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Food Safety Focus (97th Issue, August 2014) – Incident in Focus

Bacillus cereus in Processed Food

Reported by Dr. Ken CHONG, Scientific Officer,
Risk Assessment Section,
Centre for Food Safety

Recently, there were two incidents involving prepackaged food products that had been contaminated with excessive amount of Bacillus cereus. Affected products included bean milk and Chinese-style soup produced by two local food chains. This article discusses the risk of Bacillus cereus in processed food and the importance of temperature control.

Bacillus cereus and its Capability

Bacillus cereus is a spore-forming bacterium and grows best at 30°C to 37°C but stops growing at below 4°C. It is commonly found in the environment and a variety of raw foodstuffs such as fruits, vegetables and herbs but the level is generally too low to cause food poisoning. These foods usually contain less than 100 spores per gram, but higher amounts may be found in some herbs and spices.

Cooking, ironically, can give a chance for Bacillus cereus to grow into large numbers when opportunity arises. Not only the spores can survive normal cooking temperatures, the heat of cooking also activates the germination of spores of Bacillus cereus to become vegetative cells. Cooking also kills other bacteria that are not heat-resistant resulting in an environment short of competitors for the vegetative cells of Bacillus cereus to grow. If cooked food is left at ambient temperatures for a prolonged period, the vegetative cells can multiply into large numbers and/or eventually produce emetic (cause vomiting) toxin.

Temperature control to restrict the growth of Bacillus cereus
Temperature control to restrict the growth of Bacillus cereus

Since the toxin is heat-stable (can resist heating at 126°C for 90 minutes), once it is formed, it cannot be destroyed even the food is thoroughly reheated. Food poisoning caused by the toxin is characterised by vomiting within a short period of time after ingestion and has generally been associated with rice products as well as other starchy foods.

On the other hand, the bacterium can cause diarrhoeal type food poisoning which is due to the ingestion of food with large numbers of bacterial cells and/or spores that can produce enterotoxins in the small intestine. Food poisoning of this type is characterised by watery diarrhoea associated with abdominal pain, in which a wide variety of food including meats, milk, vegetables and fish have been implicated.

Temperature Control to Limit the Growth

In general, the presence of more than 100 000 cells of Bacillus cereus per gram of food can cause food poisoning; a simple way to prevent this tenacious bacterium is to limit its growth. Time and temperature control following heat treatment is of prime importance to prevent extensive Bacillus cereus growth and/or formation of emetic toxin. A rapid cooling process is required for heat treated food, followed by storage at refrigerator temperature. A set of standard cooling procedures should be established and the storage condition should be monitored. Alternatively, cooked food can be kept at above 60°C to restrict the growth of the bacterium.

Actions Taken

In response to the media reports, the Food and Environmental Hygiene Department (FEHD) has inspected the processing plants and retail outlets of the manufacturers of the two concerned local food chains and collected food samples for laboratory testing. In view of the excessive amount of Bacillus cereus detected in their products, the FEHD has requested the manufacturers concerned to review and improve the production processes. The FEHD will closely monitor the situation and step-up inspection of the manufacturers concerned.

Key Points to Note

  • Bacillus cereus is ubiquitous and presents in a variety of foodstuffs.
  • Improper storage subsequent to heat treatment can allow the bacterium to grow and form heat-stable toxin.
  • Rapid cooling followed by proper refrigeration after heat treatment helps to prevent the multiplication of the bacterium.

Advice to the Public

  1. Keep food at safe temperatures, i.e. above 60°C or at or below 4°C, if it is not consumed immediately.
  2. Consume perishable prepackaged food and beverage promptly after opening or reheating and avoid prolonged storage at ambient temperatures.

Advice to the Trade

  1. Observe Good Manufacturing Practice (GMP) in food processing to assure that the food products do not pose risk to the public.
  2. Food businesses are recommended to implement the Hazard Analysis and Critical Control Points (HACCP) system, a systematic application of good practice to the prevention of food safety problems.
  3. Implement preventive measures, particularly in food businesses with large volume throughput, to restrict the growth of Bacillus cereus in heat treated food, for example:
    • install specific rapid chilling equipment to speed up the cooling process, and
    • closely monitor the temperature of refrigerator and maintain a temperature log.
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Last Revision Date : 20-08-2014