Impact of strain variability on the risk presented to chilled foods by non-proteolytic clostridium botulinum
This PhD thesis aims to extend the current quantitative microbial risk assessment for minimally heated chilled foods by including information on variability of non-proteolytic Clostridium botulinum at strain level. The research on strain variability covers practical experiments as well as literature surveys and focuses on heat resistance properties of non-proteolytic C. botulinum spores, variability in growth at chill temperatures and variability in growth and neurotoxin formation in the presence of different carbohydrates. A strain classification pattern was developed and compared with published literature information on the genetic variation of non-proteolytic C. botulinum. Results for the growth at chill temperatures and from the carbohydrate study provide strong evidence of significant variability for strains of non-proteolytic C. botulinum that is associated with the type of neurotoxin formed. Although a literature review on heat resistance did not show correlation between the decimal reduction time and toxin type for non-proteolytic C. botulinum it provides a strong quantitative support for modelling of the thermal properties of the spores. Insights on strain variability were included in a modular quantitative microbial risk assessment (QMRA) for a model dairy-based chilled dessert using a Monte Carlo simulation technique. The model considered four steps in product manufacture that either reduced or increased the hazard associated with non-proteolytic C. botulinum: spore load in raw material, thermal inactivation of spores, distribution of spores in retail units and population kinetics for non-proteolytic C. botulinum during multi-stage storage regimes. The replacement of a commonly applied assumption on strain homogeneity with strain specific characteristics of non-proteolytic C. botulinum revealed that inclusion of information on strain variability has an important impact on estimated risks. The QMRA indicated that, for a minimally heated dairy-based dessert, a greater hazard was associated with type E strains than with those of type B and F. The findings in this thesis provide important information relating to food safety and public health and could be used by risk managers for verification of microbiological criteria for particular products originating from geographical locations with higher prevalence of type E non-proteolytic C. botulinum strains. This approach illustrates a step forward by including population details into to risk assessment which may become a significant element in the assessment of complex foodborne hazards.
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