profile analysis, confocal laser scanning microscopy (CLSM), and low-field nuclear magnetic resonance (LF-NMR). Local micro-fat separation in higher fat gels (>18% w/w fat) led to wall slip, which consequently lowered the SME from 70.3 ± 4.6 to 61.4 ± 2.9 kJ kg⁻¹. Macroscopic and CLSM images revealed an anisotropic structure in higher fat gels (>18% w/w fat), with elongated fat droplets separating the casein network. LF-NMR revealed that high fat content led to a more open protein network, with higher mobility of tightly bound (T_2.1) and less bound (T_2.2) water, due to increased number of fat droplets and serum pockets. The strain sweep revealed an increase in storage and viscous modulus for extrudates with the highest milk fat contents and an increase in the gel-sol transition temperature from 64.0 ± 0.8 to 75.5 ± 0.7 °C. Texture profile analysis revealed lower force values with increasing fat content, indicating a softening behavior at the macroscopic level. This study provides new insights regarding the effect of fat concentration on structural and rheological properties of extruded casein gels.