Survival of pathogens in low moisture foods
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Abstract
This work investigated the survival and heat resistance of pathogens (Salmonella spp and
Listeria monocytogenes) and a potential surrogate strain (E. faecium NRRL B-2354) in a
selection of low moisture foods. The pathogens and the potential surrogate bacteria were
inoculated into a selection of low moisture products (confectionery formulation, chicken
meat powder, pet food and savoury seasoning, paprika powder and rice flour) and survival
during storage as well as heat resistance were determined using glass vials and specially
designed thermal cells. This study showed that pathogens can survive well in low
moisture foods and survival was dependent on many factors such as water activity (aw),
storage temperature and food composition. It was also shown that RpoS regulon plays an
important role in Salmonella survival in low moisture foods. A strain lacking an active
RpoS was significantly less viable in low moisture foods and significantly less heat
resistant than the RpoS+ve strain. This study also showed that the use of E. faecium
NRRL B-2354 as a surrogate is feasible for process validation although it has some
limitations. It was shown that E. faecium NRRL B-2354 cannot be used as a surrogate in
products containing high levels of sugar (confectionery powder) as Salmonella was
significantly more heat resistant in this type of product than E. faecium NRRL B-2354. It
was also shown that in paprika powder and in rice flour the two most resistant Salmonella
strains (S. Enteritidis - PT 30 ATCC BAA-1045 and S. Typhimurium ST30; both RpoS
+ve) in some conditions were more resistant than E. faecium NRRL B-2354. This study
also showed that survival curves representing microbial survival during storage or during
heat processes may not always be linear. In this study, concave upwards, concave
downwards and linear curves were recorded and the Weibull model was used to fit raw
data and precisely calculate the time required for 5 log reduction in viable numbers