Abstract
Human and bovine milk differ in composition, including the concentration of antimicrobial components, which may impact the efficacy of non-thermal processes to inactivate foodborne pathogens and may also impact the growth of pathogens introduced via post-processing contamination during refrigerated storage. Two strains of Listeria monocytogenes (ATCC 35152 and OSY-328) were inoculated into human and bovine milk and treated with high-pressure processing (HPP; 400, 450, and 500 MPa for 1, 5 and 9 min) or ultraviolet-C (UV-C) irradiation (500–7,300 J/L). Differences in HPP and UV-C treatment efficacy between the two milk types were evaluated. In addition, the growth of L. monocytogenes inoculated into Holder pasteurized (HoP, 62.5 °C, 30 min), HPP-treated (500 MPa, 9 min, 16–19 °C) and UV-C-treated (5,000J/L) human and bovine milk were studied during storage at 4 °C for 22 days. Identical processing parameters for either HPP or UV-C resulted in greater reductions of both L. monocytogenes strains in human milk compared to bovine milk. Sufficient lethality (>5-log CFU/mL reduction) of inoculated L. monocytogenes, which is the target for pasteurization, was achieved when raw human milk was treated with HPP at 500 MPa for 5 and 9 min and UV-C at ≥5,000 J/L; however, a 5-log reduction was not achieved at any of the tested parameters tested in bovine milk. Across storage at 4 °C of human milk treated with HoP, HPP or UV-C, and then inoculated with L. monocytogenes, the counts of the two microorganisms decreased and reached less than the detectable limit (1 CFU/mL) after 18–20 days. However, for bovine milk under the same conditions, L. monocytogenes counts increased, reaching >7 log CFU/mL after 22 days. Overall, L. monocytogenes is more sensitive to HPP and UV-C processing in human milk than in bovine milk, and human milk displayed inherent antimicrobial activity towards L. monocytogenes, even after processing, whereas bovine milk supported growth of L. monocytogenes during refrigerated storage.