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Do you microwave food in plastic containers? Be careful of leaching chemicals

Whether it’s disposable or reusable, people are using a lot of plastic food containers lately. We asked science experts to explain how to use them safely.

Whether you get your food via Deliveroo or you go to the hawker centre to order takeaway, plastic food containers are a big part of mealtimes – especially when dining out continues to be a non-option during this post-“circuit breaker” period.

If you’re eco-conscious, you might have been opting for reusable containers, while those who are back in the office might decide to give container-lending apps such as Muuse and barePak a go. For the rest, it’s probably a case of all those disposable ones piling up at home.

But whatever you’ve been doing, have you ever wondered if there’s any difference between all these plastic containers? After all, aren’t they all transparent, come with a lid and are, well, plastic-y?

READ: No red meat, more veggies? How to know if a plant-based diet is right for you


The chemical structures of these plastics are indeed the same, and remain so, whether it’s after single use or multiple uses, said Associate Professor Suresh Valiyaveettil from the National University of Singapore’s Department of Chemistry, Faculty of Science.

However, different forms of plastic contain different additives (such as stabiliser and plasticiser) and are processed differently, which affect features such as thickness and melting temperature.

For example, the reusable ones are “usually thicker, more stable, have a high durability and will be able to withstand multiple degrees of changes in environmental conditions”, said Prof Valiyaveettil.

Disposable plastic containers, on the other hand, have a lower density than their reusable counterparts, which means that they are less resistant to heat, said Dr Henry Leung, a senior specialist in Pharmacology & Toxicology, and a senior lecturer with Nanyang Polytechnic’s School of Chemical & Life Sciences.

But regardless of disposable or reusable plastic containers, there are dos and don’ts that you should be aware of. Here’s a look at some of the common questions you may have when using these boxes.


It’s so convenient to pop that plastic container of food into the microwave oven for a quick reheat and dinner is served.

That’s actually not a good move if you’re using disposables. Not only does the plastic warp or partially melt in the oven, you’re also leaching “small molecular additives” – which give the plastic its colour, stability and durability – out of the plastic and into your food, said Prof  Valiyaveettil.

These additives include the controversial bisphenol A (BPA) and phthalates. While great for making plastic malleable during production, the chemicals aren’t great for our hormones, reproductive systems as well as foetal and infant developments

That said, the US Food and Drug Administration highlighted that “BPA is safe at the current levels occurring in foods” even as it continues to monitor consumers’ exposure to phthalates to determine if there is a risk.  

(Photo: iStock/Kanawa Studio)

But even without the heat from radiated food, the leaching of chemicals may still occur, though at a lower rate. The “leaching and solubility of the small molecules are linearly proportional to the temperature”, said Prof  Valiyaveettil. 

“While heating in a microwave oven, some of these small molecules leach into the food much faster than at room temperature.” In other words, the hotter the food is, the more unsavoury chemicals there are.


Prof Valiyaveettil advised that even if the containers are microwave-safe, it is better not to heat food in plastic containers in the microwave oven at all.

And to play it even safer, don’t let hot food sit in plastic containers – disposable and reusable alike – for long. Transfer your piping-hot mee siam or white beehoon into ceramic or metal dishes as soon as possible.

Since heat plays a part in leaching out the chemicals in plastic, it makes sense to keep your containers away from sunlight and at room temperature, recommended Prof Valiyaveettil. Also, do not use plastic containers to store foods with highly acidic ingredients such as vinegar or lemon juice.

The next precaution is to make sure the plastic container doesn’t have BPA or phthalates. But how do you know?

“In many countries, the manufacturer is required to put labels to indicate the product as BPA- or phthalates-free. Reputable brands usually put those labels to make sure that the consumers make a knowledgeable selection, which will benefit them in the long run,” said Prof Valiyaveettil.

READ: COVID-19: Food container suppliers face potential shortage during circuit breaker

Another way is to check the bottom of the container for the Resin Identification Code, usually represented with a number inside a triangle, said Dr Leung.

According to Prof Valiyaveettil, the codes are used to identify the polymer used, which makes it easier for sorting and recycling:

  • Code 1: Polyethylene terephthalate or PET
  • Code 2: High density polyethylene or HDPE
  • Code 3: Polyvinyl chloride or PVC
  • Code 4: Low density polyethylene or LDPE
  • Code 5: Polypropylene or PP
  • Code 6: Polystyrene or PS
  • Code 7: Polycarbonate or PC and; other plastics

“Containers with Codes 1, 2, 4, 5, and 6 are very unlikely to contain BPA,” said Dr Leung. “Codes 3 and 7 are likely to contain BPA.”

Prof Valiyaveettil noted that Code 6 “usually contains additives such as phthalates or BPA” as well. “Code 5 or polypropylene (PP) is the “most suitable polymer for all types of applications, including microwaving, due to its high stability,” he said.


It’s best not to reuse disposable containers, even if it’s just for storing non-heated foods such as biscuits or salad.

“The plastic used in such containers tend to be low in density and is coating-free. This increases the risk of air leakage, which could expose the food in the containers to the bacteria in the air,” said Dr Leung.

READ: 1 in 2 Singapore parents gets stressed out during mealtimes – how to deal with fussy kids

But if you’re in a pinch, Prof Valiyaveettil said that some containers can be washed and reused for simple purposes. 

“For example, a fruit container can be washed and reused to keep fruits again, but not to hold hot curry or other hot foods. The performance of such containers are only guaranteed for the first-time use. It may perform for the second and third use, but may fail more often, too,” he said.

“It is similar to the expiry date on many items such as pharmaceuticals and canned food. The quality is only guaranteed by that date.”


Any change to the container – be it shape, colour or smell – is an indication that the plastic wasn’t able to withstand the temperature and has deformed, said Prof Valiyaveettil.

“Most plastic containers designed for F&B should be able to withstand hot food,” said Dr Leung. “However, there are some containers that may still warp. In these cases, the containers should not be used.”

The same goes for lids that bulge when covering hot food but revert to their original shape when cooled. 

“The lids may look like they are back to their original shape but the quality would be affected,” said Dr Leung. “Once the lids show signs of warping, it means they have reached their limit and should no longer be used.


Compared to the seven polymers mentioned earlier, silicone products are relatively safer, said Prof Valiyaveettil. “However, silicone has a higher oxygen transport rate, which is bad for food storage.”

Another downside to silicone is that it tends to trap food odours. Those who own those collapsible silicone lunch boxes or reusable cups would know. The smell of garlic or coffee lingers despite your best effort at the sink.

The New York Times’ Wirecutter website suggests baking your silicone containers in the oven at 250 degrees Fahrenheit (120 degrees Celsius) for 20 minutes. But before you do that, make sure that the container doesn’t have food residue or oil. More importantly, remove all non-silicone components – you don’t want melted plastic ruining your oven. 

“Such a cleansing method may be worth a try if the odour is coming from external sources such as food residue,” said Dr Leung. But he suggested replacing the silicone container immediately “if the odour is of the silicone itself”.

“Discolouring and odours could be a sign of silicone ageing after repeated use. Regular household cleaning agents may not be able to remove them. When this happens, it is time to replace them,” he added.

Source: CNA/bk