Many people think that heat as well as cold temperatures will keep food safe, but this is not always the case.

A common example is the idea that leftovers are safe to eat once they are heated. However, as bacteria multiply quickly in temperatures between 5 and 60 deg C, food that has been left outside the fridge for too long may contain a lot of bacteria.

Even boiling may not be enough to kill the bacteria, said Professor William Chen, Nanyang Technological University’s Michael Fam Chair Professor and director of its Food Science and Technology Programme.

So, heating catered food that has been left out in the open for several hours before consumption would not make the food any safer to consume.

“Warming the food doesn’t kill the bacteria as the temperature isn’t high enough and some toxins are heat-stable,” said Dr Leong Hoe Nam, an infectious disease specialist at Mount Elizabeth Novena Hospital. “Under harsh (read as cooking) conditions, they form spores, which cannot be destroyed. You can flame it, but you can’t cook it.

When the temperature cools, spores grow and form toxins, said Dr Leong. “I don’t like leftovers. The repeated handling of the food (contaminated or clean) allows the introduction of bacteria. And if it is left on the table for so long, there is a higher chance for the bacteria to overgrow.”

If you plan to keep leftovers, put them aside first in the fridge, he said. Heating up leftovers to boiling point and ensuring every bit of the food is cooked at 100 deg C for a few minutes helps.

Some people buy a rice or noodle dish at lunch, store it at room temperature for hours before reheating it for dinner.

However, experts advise against this. The gravy in a packet of noodles, for instance, has a high water content and rich nutrients, making it an ideal medium for bacteria proliferation, said Prof Chen.

The likely consequence would be food poisoning as bacteria need food (protein) and moisture to survive, and will grow rapidly in moist, protein-rich foods at room temperature.

Some people have the misconception that food is safe to eat once it is refrigerated, but refrigeration will not kill the bacteria; it will only slow down their growth, added Prof Chen. As such, refrigerated food should not be consumed after a few days (as compared with two hours for food left at room temperature).

More precautions should be taken for certain groups of people. Food poisoning is especially serious and potentially life-threatening in young children, pregnant women and their foetuses, older adults and people with weakened immune systems, said Ms Chan Sau Ling, a senior dietitian from the National Healthcare Group Polyclinics.

She said these individuals should take extra precautions by avoiding the following foods:

• Raw or rare meat and poultry

• Raw or undercooked fish or shellfish, including oysters, clams, mussels and scallops

• Raw or undercooked eggs or foods that may contain them, such as cookie dough and homemade ice cream;

• Raw sprouts, such as alfalfa, bean, clover and radish sprouts;

• Unpasteurised juices and ciders;

• Unpasteurised milk and milk products;

• Soft cheese such as feta, Brie and Camembert; blue-veined cheese; and unpasteurised cheese;

• Refrigerated pates and meat spreads; and

• Uncooked hot dogs, luncheon meats and deli meats.

Originally published in The Straits Times.