IFR finds spore loads of Clostridium botulinum smaller than previously thought

This makes a difference to decision making when it comes to food safety and risk assessment, said the researchers.

Consumer demand for milder heat processing and longer shelf life is driving innovation in minimally processed chilled foods, which include ready meals.

Botulism is a serious form of food poisoning, caused by a neurotoxin produced by the bacterium Clostridium botulinum.

Quantifying risk​

To quantify risk it was necessary to get better information concerning how many C. botulinum spores are in raw food materials before they are processed.

Researchers looked at meat, fish, shellfish, cereals, fresh plant material, dairy liquid, dairy non-liquid, mushroom and fungi and dried herbs and spices.

Spores of C. botulinum are present in different environments, but only become dangerous when they germinate, and produce toxin. The origins of spore contamination in food materials are unclear.

Professor Mike Peck and Dr Gary Barker from IFR’s Gut Health and Food Safety Programme looked at the issue as part of the research project SUSSLE (Sustainable Shelf Life Extension).

An optimized protocol developed by Peck and his team, able to detect as few as 10-100 spores per kilogram, combines selective enrichment culture with PCR detection of the botulinum toxin gene.

However, the level of contamination is usually lower than this and close to the limit of detection; growth from just one spore could be enough to cause botulism.

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

Models use a Bayesian framework and represent information from a literature survey of spore loads, from positive control experiments that establish a detection limit and from microbiological tests of 483 samples of food materials from nine categories.

Identifying new safe shelf life​

Professor Peck said the work used risk assessment techniques and probability distributions to identify a new safe shelf life for minimally heated chilled foods.

“Spore distributions were generated in the nine categories of raw food materials that we tested. The literature data says the most likely concentration is 10-100 spores per kg, with higher or lower concentrations also possible, but rarer. Most samples we tested were negative and reassessing the most likely concentration is now 1-10 spores per kg.​

“When we tested foods the concentration was lower and distribution more sharply peaked so we are more certain on concentration of spores. It was an exercise in reducing experimental uncertainty.​

“These distributions of spore loadings are used as input into a risk assessment and combined with other information such as the heat treatment given, incubation and growth time to determine a safe shelf life. This research gave us a better, more reliable, estimate of the spore loading of raw food materials.”  ​

Industry employs a safe heat process (in a similar manner to the ‘botulinum cook’ given to canned foods (121 degrees for three minutes)) and keep records on every run given that gets the heat treatment.

Previously most chilled foods were prepared according to either the “10 day rule” (10 day shelf-life) or heated at “90°C/10min” (shelf-life might be as much as ca. 40 days). The SUSSLE project identified a safe intermediate shelf-life that uses a heat treatment less than 90°C/10min.

With regard to further work, for meat, fish, shellfish and fungi, additional control experiments involving expected spore loads are most valuable, but for other foods testing additional samples for spores is more effective such as the case in herbs and spices.

An output of the SUSSLE project is that products are on supermarket shelves that have been produced with lower energy input and safe extended shelf-lives.

IFR worked with the Chilled Food Association and Unilever Research 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 industry, Defra and the Biotechnology and Biological Sciences Research Council. 

Source: Applied and Environmental Microbiology 

doi: 10.1128/AEM.03630-15​

“Quantification of non-proteolytic Clostridium botulinum spore loads in food materials​”

Authors: Barker G. C., Malakar P. K., Plowman J., Peck M. W