Heat resistance of Mycobacterium avium subsp. paratuberculosis in skim milk and cream tested in a pilot plant pasteurizer

摘要

Heating experiments reported in the literature show differing results about the heat resistance of MAP in milk. Presently, it seems to be clear that small numbers may survive HTST treatment. Most of the experiments were performed with whole milk, whereas in practice skim milk and cream are very often treated separately. The main objective of this study was to investigate the heat resistance of MAP in the substrates by applying a heating technology most comparable to commercial systems. 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 liquid substrates were subjected to heat treatment in a pilot plant pasteurizer. Skim milk was heated between 67-90 degC and cream between 85-100 degC at holding times between 15-60 s. Detection by culture of surviving MAP was performed with 6-months' 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. In skim milk 54 of 93 experiments showed small numbers of survivors at any time-temperature combination tested. The reduction achieved from initial MAP concentration was between 3-7 log cycles. In 9 of 57 experiments with cream, small numbers of MAP also survived despite a reduction of up to 7 log cycles.