This dissertation research focuses on studying time-dependent texture changes in high-protein nutrition (HPN) bars and their underlying mechanisms to possibly prevent or reduce change when formulated with high-protein (≥ 80% protein w/w) milk protein concentrate (MPC). HPN bars (20-50% protein w/w) are a target application for these nutritious and flavorless powder ingredients. However, high-protein MPCs produce HPN bars with undesirable texture that harden and lose cohesion during storage. The overall goal of this work is to improve the performance of MPCs for use in HPN bars by modifying their physicochemical properties to impart softness, cohesion, and textural stability. Twin-screw extrusion and particle size reduction processes were used to modify protein structure and powder particle properties of MPC with 80% or 85% protein (i.e., MPC80 or MPC85, respectively). Extrusion up to die-end melt temperature of 120C partially denatured MPC80 and decreased its solubility, free sulfhydryl and free amine contents, and water holding capacity. Extrusion-modified MPC80 powders had higher bulk and particle densities, lower occluded air, and had an improved ability to interact with water. MPC85 (D4,3 = 86 µm) particle size reduction down to D4,3 of 8 µm by jet-milling increased bulk and particle densities, decreased occluded air, and increased its water dispersibility. Evaluation of model HPN bars (30% protein w/w) showed that extruded MPC80 and jet-milled MPC85 increased denseness and cohesion. Extrusion-modified MPC80 imparted softness whereas finely jet-milled MPC85 increased firmness, but both modifications significantly improved stability by limiting textural changes during storage. Moreover, texture profile analysis (TPA) attributes and crumbliness measured by a sieve assay correlated very well with how panelists perceived HPN bar texture. Confocal laser scanning microscopy showed that extrusion-modified MPC80 prevented macronutrient phase separation during HPN bar storage and this aligned with enhanced textural stability. SDS-PAGE showed that disulfide-based and Maillard-induced protein aggregations occurred during HPN bar storage. Disulfide bond formation was less prominent in HPN bars formulated with extruded MPC80, as these powders had lower initial free sulfhydryl content due to extrusion induced disulfide bond formation. Transglutaminase crosslinked MPC was also evaluated in a similar model HPN bar, but it did not improve overall texture or stability. Extrusion and particle size reduction by jet-milling altered the properties of two high-protein MPCs that produced texturally stable HPN bars due to their new ability to hydrate more thoroughly during HPN bar production. Increased protein plasticization allowed the HPN bars to maintain a cohesive structure during storage and loss of protein plasticization led to texture changes. While chemical changes occur during HPN bar storage, they are only partly responsible for texture change, especially, since modifications can prevent a chemical change from occurring, yet, texture changes are still noted.