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Food Safety risk analysis

An Example of Risk Analysis for Microbial Pathogens

Risk analysis is usually divided into three phases, but here we are interested only in the risk assessment phase. When conducting a risk assessment of microbial pathogens, a model is traditionally used. We will look at a typical model shortly, but first, some definitions are in order.

Infection - in discussing foodborne pathogens, it is the ability to colonize and reproduce in the intestinal tract.

Morbidity - the measurable disease response resulting from infection.

Risk assessment is usually divided into three components:

  • Dose-response - the relationship between the host and the pathogen. This may be either quantitative or qualitative, but with foodborne pathogens it usually concerns infection rather than morbidity.
  • Relative severity of the disease - the public health impact of the disease.
  • Estimation of the exposure faced by the public at large - includes both incidence and prevalence of the pathogen but is not normally interested in cumulative exposure effects.

The infectious unit for microorganisms is the single cell. Although the risk of becoming infected by a single cell is very small, it is not zero.

The key to predicting risk for the individual consumer is predicting exposure. A caveat must be added here; this discussion is directed at infectious foodborne pathogens and not toxigenic pathogens. The toxigenic pathogens like Staphylococcus aureus may be subjected to minimum dose considerations because there must be sufficient microbial growth to produce enough enteric toxin to achieve a response. So, the rest of this example will be concerned with infectious foodborne pathogens.

Exposure is predicted using a mathematical model because it is assumed that once a dose-response relationship between pathogen and host has been quantified, the key to determining risk is to determine the amount of infectious units ingested. To do this, the growth of the pathogen under the conditions present for a specific food and food process - whether there is a lethality step and its effectiveness - is calculated using a mathematical formula as a model.

The most commonly used model is the beta Poisson distribution

Pi = 1 - (1 + N/B-a )

where Pi is the probability of infection, N is the exposure (or pathogen level expressed as colony-forming units [CFU]), and alpha and beta are coefficients peculiar to the pathogen being examined.

Using this equation, the probability of infection (Pi) of Rotavirus is 3 X 10-1, and that for Shigella spp. is 1 X 10-3, making Rotavirus more infectious than Shigella spp..

This methodology has been used extensively in the canning industry since this industry began; it is especially useful in low-acid canning processes because pH is not an inhibiting factor for Clostridium botulinum as it is in the normal canning processes.  In the canning industry, there are Process Authorities that determine time and temperature requirements for retorting (cooking the product in its container in, typically, a water bath), taking into account product viscosity, product composition, fill times, head space, et cetera.  All use mathematical models.

 

 

 

An independent study project for Environmental Health 511,
summer quarter 2000, taught by Dr. Bill Daniell

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