Effects of UV induced photo-oxidation on the physicochemical properties of milk protein concentrate

•UV induced modification of MPC70 indicated by carbonyl formation and diffused mass.•Unstructured casein was more susceptible to photo-oxidation than β-lactoglobulin.•At low RH, photo-oxidation promoted lactosylation of β-lactoglobulin.•Photo-oxidation induced protein aggregate by non-disulfide covalent crosslink.•Photo-oxidation caused discoloration and solubility loss of MPC70.

The objective of this study was to investigate the effects of UV induced photo-oxidation on protein modification and corresponding changes in color and solubility of milk protein concentrate powder (MPC70) stored at 25 or 40 °C and at 11, 33 or 75% relative humidity (RH), respectively. Protein modifications in the soluble fractions were determined using liquid chromatography-mass spectrometry (LC-MS) and those in insoluble fractions using gel electrophoresis. The results showed that photo-oxidation caused significant decreases in L⁎ value. Moreover, the solubility of photo-oxidized MPC70 decreased significantly, while the MPC70 samples without UV exposure retained their solubility, except at 40 °C and 75% RH. For the soluble fractions of photo-oxidized MPC70, the MS spectra showed multiple additional fractions besides the un- and lactosylated intact protein fractions compared to control suggesting that the photo-oxidation induced further modifications of the proteins, which coincided with the formation of carbonyl groups in the protein. At low RH, photo-oxidation promoted lactosylation of β-lactoglobulin. The insoluble fractions of photo-oxidized MPC70 were mainly caseins as well as high molecular weight aggregates, resulting mainly from non-disulfide covalent crosslinks induced by photo-oxidation. The results suggested that photo-oxidation could induce the modification of protein that eventually contributed to the solubility loss and discoloration of MPC70.