Dairy bacteriophages isolated from whey powder: Thermal inactivation and kinetic characterisation

Fifty three bacteriophages of three lactic acid bacteria species (i.e., Lactococcus lactis, Leuconostoc pseudomesenteroides, Streptococcus thermophilus) isolated from whey powders and whey powder formulations, were tested for their thermal stability. Complete inactivation of Ln. pseudomesenteroides and S. thermophilus phages required 1 min heating at 85 °C and 90 °C, respectively. L. lactis phages exhibited the highest heat resistance and 16% of them were still detectable after heat treatment at 95 °C for 1 min. The most thermo-resistant phages of L. lactis (P956), Ln. pseudomesenteroides (P973) and S. thermophilus (P1008) were selected to determinate their kinetic parameters and lines of equal effects for 9-log inactivation. High D-values of 105 min (phage P956), 74 min (P973) and 25 min (P1008) were determined for thermal treatments at 75 °C, confirming previous findings that pasteurisation conditions are insufficient for adequate inactivation of thermo-resistant dairy phages.