Tailoring Thermoplastic Whey Protein with Polyethylene Glycol and Calcium Stearate: A Systematic Design of Experiments Investigation of Structure, Thermal Behavior, and Rheology

Abstract The growing environmental concern related to plastic materials has driven the search for sustainable alternatives. In this context, whey protein isolate (WPI) has emerged as a promising option due to its adequate oxygen barrier properties for food packaging. However, WPI lacks mechanical resistance, requiring plasticization to increase protein chain mobility, flexibility, and processability. WPI plasticized during its denaturation is called thermoplastic whey protein isolate (WPIT). This study evaluates the WPIT modification using a design of experiments (DoE) with two factors and a center point: calcium stearate, a largely commercially used lubricant, and the plasticizer poly(ethylene glycol) (PEG). To assess the occurrence of plasticization and the proposed stabilization, the structural, rheological, and thermal characteristics of the modified WPIT were investigated. Fourier-transform infrared spectroscopy (FTIR) showed no significant increase in the α helix secondary structure, suggesting incomplete plasticization. The parallel plate rheology revealed that the complex viscosity followed power law behavior, and there was no Newtonian plateau. WPIT samples with calcium stearate and without PEG, and the center point presented crossover of storage and loss moduli in the terminal zone. The sample with maximum calcium stearate and no PEG showed evidence of cross-linking. Differential scanning calorimetry (DSC) analysis did not allow for the identification of the glass transition temperature (Tg) for WPIT samples. Thermogravimetric analysis (TGA) results indicated decreased thermal stability in the presence of PEG, likely due to its plasticizing effect. The results suggested that the presence of calcium ions may have promoted possible cross-linking between the protein chains.