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UK Salmonella outbreak sourced to chocolate bars

Chocolate products manufactured in England have been implicated as the most likely source of an outbreak of Salmonella Montevideo that affected 37 people in the UK from February to June 2006.

A leaking pipe was discovered by the manufacturer at one of their plants in January. Waste water had been dripping into a milk chocolate crumb mix, a base ingredient in several chocolate bar varieties. Tests subsequently performed by the company revealed the presence of the rarely found Salmonella strain in some products, but the company failed to report these findings to food regulators or to instigate a recall. It was not until the UK Food Standards Agency (FSA) published the results of their own outbreak investigation in June that the company disclosed their findings and withdrew seven products (1 million chocolate bars) from the UK market.

The manufacturer stated in an official media release that the reason they did not initially report their contamination findings was because the low levels of microorganisms found did not warrant such an action.

A report released in July by the Advisory Committee on Microbiological Safety of Food (ACMSF), an expert committee that advises the FSA, responded with the statement that "the presence of Salmonella in ready-to- eat foods such as chocolate is unacceptable at any level". The company implicated in the outbreak have since released a further statement declaring that they have changed their protocols and that any products showing traces of Salmonella, regardless of how low the levels are, will be destroyed.

The potential hazards associated with Salmonella and chocolate confectionery have long been known (Journal of Food Protection 1977 40, 718-727). The specific hazards are: the low moisture and high sugar content in chocolate increase thermal resistance of bacteria; and the high fat content provides protection for Salmonella against stomach acid resulting in the low infective dose of the microorganism when consumed in chocolate.

Heat treatments used during chocolate processing (e.g. conching) have been shown to be ineffective at inactivating salmonellae in high fat products, one study found that even at 90°C it took more than 1 hour to inactivate 90 percent of Salmonella Typhimurium in molten chocolate (Journal of Food Protection 2000 63, 779-795). Salmonella outbreak investigations involving high fat products have shown that as few as 1–2 cells per gram are sufficient to cause illness. In a 1985 outbreak associated with Salmonella Nima in chocolate the infective dose was reported to be as low as 0.005–0.025 cells/g (Journal of Food Protection 1989 52, 51- 54). Investigations into a 1996 Salmonella Mbandaka in peanut butter outbreak revealed that illness may have been caused by as little as 3 cells/g (Journal of Applied Microbiology 2000 89, 472-477).

The July 2006 ACMSF report also stated that the company had failed to follow new European Unionwide guidelines and implement a Hazard Analysis Critical Control Point (HACCP) program. Cordier (Food Control 1994 5 (3), 171-175) uses the HACCP program approach to outline the microbiological hazards associated with chocolate manufacturing. The areas discussed include the quality of raw materials, the risk of water or air contamination, validation of thermal processes and the potential for post-process contamination. This 2006 UK outbreak highlights the potential issues associated with the layout of production plants if exposed ingredients are situated in the vicinity of waste water pipes.

The benefit of having an effective HACCP program is that the focus is on the prevention of microbial contamination in the finished product, rather than on detection. This is particularly important in products such as chocolate where underestimation of numbers or even failure to detect microorganisms can result from a lack of sample heterogeneity and limitations in testing procedures.