Extreme thermal stability of Lactococcus lactis bacteriophages: Evaluation of phage inactivation in a pilot-plant pasteurizer

Thermal inactivation of highly heat-stable Lactococcus lactis phages P680 and P1532 of the 936 group and P635 of the c2 group was investigated in a pilot-plant pasteurizer after suspension of the phages in raw milk (107 pfu/ml). The heat-sensitive reference phage P008 was tested as a control. The titers of the phages P008 and P635 decreased ≥99.99% after heating for 32 s at 75 °C and 80 °C, respectively. To achieve a similar inactivation of ≥99.99%, phages P680 and P1532 had to be treated for 39 s at temperatures of 95 °C and 97.5 °C, respectively. D- and z-values for thermal inactivation were calculated for each of the phages. When comparing thermal inactivation data of phages in the pilot-plant pasteurizer obtained in this study with literature data on phage heat inactivation in stainless-steel test tubes in a water bath, the pilot-plant pasteurizer was notably more efficient.