Objective
Heating experiments reported in the literature show differing results for the heat resistance of Mycobacterium avium subsp. paratuberculosis (MAP) in milk though it appears that small numbers may survive HTST treatment. Most of the past experiments were performed with whole milk. In practice, they are often performed separately in skim milk and cream. The main objective of this study was to investigate the heat resistance of MAP in skim milk and cream by applying a heating technology most comparable to commercial systems.
Materials and methods
Skim milk and cream were inoculated with a cocktail of five MAP strains of bovine, caprine and human origin at colony counts between 10
3-10
5 cfu ml
-1. Both substrates were subjected to heat treatment in a pilot plant pasteuriser. Skim milk was heated between 67-90 °C and cream between 85-100 °C at holding times between 15-60 s. Cultural detection of surviving MAP was performed by application of 6 month resuscitation/enrichment in modified Dubos medium and subsequent culture onto Herrolds egg yolk medium for another 6 months. Survivors were identified by acid fast staining and IS900 based PCR.
Results
In skim milk 54 of 93 experiments showed a reduced survival rate at almost any time-temperature combination tested. The reduction achieved was between 3-6 log cycles. In cream 9 of 57 experiments revealed a reduction of up to 6 log cycles.
Conclusions and perspectives
Surviving MAP were detected in all types of experiments performed. According to statistical analysis survival in skim milk is dependent on holding time, applied heating temperature and on seeded initial count. No significant dependencies were found in cream. At present no physiological properties of MAP are known to explain the exceptional heat resistance. Despite these uncertainties regarding possible survival, a reduction of up to 6 log cycles could be demonstrated.
