Reactive supercritical fluid extrusion (RSCFX) is a novel integrated process for controlled chemical reactions and continuous generation of expanded extrudates of modified functionalities. Twin screw extruders are ideally suited for highly viscous materials due to their excellent mixing abilities which help maximize reaction rates. Beyond their superior nutritional qualities, whey proteins are also utilized for thickening, stabilization and emulsification of food formulations following pH adjustment and heat treatment to induce protein denaturation and aggregation. Creating cold-gelling and thickening functionalities in whey protein for use in food system where heating is undesirable remains a challenge. Also, replacing starch-based thickeners with a whey protein ingredient may be attractive to diet -conscious consumers. The aim of this work was to alter and quantify the functional properties of whey proteins by RSCFX proc essing to create novel dairy ingredients for food applications. Texturized whey protein concentrate (TWPC) at acidic condition (pH 3.0) were made by RSCFX and the effect s of addition of starch, calcium (0.3 and 0.6 %, w/w), and extrusion temperature (50, 70 and 90 °C) on selected physicochemical properties of the modified protein were evaluated. TWPC exhibited 200 - to 300-fold higher viscosities than non-texturized WPC at various concentrations (6-26%, w/w TWPC) and formed cold-set gels at 20 % (w/w) upon reconstitution in water. Starch-containing samples (TWPC-S) were less soluble, consisted of larger protein aggregates (1.527 [mu]m), but had 1.2-1.4-fold higher apparent viscosity than TWPC alone because of synergistic interactions between the two biopolymers. TWPC without the starch and at lower calcium level (0.3%) ha d smaller protein aggregates (0.996 [mu]m) with higher solubility. TWPC alone extruded at 50 °C (TWPC-50) and 70 °C (TWPC-70) formed soft-textured aggregates with high solubility in water (77-79 %) than that extruded at 90 °C (TWPC -90) with a solubility of 24%. Total free sulfhydryl contents and solubility studies in selected buffers indicated that non-covalent interactions were prevalent in stabilizing the TWPC aggregates. TWPC extruded at 90 °C showed an increase in aromatic hydrophobicity and a decrease in aliphatic hydrophobicity indicating changes in protein structures. Secondary gelation occurred in TWPC -50 and TWPC-70 when the cold-set gels were heated to 95 °C, while TWPC-90 showed excellent thermal stability. Factors such as the degree of protein denaturation, exposure of hydrophobic groups, and cross -linking influenced the intermolecular associations and improved the cold-set and second-stage heat-induced gelation of TWPCs. Compared to non-texturized WPC, TWPC formed stable oil-in-water emulsions at lower protein concentrations. However, the presence of starch in TWPC impeded emulsifying properties. TWPCs were able to form cold-set emulsion gels containing 40 to 80% (w/w) that were stabilized by 4 to 12% (w/w) protein and had smaller mean droplet sizes as opposed to the non-texturized WPC. TWPC-90 emulsions showed excellent stability during storage (30 days at 4 and 25 °C) and heating (70-90 °C for 20 min) due to the extensively denatured proteins. Heat treatment [GREATER-THAN OR EQUAL TO] 80 °C of non-texturized WPC and TWPC-70 emulsions increased the droplet sizes and loss moduli (G'), indicating emulsion destabilization due to aggregation of native proteins. TWPC emulsions had higher adsorbed proteins (6.0-23.3 mg/mL) in contrast to the nontexturized WPC emulsions (2.17-6.33 mg/mL). SDS-PAGE of the adsorbed TWPC showed greater intensity of [alpha]-la ([alpha]-lactalbumin) and the presence of high molecular -weight protein aggregates. The adsorbed proteins of TWPC -70 emulsions underwent time-dependent polymerization, but they remained stable in TWPC-90-based emulsions. The superiority of TWPC stabilized emulsions may be due to combinations of a stable protein gel matrix that formed the continuous phase of emulsion, greater surface hydrophobicity, and intra-film protein polymerization that conf erred strength to the protein interfacial layer. TWPC-90 that contains a higher degree of denatured protein offers the best potential to serve as a novel, whey protein-based food emulsifier and stabilizer. The RSCFX process provides a new approach by which functional characteristics of TWPC ingredient may be advantageously designed by altering the formulation composition and extrusion operating parameters. These new ingredients may be utilized in different products requiring targeted physicochemical functionalities and a cleaner, all-dairy label.