Heat resistance of thermoduric enterococci isolated from milk

Enterococci are reported to survive pasteurisation but the extent of their survival is unclear. Sixty-one thermoduric enterococci isolates were selected from laboratory pasteurised milk obtained from silos in six dairy factories. The isolates were screened to determine log10 reductions incurred after pasteurisation (63 °C/30 min) and ranked from highest to lowest log₁₀ reduction. Two isolates each of <i>Enterococcus faecalis</i>, <i>Enterococcus faecium</i>, <i>Enterococcus durans</i> and <i>Enterococcus hirae</i>, exhibiting the median and the greatest heat resistance, as well as <i>E. faecalis</i> ATCC 19433, were selected for further heat resistance determinations using an immersed coil apparatus. D values were calculated from survival curves plotted from viable counts obtained after heating isolates in Brain Heart Infusion Broth at 63, 69, 72, 75 and 78 °C followed by rapid cooling. At 72 °C, the temperature employed for High Temperature Short Time (HTST) pasteurisation (72 °C/15 s), the D values extended from 0.3 min to 5.1 min, depending on the isolate and species. These data were used to calculate <i>z</i> values, which ranged from 5.0 to 9.8 °C. The most heat sensitive isolates were <i>E. faecalis</i> (<i>z</i> values 5.0, 5.7 and 7.5 °C), while the most heat resistant isolates were <i>E. durans</i> (<i>z</i> values 8.7 and 8.8 °C), <i>E. faecium</i> (<i>z</i> value 9.0 °C) and <i>E. hirae</i> (<i>z</i> values 8.5 and 9.8 °C). The data show that heat resistance in enterococci is highly variable.