## 5 CFD in Extrusion Processes  
### Extrusion Mechanisms in Food Processing  
Extrusion is a high-intensity thermomechanical process that transforms raw food materials into structured, texturized, and shelf-stable products by forcing them through a shaped die under precisely controlled conditions of heat, pressure, and shear. It integrates several unit operations--mixing, cooking, shaping, and expansion--into a single continuous system, making it one of the most versatile technologies in modern food processing. Common applications include the production of breakfast cereals, snacks, pasta, pet food, and increasingly, plant-based meat analogues. Understanding the underlying mechanisms of extrusion is essential for accurate process control, effective product design, and reliable computational modeling. These mechanisms arise from complex interactions between material properties, screw configuration, process parameters, and equipment geometry (Table 2) [34]. The extrusion process can be conceptually divided into several sequential zones. In the feeding and conveying zone, raw materials, typically in powder or granular form, are introduced into the extruder via a hopper. Here, the rotating screw transports the material forward, often resulting in compaction and increased bulk density, though no significant thermal transformations occur at this stage. In the compression and melting zone, the channel depth of the screw decreases, leading to increased pressure and shear. Mechanical energy is converted into heat, inducing partial melting or plasticization of the food matrix. Water within the system serves both as a lubricant and a medium for heat transfer, and under pressure, superheated steam may form [89]. As the material advances, it enters the mixing and homogenization zone, where shear elements or kneading blocks enhance both dispersive and distributive mixing, ensuring uniform temperature distribution and consistency. In twin-screw extruders, especially with co-rotating intermeshing screws, intense mixing is achieved through controlled residence time and precise flow management. Further along the barrel, thermomechanical energy initiates a series of physicochemical transformations--such as starch gelatinization, protein denaturation, lipid melting, and Maillard or enzymatic reactions--which are critical to the development of product texture, nutritional quality, and flavour [52]. In the final pressure build-up and shaping zone, the material is compressed against the die, creating the high-pressure conditions necessary for flow through the restricted orifice. Upon exiting the die, a sudden pressure drop triggers flash evaporation of water, causing the product to expand and acquire a porous structure. The extrudate is then shaped and cut to the desired length using rotating knives located at the die face. Subsequent cooling stabilizes the structure, preventing collapse or deformation and locking in the final texture [90]. Mechanical and thermal interactions within the extruder are highly coupled and nonlinear. Shear forces generated by screw rotation promote mixing, reduce apparent viscosity, and contribute to internal heating via viscous dissipation. Friction between the material, screw, and barrel further elevates the temperature. External barrel heaters are used to establish and control temperature profiles, especially in low-moisture formulations. The rate and efficiency of heat and mass transfer are determined by the material's viscosity, thermal conductivity, and specific heat capacity. Many food materials exhibit shear-thinning or viscoelastic behavior, adding additional complexity to the control and modeling of extrusion dynamics [91]. Screw design has a profound influence on process performance. Key parameters include screw diameter, pitch, and channel depth, as well as the configuration and sequence of conveying, mixing, and reverse elements. The screw speed and the length-to-diameter (L/D) ratio also affect energy input and residence time.