Quick Answer: You can microwave cardboard if it has the microwave-safe label. Some cardboard may not be safe to microwave, and these include the likes of waxed cardboard, cardboard that has been printed with inks, and assembled cardboard trays and cartons that have been constructed with adhesive or metal. If you would like more detailed information, please read on.
A few years ago, I had to up sticks and moved away from home and family for a few months to work in a distant state. My accommodation was little more than a shipping container with electric power.
I bought myself a single burner camping-gas ring and a microwave to heat food. I couldn’t afford to eat out. I used to get the occasional take out Chinese or Indian.
Boy, did I become an expert with the microwave, or what?
Anyway, I learned a lot about microwaving during that time, particularly about cardboard and microwave containers, and I am more than happy to share what I learned with you. So, here it goes.
Can I warm up food in cardboard?
As a rule of thumb, you can put pure cardboard containers bearing foods or drinks into the microwave.
We would recommend, however, that youngsters should only do so when they are being supervised.
Some cardboard may not be safe to microwave, and these include the likes of waxed cardboard, cardboard that has been printed with inks, and assembled cardboard trays and cartons that have been constructed with adhesive or metal.
Never expose cardboard containers to too much heat, and don’t cook food in them. The bottom line is always to read the label.
If you would like more detailed information, please read on.
A lot of take-out food establishments use Styrofoam containers. They are light and have reflective insulation properties that keep hot food warm.
But if you reheat food in a Styrofoam container or tray in your microwave – it will melt. Not only that, but chemicals can get into the food from the melted foam.
Some take-outs now supply their foods in cardboard trays and cartons. But is this suitable to heat up in the microwave? Well, as I’ve stated above, you have to be careful.
But of course, it’s your food we are talking about. So, to help you to make a more informed choice, I’ve carried out some research and, if you carry on reading, I would like to share my findings with you.
Some facts about cardboard
Cardboard comes in various types and grades. Some are designed for packaging and maintaining the integrity of an item.
This will be explained in a moment. I will also offer you some vital information about safety labels. Also, I will give you some useful tips on how to microwave safely.
Materials to avoid putting in a microwave
If you are new to microwaving, there are many materials you must avoid using. These include aluminum foil, ordinary plastic bags, metal, Styrofoam, and plastic water bottles.
If you do inadvertently put any of these things into a microwave, one of three things will happen, depending on what the material is.
- The item will get damaged.
- Whatever is in the item will get damaged.
- Your microwave oven will get damaged.
Whichever one or more of these items is microwaved, the results could be catastrophic. It is, therefore, crucial that you check exactly what it is you are about to microwave.
Nature of cardboard
Cardboard is a heavy paper-based product that is produced in various thicknesses. It is also sometimes referred to as paperboard. Its most common uses are for packaging and transporting.
However, you will probably find several cardboard items in your home. These include things like:
- Cartons containing cereals
- Kitchen paper rolls
- Shoe boxes
- Food packaging
You may also have corrugated cardboard at home. Basically, it’s two pieces of flat cardboard sandwiching a grooved piece of cardboard in between them.
It’s a very useful protective packaging material. Shipping boxes and beer six-packs are often made from it.
As far as cooking and cardboard goes, you have to be a little careful. Several cardboard products (particularly those not associated with foods) could have chemicals in their make-up. It’s not the sort of material you want to risk putting in your microwave.
The trend to minimize the amount of disposable plastic that finds its way into our oceans means that cardboard containers are used more and more, and many people are uncertain about whether they are safe to microwave.
Various types of cardboard containers and their uses
Cardboard containers used in the food industry come in all shapes and sizes and are designated for various uses. These include:
- Containers for take-out foods
- Pizza boxes
- Bowls that can be recycled
- Disposable cups
- Compartmentalized food trays
- Eco-friendly food packaging
There is even a cardboard piece inside your microwave. It’s there to mask the waveguide and must not be removed under any circumstances.
How does microwave warm up cardboard?
To get to the bottom of what happens when you microwave cardboard, it’s necessary to understand some of the basic science behind the process.
Without wishing to state the obvious, microwave ovens work on the principle of generating microwaves. So, let’s begin by explaining what microwaves are.
Microwaves are a form of electromagnetic wave. Their wavelength is pitched between infrared radiation waves and radio waves. Inside each microwave, there is an inbuilt vacuum tube known as a magnetron.
It is this device that generates the microwaves to a frequency of 2450 MHz.
These microwaves get absorbed by the fat, sugar, and water molecules of which the food is comprised. This absorption makes the molecules begin to rotate, and in doing so, they interact with other molecules.
