Synthesis of hydrophobic whey protein isolate by acylation with fatty acids
Due to its high availability, whey protein isolate (WPI) is an attractive feedstock for the production of bio-based materials. In the current study, different methods of synthesis are compared for the acylation of WPI with fatty acids. The aim is to synthesise hydrophobic, bio-based WPI derivatives for thermoplastic processing. Contrary to the acid chloride/pyridine acylation procedure, homogeneous acylation with carbonyldiimidazole (CDI)-activated fatty acids in dimethylsulphoxide (DMSO) enabled organic solvent soluble WPI derivatives to be synthesized. The substitution pattern and the degree of substitution (DS) were estimated by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and 1H NMR spectroscopy. The ATR-FTIR analyses showed qualitative differences in the degree of acylation of WPI achieved by the different synthesis methods. Solutions of WPI-laurate and -oleate in THF and CHCl3 were studied via dynamic light scattering (DLS), showing the influence of the type of esterified fatty acid on the particle size distribution. Water uptake experiments demonstrated the hydrophobicity of WPI-oleate. The thermal behaviour was studied by Differential Scanning Calorimetry (DSC) and Thermal Gravimetric Analysis (TGA). Thermoplastic behaviour of WPI-oleate was detected at 150 °C, with 30% oleic acid (OA) as the plasticiser. Extrusion processing was tested using a WPI-oleate/OA mixture and using starch laurate as a blend component. In summary, a new type of WPI derivative has been synthesized and shows potential for thermoplastic processing.