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Quantifying C. botulinum Spores

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A study from the BBSRC strategically-funded Institute of Food Research has provided new evidence on the background levels of food-poisoning bacteria in raw food to help the food industry deliver safe chilled foods more sustainably.

Botulism is a serious form of food poisoning, caused by a deadly neurotoxin produced by the bacterium Clostridium botulinum. The neurotoxin is so poisonous that eating even the tiniest amount of food in which C. botulinum has germinated, grown and formed neurotoxin can result in severe illness and death. Because of this, the food industry uses high quality raw materials and good hygienic manufacturing practice along with a strict set of safety criteria when producing foods. The safety criteria include precisely defined cooking conditions and times, as well as controlled storage temperatures and use by dates that keep our food safe. The rare outbreaks of foodborne botulism have occurred when these criteria haven’t been followed correctly.

Although these criteria are well established, consumer demand for reduced preservatives, milder heat processing and longer shelf lives is driving continuous innovation in minimally processed chilled foods, which include ready meals and similar prepared items. But delivering this needs a full understanding of how these changes affect the germination and growth of food poisoning bacteria, especially C. botulinum.

Professor Mike Peck and Dr Gary Barker from IFR’s Gut Health and Food Safety Programme have played a key part in a very successful research project called SUSSLE (Sustainable Shelf Life Extension (SUSSLE). IFR collaborated with the Chilled Food Association and Unilever Research to enhance the sustainability of minimally processed chilled foods by using quantitative risk assessment to set a safe shelf life with respect to C. botulinum, and reduce energy usage for minimally processed chilled foods. The SUSSLE project was co-funded by the food industry, Defra and BBSRC.

To quantify the risks it was necessary to generate new information about how many C. botulinum spores are in raw food before processing. Spores of C. botulinum are present in lots of different environments, but only become dangerous when they germinate and produce their deadly toxin. Minimising this risk starts with an assessment of the initial spore load in food. A protocol developed by Prof. Peck and his team is considered a very sensitive enumeration method for spores in food materials, able to detect as few as 10-100 spores per kilogram. However, the actual level of contamination with spores is usually lower than this, and just one spore could be enough to germinate and cause botulism.

To overcome the problem of measuring spore loads that are close to the limit of detection, the researchers combined information from hundreds of historical studies with dedicated experimental and statistical approaches to quantify typical spore loads for different food types.

The researchers reviewed over 700 studies to get an estimate of spore concentrations in different food types, as well as the variability and uncertainty associated with these. These data were then combined with experimental results taken from sampling almost 500 raw foods, prior to any treatment that would reduce the spore load. This information is combined with results from controlled experiments that established the detection limit for spores in the different types of raw food.

Together this gives a statistical framework to provide unbiased estimates of the spore loads, based on statistical uncertainty and targeted evidence collection. Most of the tests were negative, which is significant information to feed into the model. In conjunction with the finding that for many foods it was possible to detect just a few spores of C. botulinum, the research also provides an appreciation of the value of the information set and potential conflicts within the data.

“The evidence shows that for many raw food materials, typical spore loads are smaller than previously reported” said Professor Peck. “This makes a big difference to how we make decisions about food safety and risk assessment.”

These findings can be used to improve active surveillance of foods, by showing how resources could be best used in this area. For meat, fish, shellfish and fungi, additional control experiments involving expected spore loads are most valuable, but for other foods testing additional samples is more effective. In particular, additional tests for spores in herbs and spices would be most beneficial.

Through the SUSSLE project products are now on supermarket shelves that have been produced with lower energy inputs, with safe extended shelf lives, and improved consumer acceptability.