Abstract:
Listeria monocytogenes, which is a food-borne pathogen often isolated from milk
and milk products, causes listeriosis in pregnant women, newborns, older adults and
immuno-suppressed people. Listeria is said to be more resistant to heat treatments
and the present study investigated the effects of current heat treatments on
inactivation of Listeria in milk. Two isolates of L. monocytogenes, namely FSTLC2
and FSTLC55, were introduced to sterilized milk at population levels of 102 and 107
cfu/ml and were subjected to various temperature-time combinations. Listeria was
enumerated using the FDA Listeria Enrichment Broth (LEB; Oxoid Ltd) and
Modified Oxford Agar (MOX; Oxoid Ltd) medium. The Thermal Death Time
(TDT) and Thermal Death Rate (TDR) curves were also constructed using thermal
resistance data. The slope of the TDT curve (z) and the slope of the TDR curve (D;
Decimal Reduction Time) were also determined using the statistical software
package Minitab (release 8.21). It was observed that the normal pasteurization
treatments of 62.8 °C for 30 min (LTLT; Low Temperature Long Time) and 71.7 °C
for 15 sec (HTST; High Temperature Short Time) appear to be adequate to destroy
a Listeria population of 102 CFU/ml in milk, but not a population of 107 cfu/ml. A z
value of 7.6 °C and a D71.7 °C value of 2.9 sec were observed for L. monocytogenes
in milk. The current heat treatment techniques used in the dairy industry need to be
reviewed and necessary modifications introduced to ensure a complete elimination
of this pathogen. Moreover, the z and D71.7 °C values observed in the present study
may be used in devising sound heat treatment techniques as well as suitable Hazard
Analysis Critical Control Points (HACCP) systems in the dairy industry.
