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Aioli using raw egg: Salmonella Typhimurium

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On 18 January 2010, NSW Health notified NSW Food Authority of a gastroenteritis outbreak in 20 people who had eaten from a retail burger bar on 14 and 15 January 2010. The burger bar was a popular eatery - by 28 January 2010 NSW Health had linked a total of 179 Salmonella Typhimurium phage type 9 cases to the business over the 2 days in question.

Interviews of Salmonella cases determined that aioli was a common food served over the exposure period. The business prided itself on its homemade burgers and ingredients. The aioli was prepared on the premises.

DNA fingerprinting using MLVA identified a match between clinical isolates and Salmonella isolates detected from the burger bar premises.

Environmental investigation

On 18 January 2010, the environmental health unit of the local council inspected the premises after also being notified by NSW Health. A batch of aioli in use at the time of inspection was sampled on the basis of preliminary epidemiological evidence.

An initial interview of the proprietor confirmed the aoli had been prepared using raw eggs and did not receive any further cooking or processing. The business was instructed to cease serving this type of product.

Additional food and environmental samples were obtained for testing on 19 January 2010. In total there was a sample of the aioli, eggs, a cleaning cloth, swabs of a food preparation bench, food preparation cutting boards, a food storage area, and the hand washing area. As there were no new cases linked to eating at the premises after 15 January 2010 the business was allowed to continue operating.

On 22 January 2010, the Food Authority was notified by NSW Health of another 2 possible cases who had consumed food from the business on 19 January 2010. On the basis of this information the Food Authority contacted the business and requested that they cease trading immediately. The Food Authority served a formal Prohibition Order on the business that day ordering that no food for sale be handled on the premises until a clearance certificate was issued.

On 24 January 2010, test results showed that Salmonella was present in the aioli sample and a swab of a plastic chopping board.

A further premises inspection was undertaken on 25 January 2010 and a number of swabs were taken from various areas and tested for Salmonella. These were subsequently negative.

The prohibition order on the business was lifted after two weeks on the basis of the later test results, demonstration by the staff of appropriate food safety skills and knowledge, and improvements in cleaning and sanitising procedures.

Food safety implication 1: preparation of aioli from raw eggs, poor food handling

Many outbreaks of salmonellosis have been linked to products using raw eggs.

  • The practice of pooling large quantities of eggs to produce mayonnaise, desserts, or other products increases the likelihood of Salmonella being introduced from the surface of an egg shell to a food which does not receive a further ‘kill’ step.
  • Most outbreaks involving the use of raw egg products also involve poor food handling. This outbreak occurred during summer when ambient temperatures at the burger bar were high (>30°C) and eggs were not stored in a refrigerated environment. Under these conditions eggs may ‘sweat’ which reduces their shelf life and increases the potential for penetration by Salmonella from the outside of the egg shell.
  • Analysis of the aioli found that it had a pH of 5.8 which is not sufficient to prevent the growth of any Salmonella present, particularly at the high ambient temperatures current at the time of the incident.
  • In this incident the business was sourcing eggs from a local hobby farm rather than a dedicated egg supplier. The hobby farm did not have any system for quality control such as candling or crack detection, and eggs were not safely washed prior to sale. Eggs were also placed into re-used cartons which increased the potential for cross contamination of Salmonella to the outside of shells.

What should the business have done?

  • use a pasteurised egg product or commercial mayonnaise in place of raw egg ingredients
  • stored eggs under refrigeration: below 5°C
  • sourced eggs from a recognised commercial supplier.

Food safety implication 2: insufficient sanitising of food contact surfaces

  • investigation of cleaning practices revealed that an antibacterial surface spray/wipe product was used by the business. This product had a low ethanol content and was inadequate for commercial use as a sanitiser
  • there was no document or schedule outlining a cleaning regimen of how and when equipment such as bench tops, floors, chopping boards and other equipment should be cleaned and sanitised.

This may have contributed to the Salmonella finding on a chopping board.

What should the business have done?

  • obtain an appropriate chemical from a supplier for use in a food service facility, such as a hypochlorite or quaternary ammonium compound
  • ensure that a well understood, documented cleaning regime is in place and adhered to rigorously.
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