Evaluating secondary structure changes in beta-lactoglobulin induced by supercritical CO₂ treatment

The focus of this research was to evaluate changes in the secondary structure of β-LG induced by supercritical CO₂ (ScCO₂) treatment of whole milk powder (WMP). Processing conditions of 63°C/200 bar and 75°C/300 bar were chosen for further investigation because they produced the highest reduction in antigenicity, 42.9% ± 2.83% (±SD) at 63°C/200 bar and 54.75% ± 2.43% at 75°C/300 bar, respectively, via sandwich ELISA. Orbitrap fusion liquid chromatography-MS/MS detected the presence of capric acid under processing conditions of 63°C/200 bar and lactose molecules at 75°C/300 bar, and these post translational modifications were established to be unique to ScCO₂ processing. The β-LG was isolated from reconstituted WMP via isoelectric precipitation and membrane filtration, and secondary structure analysis was conducted via UV-absorption, intrinsic and extrinsic, circular dichroism, and Fourier-transform mid infrared spectroscopy. Results indicated the unfolding of the β-LG molecule, with exposure of Trp residuals to the exterior and an increase in surface hydrophobicity of the protein molecule. Circular dichroism spectroscopy results highlighted an increase in α-helices and random coils with a reduction in β-sheets characteristic to β-LG and thus highlighting significant changes in the secondary structure of the protein. Glycoprotein formation and caprylation had the most significant effects on the amide I and II regions of β-LG, indicative of posttranslational modifications and were found to be unique to ScCO₂ processing.