Thermal Inactivation of Listeria monocytogenes in Bovine and Non-bovine Milk Pasteurization

Introduction: Non-bovine milks are becoming popular for fluid milk consumption and artisanal cheese production.  Listeria monocytogenes (LM) is one of the most common pathogens in raw milk and has been found in 50% of commingled raw bovine milk silos in the US.  Bovine milk pasteurization conditions should be adequate for proper treatment of non-bovine milks, however little research has been completed to define the differences in thermal resistance of LM in non-bovine milks.  

Purpose: To compare the thermal inactivation of Listeria monocytogenes in bovine, goat, camel, and water buffalo milks using standard HTST pasteurization conditions.

Methods: A preliminary screening of 25 LM strains was conducted by heating each in 6 ml TSB+YE at 60°C in submerged glass vials for up to 30 min with quantitation on TSA+YE.   The most resistant strain was used for inactivation studies in which 5 ml milk in glass vials was pre-heated at 71.7°C for 3 min prior to inoculation with 20 µl of ~10¹⁰ CFU/ml LM via syringe.  After heat treatment, the vials were moved to an ice slurry for 30 s followed by dilution and plating on TSA+YE. Four trials per milk, with duplicate vials for each treatment time (0, 2, 4, 6, 8, 10, 12, 14, and 16 s) were quantitated and the time to reduce LM by 1 log (D-value) was determined.

Results: Average D-values were 5.92±0.56 s, 5.36±1.10 s, 4.58±0.82 s, and 3.67±0.20 s for LM in water buffalo, goat, bovine, and camel milks, respectively.  D-values increased with increasing levels of fat, protein and solids. Only buffalo milk was significantly different (P<0.05) than bovine milk, and contained the highest percentage of fat, protein and solids.

Significance: LM in goat and camel milks was inactivated similarly to LM in bovine milk while buffalo milk may require a longer treatment time or a higher temperature to result in the same reductions.