Course:FNH200/Lessons/Lesson 06/Page 06.4

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6.4 Margin of Safety

Inherent in the thermal processing of foods is the concept of a margin of safety which refers to the probability that a container of food could still contain a viable spore of Clostridium botulinum after the completion of the thermal processing. Obviously, the goal is to ensure that the margin of safety is as large as possible which means that the probability of survival of spore of Clostridium botulinum after thermal process is as low as possible without causing undue heat damage to the quality factors and nutrient value of the food.

Since Clostridium botulinum is ubiquitous in the environment where food materials are grown and harvested, the assumption is made that all foods to be preserved are potentially contaminated with Clostridium botulinum spores and thus must be processed accordingly.

As the spore population in a food system is increased, the total time required at a particular temperature to kill ALL the spores increases. This phenomenon is inherent in the logarithmic order of death of microbial cells and spores. During preparation of foods for processing, efforts are made to minimize the microbial population in the foods to be processed. Microbial and spore populations can be minimized by proper handling techniques, the use of extensive washing, and the use of peeling and trimming procedures to remove as many as possible of the spores that may be present on the food.

  • Typically, for low acid foods (pH greater than 4.6) a margin of safety of 12D is applied. This means that the low acid foods (e.g. milk, meat, poultry, fish, vegetables) are subjected to a thermal process so that the slowest heating portion of the food is exposed to an amount of thermal energy (heat) such that the microbial spores present in the food will experience the equivalent of 12 successive decimal reduction times. This has the capability of killing 1012 (one trillion) spores of Clostridium botulinum per container. Since the natural levels of contamination of foods with C. botulinum are much lower than that, a large margin of safety is introduced for thermally processed foods. The D-value is temperature dependent. Thus, the higher the temperature, the lower the D-value and the less time taken to achieve the 12D "botulinum cook" for low acid canned foods.
  • Another important factor to be considered is that C. botulinum is very sensitive to acid and it will not grow in foods at pH 4.6 or below. Therefore, the 12D heat treatment would be excessive and unnecessary for acid foods (pH of 4.6 or less). With acid foods, temperatures at or below 100°C for a few minutes should be an adequate heat treatment. Typically, a 5D thermal process is usually used for acid foods.
Want to learn more?
  • If the spore population in a container of food were increased from 100 spores to 10,000 spores, would the margin of safety of a 12D thermal process increase or decrease?
  • What is the effect of a higher initial microbial load on the margin of safety?