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Chicken rice, cai png and roast meat: Is it safe to eat food left out in the open?

Tan Ee Lyn on 12 Oct 2019

The Straits Times


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SINGAPORE - A key hygiene rule requiring food to be kept hot - above 60 deg C - before eating to prevent poisoning has thrown up interesting questions.


How have generations of Singaporeans managed to survive common fare like Hainanese chicken rice, cai png (economy rice) and roast meat, when some items like the chicken itself are left hanging for hours or even an entire day, without massive food poisoning events?


Scientists and food science experts The Straits Times spoke with offered various explanations: From minimal handling of the food once it is cooked to prevent contamination, to keeping the food clean right from the start to minimise the microbial load.


Dr Ch'ng Jun Hong from the department of microbiology and immunology, National University of Singapore (NUS) Yong Loo Lin School of Medicine, explained that thorough cooking kills microbes in food and contamination primarily occurs later from exposure in the external environment, such as in the handling of cooked food with tainted hands or using non-sterilised utensils.




"If a freshly roasted chicken is hung out whole for a few hours before it is chopped, it will probably contain much less bacteria than if the same chicken was chopped up first prior to being stored for the same amount of time," said Dr Ch'ng, adding that utensils involved such as the chopping board, gloves and cleaver may not be clean.


The early introduction of microbes in the latter instance and the time given for them to grow can give rise to large numbers of toxins and subsequently, food poisoning.


"This could explain why contaminated pre-packed individual portions (for example, bento boxes), which are then stored before consumption, are likely to be contaminated with more bacteria than a large portion of freshly cooked food, (for example, cai png) which is portioned to the customer only on order," said Dr Ch'ng.


The discussion follows an ST article last week which found that food poisoning is more likely to occur from dining at restaurants and consuming catered food than from eating at places such as food courts and coffee shops. Contrary to popular perception, eating at hawker stalls is least risky.


In that article, experts stressed the importance of keeping cooked foods at over 60 deg C to prevent microbes from multiplying and causing food poisoning. Readers wrote in afterwards to ask how foods like Hainanese chicken rice, roast meat and cai png - which are left in the open for hours - appear not to cause any problems.


Assistant Professor Li Dan from the department of chemistry's food science and technology programme at NUS raised the possibility of under-reporting of food poisoning incidents, especially when they happen sporadically to customers of individual hawker and food court stalls.


"Even if they (customers who fall ill from food poisoning) go to the doctor, they may not report to the Government," said Dr Li.


On the other hand, food poisoning events at restaurants and catering outlets are invariably on a larger scale, drawing more attention, said Professor William Chen, the Michael Fam Chair Professor and director of Nanyang Technological University's food science and technology programme.




The experts agreed that pre-cooked food, like chicken and roast meat, would carry much less microbial contamination than, say, raw food like salads and sushi. Foods like char siew (roast meat), which are relatively dry and sweet, also appear not to support microbial proliferation.


Dr Li said: "If you have food with low water activity, either it's very dry or has a lot of sugar or salt, these foods don't support the growth of microorganisms."


Prof William Chen said: "All microbes need not only nutrients but more importantly a watery environment to proliferate. The delicious hor fun would be a more conducive environment for microbial growth and (carry) higher risk of food poisoning compared with french fries."


Assistant Professor John Chen at the department of microbiology and immunology, NUS Yong Loo Lin School of Medicine, explained that foods that are very salty or sweet will block the growth of microorganisms and prevent spoilage.


"These would be any pickled foods, which are high in salt, in acidity. Some foods are too high in sugar content, which would be any kind of sweets or even pure honey would be a good example - this is why you can leave honey or confections at room temperature and not have to worry about them spoiling," said Prof John Chen.


Experts stressed the importance of using clean ingredients right from the start to minimise the load of microorganisms. "We don't only get sick from live bacteria, we also get sick from toxins. When bacteria accumulate and grow to high numbers on food left at room temperature, they start to produce toxins. Toxins are a lot more heat resistant than bacteria," said Dr Li, who added that toxins manage to stay stable even at 120 deg C.


Prof William Chen said: "It's important to reduce the bacterial contamination from the start, such as thorough washing of vegetables and separate preparation of meat from other foods."




Dr Ch'ng called for food to be kept behind barriers because of contamination from food handlers who may themselves be carrying pathogens.


"Food that has been improperly handled (contaminated) by someone who is ill is more likely to result in a high load of contamination by a highly virulent pathogen. Such contamination, whether just before or long before consumption, will give rise to food poisoning," he said.


"Aerosolised droplets containing pathogens, from say someone sneezing, could also land on food and start growing - hence the need for food to be as isolated as possible (for example kept behind glass, with food handlers wearing face masks)."


Dr Ch'ng offered a possible explanation for the mystery surrounding Hainanese chicken rice, a meat that is sometimes left slightly pink and which is submerged for a while in ice water right after cooking - a step that some chefs explain locks in the juices of the chicken.


"As for food which may not be completely sterile after cooking (for example "white" chicken rice where the meat is not cooked through thoroughly), the practice of keeping most of the chickens in ice water will also slow the growth of microbes," said Dr Ch'ng.


Bacteria such as salmonella can be found in different foods including beef, chicken, eggs, fruit, pork, sprouts and vegetables, but they are safe to consume if they are stored and handled properly and cooked thoroughly.


Dr Ch'ng said it is still safer to eat food that is completely cooked.


"People are more likely to get ill from consuming uncooked and partially cooked food," said Dr Ch'ng.


Asked for its comment, Singapore Food Agency said all retail food stalls and establishments must ensure that food hygiene standards are adhered to. They should adopt key control measures for food-borne pathogens, including observing guidelines on time-temperature control. (More information can be found at: https://www.sfa.gov.sg/food-retail/food-hygiene-practices-guidelines)


Source: The Straits Times © Singapore Press Holdings Limited. Reproduced with permission.



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