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Food Safety Focus (181st Issue, August 2021) – Incident in Focus

Staphylococcus aureus and Siu-mei

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

Last month, takeaways from a siu-mei shop were involved in several food poisoning clusters, in which the victims developed symptoms a few hours after consuming the food. Unsatisfactory levels of coagulase-positive staphylococci were detected from the plain chicken and siu-mei samples collected from the shop upon follow-up investigation. Among coagulase-positive staphylococci, Staphylococcus aureus is the predominant one causing food poisoning. In this article, we will discuss how S. aureus is spread to siu-mei and the relevant preventive measures.

Risk of S. aureus due to Improper Handling and Storage

In Hong Kong, siu-mei and plain chicken are among the common food vehicles related to staphylococcal food poisoning, often due to contamination during manual handling and prolonged display at room temperature. S. aureus is a bacterium commonly found on the hair and skin and in the nasal cavity, throat, and wounds of humans. Typically, S. aureus contaminates food through contacts with food handlers’ hands, especially in scenarios where the food is handled manually after cooking. While cooking eliminates other microorganisms competing for growth, it creates a favourable environment for S. aureus from contamination to proliferate. Compounded with prolonged storage at room temperature, the bacterium can grow exponentially and form heat-stable toxins. Therefore, consuming the incriminated food can still make people sick even after re-heating.  It is not unusual to see some food businesses hang and exhibit siu-mei in a display case before the storefront at room temperature for a few hours. This practice calls into question whether the displaying is proper from the perspective of food safety.

Hygienic Handling and Display of Siu-mei

The traditional practice of displaying siu-mei before chopping for a few hours is unlikely to have microbiological concern, as noted from a study conducted by the Centre for Food Safety. The study assessed the changes in the microbiological quality of siu-mei, particularly roasted meats, kept at room temperature for up to eight hours. The study revealed that whole intact piece of siu-mei before chopping did not favour bacterial growth, possibly due to the protective effects of surface drying and seasoning.  That said, food handlers should follow good hygiene practices to minimise contamination with bacteria, including those disease-causing ones. In three out of the eight shops participating in the study, S. aureus was detected at low levels from all the siu-mei samples collected from those shops. This implies that S. aureus could cause widespread contamination at shop level. It is important to maintain a high standard of personal hygiene and cleanliness throughout the preparation of siu-mei.

It is worth noting that chopping siu-mei for display can facilitate bacterial growth. The study showed the total bacterial counts of chopped siu-mei, especially those wrapped in cling film, increased to unsatisfactory levels after being left at room temperature for hours. It is possibly the result of (1) the contamination of meats from the chopping process; (2) the increase in surface area for bacteria to grow; (3) exposure of protein-rich surface; and (4) retention of moisture in food that favours bacterial growth. Therefore, for siu-mei that are cut and packaged for display at room temperature, it is advisable to sell them within two hours, whereas consumers should consume these siu-mei as soon as possible.

Figure 1: Safe preparation and display of siu-meiFigure 1: Safe preparation and display of siu-mei

Key Points to Note

  1. S. aureus are commonly found on body parts including the skin, nose and wounds, and can contaminate siu-mei during manual handling, including the process of chopping siu-mei.
  2. Uncut siu-mei do not support bacterial growth well and hence can be displayed for hours at room temperature. That said, food handlers have to maintain personal and environmental hygiene to minimise cross-contamination.
  3. Cut siu-mei, especially in packaged forms, provide a favourable environment for bacteria to grow to high levels. These siu-mei are recommended to be sold within two hours and be consumed as soon as possible.

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