Heat resistance of Enterococcus durans and E. hirae isolated from pasteurised milk

Some enterococci are regarded as beneficial microflora in artisanal cheeses produced in some countries in Europe and have been considered as possible adjunct starters. In contrast, the presence of enterococci in milk being processed into dairy products has also been regarded as undesirable due to their possible indication of faecal contamination of milk and the equipment and environment in which milking is conducted. While it has been reported that enterococci can survive milk pasteurisation, there is limited thermal inactivation data on wild-type strains that have been isolated from milk and dairy products. Also, the occurrence of Enterococcus faecalis and E. faecium in nosocomial infections has been well documented in Australia and elsewhere and the incidence of infections due to E. durans and E. hirae has been reported to have increased. The purpose of the current investigation was to isolate wild-type enterococci from Australian milk and to characterise the thermal resistance of the main species detected. In the current study, enterococcal species were isolated from factory pasteurised (72°C/15 s) and laboratory pasteurised (62.5°C/30 min) Australian bulk raw milk. E. durans was the main species detected from both sources of pasteurised milk and E. faecalis, E. hirae and E. asini were also detected. The thermal resistance of E. durans and E. hirae isolated from laboratory pasteurised milk (62.5°C/30 min) was determined by calculating D and z values in Brain Heart Infusion Broth. A collection of 28 isolates of E. durans and E. hirae were screened for resistance to laboratory pasteurisation by determination of log10 reduction in viable count and ranked in order of resistance to thermal inactivation. A total of four isolates, including those with the highest and an intermediate level of heat resistance, for each of E. durans and E. hirae (in the populations tested) were selected for determination of D and z values. The D values were calculated from survival curves determined at 63, 69, 72, 75 and 78°C and thermal death curves (log10D vs Temp) were used to calculate the z values. The most heat resistant isolates of E. durans and E. hirae had z values of 9.0 and 8.9°C, respectively. Since the enterococci isolates were selected as a result of their ability to withstand pasteurisation, the results are indicative of thermoduric enterococci and not all enterococci that might be isolated from raw milk.