Purpose: To quantitate Coxiella burnetii and Micrococcus luteus (ML) thermal inactivation at 71.7ºC in bovine, camel and goat milks and evaluate ML as a potential surrogate.
Methods: Milk (6 ml) was pre-heated in a water bath to 71.7ºC in vials for 2.5 min. prior to co-inoculation of CB and ML at a final concentration ~ 6.5 log ge and CFU/ml using a syringe. Inoculated vials were heat-treated at 71.7ºC up to 16 s, cooled in an ice slurry and serially diluted. Viable ML was quantitated by plating on BHIA. For CB enumeration, 1 ml each dilution was inoculated into 9 ml Acidified Citrate Cysteine Medium-2 (ACCM-2) in triplicate T-25 flasks to produce a 3-tube Most Probable Number (MPN) assay and incubated for 14 days at 37ºC under modified atmosphere. The number of flasks showing growth of > 0.5 log ge/ml by qPCR were used to calculate the CBMPN/ml milk. Two trials with two vials per treatment time were completed for each milk.
Results: Average D-values were 1.99 ± 0.21 s, 2.06 ± 0.71 s and 1.43 ± 0.30 s for CB, and 5.47 ± 0.94 s, 5.34 ± 1.54 s, and 3.48 ± 0.83 s for ML in bovine, goat, and camel milks, respectively. The D-values of the non-bovine milks were not significantly different (P<0.05) than bovine milk for both strains. ML and CB were inactivated to a greater degree in goat milk than camel milk.
Significance: CB inactivation in non-bovine milks was similar to that of bovine milk. Although conservative, ML may act as an appropriate surrogate for estimating CB inactivation in milks.