A unique characteristic of the world-popular mozzarella cheese is its fibrous structure, which is traditionally created during the cooking-stretching process. During this process, the shear forces applied to the cheese curd during kneading and extrusion play a key role in the final structure of the cheese. Recently, extrusion technology has gain industrial interest to structure plant protein products, while applications to dairy products like cheese are still limited. Developing dairy products with innovative structures to bring new experiences to consumers, is of great importance to the dairy industry. To our knowledge, limited studies are conducted regarding the effect of shear forces induced in the cooker-stretcher or twin-screw extruder on mozzarella structure formation. Hence, this PhD project focuses on understanding structure formation in mozzarella cheese curd during shear processing in a traditional cooker-stretcher and during extrusion in a twin-screw extruder. A variety of methods were developed to gain insight into curd structure and its relationship with functional properties of the cheese. To understand the traditional mozzarella cooking-stretching process effect on cheese properties, the cooking residence time (CRT), and the stretching residence time (SRT), of the cheese curd in a cooker-stretcher, which not only represent the interaction between water temperature and screw speed, but also reflect the applied thermal energy and shear forces, were quantified. The results showed that cheese composition, anisotropy and rheological property (storage modulus at 1Hz G’1) were linearly correlated with CRT, while non-linear correlations were observed for fat globule size, oiling-off and meltability with both CRT and SRT. Compared to CRT, SRT showed a less significant effect on cheese properties in the equipment used, which was attributed to the low values of SRT and specific mechanical energy (SME) that are characteristics of equipment design. Based on the results obtained with the cooking-stretching process, the cooking and stretching processes were subsequently investigated separately to better understand the structural changes in the curds. For this purpose, the changes in composition and structure of cheese curd during water cooking at varying temperature (60-90 ºC), salt concentration (2.5-20%, w/w) and pH (5.2 and 6.6) were determined. Cooking of curds at high temperature and low pH accelerated dry matter loss, of which the main component lost was fat. Intensive heat treatment (temperature > 70 °C and time > 8 min) and concentrated brine (20% NaCl, w/w) impaired the ability of curd cubes to fuse during heating. Thus, higher values of G’1 and crossover temperature Tx were obtained with higher salt concentration, while acidification of water to pH 5.2 led to reduced G’1 and Tx of curd. Oiling-off of the cooked curds was extensively enhanced by increasing the cooking temperature (from 60 to 80 °C) and decreasing pH (from 6.6 to 5.2), while meltability was to a less extent influenced by these parameters. Application of high-shear to the curds using a twin-screw extruder showed that extrudates with a variety of characteristics can be produced. Controllable extrusion parameters – heating and cooling temperatures – are crucial to create fibrous curd structures. With increased temperature the curd viscosity decrease allowing the flow of the curd through the extruder (low shear forces) thus a higher mass flow was obtained. In similar way, the decrease of temperature in the die, increased the viscosity of the curd increasing the shear forces, which led to fiber formation. A balance between these effects is important to structure the curds. Exit temperature Texit, residence time RT and SME are promising indicators for a better understanding of the extrudate properties, since they can comprehensively summarize the influence of multiple controllable parameters and their interactions. At higher Texit (50-54 °C), shorter RT (55-60 s) and lower SME (23-27 kJ·kg−1), finer and longer protein fibers were obtained. The effect of cooking of curd on further stretching/shearing process in a twin-screw extruder was studied to assess its impact on fiber formation and cheese properties. The cooking process significantly enhanced meltability and oiling-off of the curd, which resulted in faster extrusion at lower SME. However, cooking caused modification in the proteins matrix, observed for instance as cracks in protein matrix, ultimately leading to a less fibrous protein structure. Calcium-protein interactions were found to increase as a result of the extrusion process. Additionally, by combining confocal laser scanning microscopy and X-ray micro-tomography techniques, the microscopic observations at different length scales showed not only clearly distinguished protein, fat and serum phases, but also provided complementary details e.g. salt distribution in the protein matrix and anisotropic characteristics of the structure. Overall, the resuts of this thesis show the possibility to create a variety of cook-stretched and extruded dairy products by exploring the relationships between controllable parameters and curd composition and structural properties. Knowledge about structure formation in cheese curd during shear processing, provides new insights to improve the equipment design and to produce structured cheese products with customized behavior.