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 Enterococcus faecalis, Enterococcus faecium, Enterococcus durans and Enterococcus hirae, exhibiting the median and the greatest heat resistance, as well as E. faecalis 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 z values, which ranged from 5.0 to 9.8 °C. The most heat sensitive isolates were E. faecalis (z values 5.0, 5.7 and 7.5 °C), while the most heat resistant isolates were E. durans (z values 8.7 and 8.8 °C), E. faecium (z value 9.0 °C) and E. hirae (z values 8.5 and 9.8 °C). The data show that heat resistance in enterococci is highly variable.