Clostridium botulinum and shelf life

Results: 

Combase predictions indicate that meats with pH values of 5.5, 5.75, 6.0 and 6.5 would have to be stored below 8, 7, 6 and 5°C respectively to prevent growth within 10 days. Only meat with a pH of 5.5 stored at 4°C was predicted not to support growth within 28days. There is very little information in the literature on growth and toxin production by non-proteolytic C. botulinum in fresh red meat. Although the data is limited, and some is of questionable quality, what has been published shows growth can occur in fresh within 28 days at 5°C.

In a challenge test, samples of raw beef, pork and lamb were inoculated with spores of a mixture of strains of non-proteolytic Clostridium botulinum types B and E. After vacuum packing, the samples were incubated at 5, 8 or 10°C and samples were taken for analysis of toxin formation after selected times of up to 63 days. A majority of the packs showed spoilage as judged by visible gas formation and off colour.

Non-proteolytic C. botulinum was able to grow and produce high concentrations of toxin within 10 days at 8 or 10°C in a sterile meat based culture medium but toxin was not detected in any of the samples at 5°C. Toxin production in the raw meat samples was very sporadic but was observed in 28 days in lamb at 5°C and within 28 days in beef, pork and lamb at 8°C. At a nominal temperature of 10°C toxin was first observed after 10 days in pork and lamb and 28 days in beef. In view of the observation of toxin formation at 10 days at 10°C (the first day on which toxin was tested for), a shelf-life of more than 10 days at 8-10°C would appear to leave only a small margin of safety to allow for possible time and/or temperature abuse by consumers. Thus, these data do not support assignment of a shelf-life of more than 10 days at 8-10°C. When raw meat was stored at 5°C, toxin was detected after 28 days, but not at day 24 or earlier. A commonly used approach to food safety is that the product should be demonstrated to be safe for 150% of its shelf-life at the test (or even an abuse) temperature. The demonstration of no toxin formation within 24 days at 5°C may indicate that a shelf-life of 16 days at 5°C is justified.

In conclusion, the data available on growth of non-proteolytic C. botulinum in fresh meat is limited and confused. It suggests that the probability of growth and toxin formation in refrigerated raw meat may be low but it can occur. At this time there does not appear to be sufficient scientific evidence to support a shelf-life of greater than 10 days at 8°C in fresh red meat. The position at 5°C is less clear. The challenge test work indicates that a 16 day shelf-life might be justified, while ComBase predicts an increase in viable count of 0.5 log unit at pH = 6.0, and 2.0 log unit if pH = 6.5 after 16 days at 5°C. Neither of these increases in viable count are considered acceptable. Literature data indicate growth and toxin formation in raw meat in 21-27 days at 4°C. A shelf-life of 21-28 days at 5°C does not appear to be justified.

Planned activity:

The results do not support strong recommendations for changes to practice and a proactive communication plan is not considered necessary. The possibility of further research is under consideration.

The Problem:

FSA guidelines on the shelf life of vac packed and modified atmosphere packed foods in relation to Clostridium botulinum  places a maximum shelf life of 10 days on such products unless one of the specified “controlling factors is in place. In respect of fresh meat none of these apply.  In practice this means that the guidelines indicate:

1. Vacuum packed meat held below 3°C does not have shelf-life restricted by the possible growth of C. botulinum
2. Once temperature exceeds 3°C a shelf life of 10 days is indicated.

Although the FSA guidelines are not legal requirements, they are used by enforcement officers (particularly Environmental Health Officers) to challenge businesses. This presents difficulties for operators who rely on vacuum packed storage for maturation or stock control of raw fresh meat.  Many smaller/medium sized businesses do not have chillers capable of operating at <3°C.  Furthermore the guidelines could reduce the benefit of investing in innovative retail packaging strategies (such as Dar-fresh) because the retail shelf life would be restricted to 10 days. An important part of the cost-benefit of these is extended shelf life (normally at >3°C).

While these guidelines are based on the best available evidence, they cover all foods and it is well known that the risk of C.botulinum toxin formation is not the same for all foods. As far as can be ascertained there is no known case of C.botulinum toxicity arising from fresh vac-packed meat. It was thought probable that a combination of factors present in raw vac-packed meat control the growth of C.botulinum and these could be defined as inherent controlling factors.

Project Aims:

1. To demonstrate whether normal conditions in vacuum packed raw meat provide conditions under which the growth and toxin production of C.botulinum is inhibited.
2. To report the results in a form that could be presented to FSA to modify their guidelines or be used by food business operators to demonstrate an extended shelf life is acceptable within the “any other combinations part of the guidelines.

Approach: 

The project used a combination of literature review, microbiological modelling and practical research (challenge testing).