This movement of the molecular structure generates heat, and this heat warms the food and liquid. If the foodstuff is inside a cardboard tray or carton, the cardboard will also heat.
Particular types of plastic also soak up microwaves, which is why you have to be very careful to only put certain plastic articles in the microwave.
Way that cardboard reacts to microwaves
Cardboard gets used in various industries and appears in multiple forms. Besides the foodstuff packaging that we already referred to, cardboard is used in things like pen cases, packaging medicines, and a whole host of other applications.
Cardboard is formatted in many ways and ends up as being corrugated cardboard or double or triple wall cardboard, etc. But, at the end of the day, whatever the final format, they all share one basic fact. They are all created from cellulose (wood fiber), as is paper.
The average moisture content cardboard is between 9% and 12%. When it is heated in a microwave oven, the moisture elements will begin to decrease and can get a lot as low as 1%, depending on two things. The first is the surface area of the cardboard, and the second is for how long it is microwaved.
This dehydration for want of a better word takes place quite rapidly. If you were to microwave it too long inadvertently, the cardboard would become completely parched and would ignite.
However, it should be noted that this only happens when the cardboard is microwaved in isolation without any foodstuffs.
The take-away from this is not to microwave cardboard in isolation because under certain scenarios, it could lead to a fire. What it boils down to is that anything that is cardboard is, in essence, flammable.
You may come across some articles or communiqués that maintain that it is safe to microwave cardboard (even in isolation) for drying purposes, for example.
The point is, however, if such a process is not very carefully monitored, it can be risky. While the risk depends on the type of cardboard being used, it is recommended not to microwave cardboard in isolation.
What if the cardboard tray or carton contains foodstuffs?
Having concluded that microwaving cardboard on its own is not recommended, let’s take a look at what happens when the cardboard contains foodstuffs and is microwaved.
As we have already stated, there are numerous types of cardboard boxes, trays, and containers. The rule of thumb is that cardboard boxes containing food and nothing else are generally safe to microwave.
The reason is that when you heat foodstuffs in a cardboard container, the microwaves initially get absorbed by the foods. Therefore the amount of moisture in the cardboard won’t get depleted to hazardous levels.
In other words, it is not usually risky to heat foodstuffs in a pure cardboard container unless the label warns you otherwise.
Bearing in mind, though, that we are talking about merely reheating. You should not cook or boil foodstuffs in cardboard containers.
If you allow the cardboard with foodstuffs to be cooked for an extended period of time, it will damage both the cardboard and the food.
Instances when cardboard shouldn’t be microwaved
It’s essential to bear in mind that some cardboard food containers have a plastic or wax coating or that their construction includes these materials.
You should not microwave these types of cardboard containers unless the label expressly states otherwise.
1 Beware of waxed containers.
The wax that is used to coat cardboard and paper containers is, in most instances, polyethylene or PE for short. When heated at high temperatures, this type of plastic will melt, resulting in chemicals leaching into the foodstuff.
This cannot be allowed to happen. Wax coated food containers are at least easy to spot because they are shinier and smoother than an ordinary cardboard.
2 Look out for containers that carry ink
Containers that carry ink or print can also present a hazard. It is, of course, possible that not all printed cardboard containers are dangerous.
To stay on the safe side, you should read the label carefully and only microwave containers classed as microwave-safe.
3 The concern about adhesives
Quite a lot of packaging is assembled using adhesives. You should not microwave such containers. The microwave process can result in toxic gasses being emitted, and if this migrates into the foodstuff, it will represent a hazard.
4 Metal in food take out containers
A lot of take-out establishments (Chinese take-outs, for example) use trays that contain metal. Putting metal of any sort in a microwave is a no-no.
Not only could it damage be microwave but it could also cause a fire.
There will be instances when you’re not sure whether to microwave something in a cardboard container.
The golden rule is that if you are unsure, follow the instructions on the label. If you are still unsure, don’t microwave it.
You should also make a note that it is not recommended to microwave food that is in brown paper bags. They can easily catch fire, and any glue or print they contain could create toxic fumes.
Never forget that when you read eat foodstuffs, they, as well as the cardboard containers, can be hot enough to burn or scald your skin. Test them carefully before handling and consuming.
Recommendation for safe microwaving
If you are microwaving foodstuffs you’ve precooked yourself, you will need to put them into some sort of container before microwaving.
It gets a bit messy if you don’t! Let’s take a look at some of the container options, starting with the ones you can use.
1 Containers made from glass
In my view, glass containers are great. There is no question that they are perfectly safe to microwave. The only thing to be aware of is that the glass can get quite hot if it’s microwaved for any length of time.
