Reader Kelvin Leong wrote in to ask about reusing oil for cooking.

He said: "I keep the oil (rice bran oil) after deep-frying meat or seafood, and use it for a few rounds. I keep it clean by straining it.

"I am aware that the quality will deteriorate with each reuse, but it is such a waste to pour the oil away after one or two times. Please tell me how many times I can safely reuse the oil." Food writer Eunice Quek finds out.

According to the Health Promotion Board, one should avoid reusing oil more than twice. Reusing oil too many times can increase exposure to free radicals, which can cause cell damage, says Ms Bibi Chia, who is principal dietitian at the Raffles Diabetes & Endocrine Centre.

"Heat-sensitive vitamins and antioxidants will be reduced after each use," she notes.

Her advice is to check the colour of the oil before reusing - discard it if it has turned dark or black. It should also be discarded if it smells rancid or has turned thick and sticky.

To remove crumbs from the oil after frying, use a strainer or cheesecloth. Allow the oil to cool before storing in an airtight container, away from exposure to air and light.

The same storage method applies whether the oil is peanut, corn, sunflower, grapeseed or olive.

However, whether you should reuse the oil depends on its smoking point. That refers to the temperature at which the oil starts to smoke and break down rapidly. The smoking point is reduced each time the oil is reused.

Only oils with higher smoking points can be reused, says Ms Chia. Extra virgin olive oil, for example, has a low smoking point, and is not recommended for reuse as there could be exposure to harmful chemicals.

Ms Chia says: "When choosing oil, think of the temperature needed to prepare the food, and the flavour you want the food to have. Use only one or two types of oil at a time, to keep them fresh. Heat, light and exposure to air can cause oils to turn rancid or taste bad."