Purpose: The efficiency of UV-C irradiation as a nonthermal processing method for milk was investigated. Milk inoculated with two surrogate viruses (MS2, T1UV), Escherichia coli (ATCC 25922), and two pathogenic bacteria (Salmonella Typhimurium, Listeria monocytogenes) was treated with UV-C irradiation.
Methods: Optical properties of the samples were measured using a spectrophotometer connected to a single integrating sphere. Reduced scattering and absorption coefficients were mathematically extracted and accounted for in the dose calculations. Biodosimetry techniques were used to calculate the reduction equivalent dose (RED). Milk was irradiated using a continuous-flow reactor with the fluid pumped around a central low-pressure mercury UV lamp (40 W) emitting at 254 nm wave-length; this system achieved good mixing and uniform fluence. A series of known UV doses (0 – 50 mJ⋅cm-²) were delivered to the samples, in triplicate, followed by duplicate plating and enumeration.
Results: The populations of MS2, T1UV, E. coli, S. Typhimurium, and L. monocytogenes were reduced by more than five log10. At the highest dose of 40mJ⋅cm-2, our results showed that UV-C irradiation effectively inactivated viruses and pathogenic microbes in milk. The inactivation kinetics of all microorganisms were best described by log linear models with a low root mean squared error (RMSE) and higher coefficient of determination (R²>0.95).
Significance: This study demonstrated that high levels of inactivation of pathogens can be achieved in milk (highly opaque fluid) and suggested significant potential for UV-C treatment of milk.