Effect of UV irradiation on the properties of whey protein solutions treated using a novel UV light reactor

The objective of this study was to investigate the structural changes of whey proteins in solution during continuous flow through a novel UV reactor that was designed to minimize poor UV penetration and heterogeneous UV dose distribution. Varying UV irradiation doses were obtained by controlling the flow rate and the mixing speed of the liquid through the reactor. Aqueous solutions of whey protein isolate (WPI) at concentrations of 1 and 5% (w/v) were circulated at flow rates of 30, 40, 70 130 and 800 ml/min (samples designated as UV 30, UV 40, UV 70, UV 130, UV 800, respectively). Lower flow rates resulted in greater sample exposure to UV irradiation. The effect of UV light exposure to the properties of WPI was investigated by fluorescence spectroscopy, size exclusion chromatography (HP-SEC) and spectrophotometry. Fluorescence spectra of the irradiated WPI solutions were red-shifted compared to that of the untreated control solution indicating changes in tertiary structure of proteins. The greatest shift was observed in UV 30 samples (12 and 7 nm for 1 and 5% solutions, respectively). UV treatment increased the concentration of free SHgroups (expressed as % of total thiol groups) compared to control for both 1 and 5% WPI solutions, with the greater increase observed for the UV 30 sample. HP-SEC data showed the formation of UV-induced aggregates and Nformylkynurenine, an oxidation product of tryptophan; the highest extent of change was observed for 1% WPI sample with the highest UV exposure. In addition, the surface hydrophobicity of proteins decreased with UV exposure, particularly for the 1% WPI solutions. The speed of mixing of UV 30 WPI 1% solution in the reactor (50 rpm or 250 rpm) did not cause significant differences in the properties of proteins when compared to the unstirred UV 30 WPI 1% solutions indicating that the rate of flow played a major role in affecting the protein structural changes. It can be concluded that when operated under mild exposure conditions this novel UV technology has a potential to be applied to whey based products. Studies on the effectiveness of such conditions for milk pasteurization should also be conducted, to determine the applicability to more complex dairy matrices.