So, if you want to avoid turning the air blue with a few expletives, handle the heated container with oven gloves.
Paper plates, towels, and napkins may be used with care, but only if you are going to be warming something gently, so it is Luke-warm.
I often wondered who Luke was and why he was warm? Maybe he was just a friendly guy? Anyway, back to the point. Microwave foodstuffs in these materials too long and the material could catch fire.
Some of the products are plastic coated. Just to be sure, follow NSF guidance and check the label to see if they are microwave safe.
2 Ceramic containers
Good quality ceramic products are glazed in an oven and should be microwave friendly. However, watch out for ceramics made in China.
No, don’t put that Ming vase in the microwave!! It’s worth far too much. No, in all seriousness, the Today Show ran an item about China not properly regulating its ceramics industry.
Some of the glazes they use have high quantities of arsenic and lead in them. Neither of these elements makes particularly good condiments if they leach into the foodstuff.
Not to mention other undesirables that could be included if the factory is situated in Wuhan. Some ceramics might even have metal particles in them, which can cause worrying sparking.
3 Waxed paper and parchment
The NSF also advises that waxed paper, and parchment, should also be safe to microwave as long as they contain foodstuffs.
Materials to avoid microwaving
So, now you have a good idea about what should be safe to microwave. But what about the other container materials you might come across? Here’s what you need to avoid.
1 Aluminum foil
Technically speaking, you ought to be able to microwave foodstuffs in aluminum foil. Personally, I would avoid doing so. Did you know that “Hot Pockets” packaging has an aluminum lining?
Well, it has. I would suggest that you might have more than hot pockets if you leave it microwaving for too long. They might ignite. Talk about pants on fire!!
2 Containers made from metal
I’ve already given you my opinion on this one. Metal is an absolute no-no. You might get some pretty fireworks for a few seconds before your microwave oven irretrievably breaks down.
3 Brown paper bags
Some brown paper bags from stores and supermarkets are plain, and some are printed. Whatever type they are, the USDA says they are unsanitary, flammable, and may even emit nasty fumes when they burn or smolder. You can buy oven cooking bags. Use these instead.
4 Bespoke foodstuff containers
The things we are talking about here are the plastic containers and tubs that foodstuffs like butter, cottage cheese, margarine, and yogurt, etc. are sold in.
They are all made from some type of plastic. Unless it says otherwise on the label, don’t put them into your microwave, unless it’s on the de-freeze setting, and even then, only for a few seconds.
5 Plastic rubbish bags etc
Now c’mon. Would you seriously consider microwaving food in a plastic rubbish sack? Who in their right mind would?
Even for those people who might think of trying, (and this includes other materials like garbage cans and film canisters), the University of Nebraska warns you not too.
6 Other plastics
Some plastics have been linked with causing cancer. Surely, not something you want to risk by allowing melted plastic to leak into your food. Although the debate rumbles on, my advice is to stay safe rather than be sorry.
What the professionals say
The EWG has a plain, simple message for us. Don’t microwave any foodstuffs in plastic containers, full stop. However, the FDA says that some plastic containers can be microwaved based on a report issued by Harvard Health on chemical leaching.
But let’s face it. Who has the time or inclination to study such research? Well apart from me, but I am a bit of a saddo anyway.
But without going into the laborious facts and figures, trust product labeling. If it’s been tested and approved and bears the microwave-safe icon, you can microwave it. If it doesn’t – don’t.
Should you decide to ignore this advice, at least make sure, the containers aren’t cracked, old, or discolored. Oh, and one other thing. If you cover foodstuffs with film wrap, try to ensure the plastic doesn’t come into direct contact with the food.
Foods and drinks you should and shouldn’t microwave
Generally speaking, microwaving foods is safe and helps them to retain their nutrient content. But some should be avoided.
Foods on the approved list
Here’s the good news. The following foodstuffs are all suitable for microwaving.
- Bacon – although it’s safer if you invert a bowl over it
- Frozen pizza – thank goodness for that!
- Scrambled or poached eggs
- Kale – a healthier choice than potato chips
- Potato chips – not as health as Kale but a heck of a lot more satisfying
- Various veggies – usually with a sprinkling of water
- Deserts such as brownies and sponge cake. Other items not listed among the baddies show below
Foods NOT to microwave
- Grapes. Believe it or not, they can explode
- Eggs intact in their shells. Unbroken eggshell is an airtight package. Heat it, and pressure will build-up to the point where it explodes. The same goes for shelled, but intact boiled eggs.
- Dried spicy peppers. Capsicum is unstable and can ignite.
Dangers to watch out for
Some people reheat cups of tea or coffee in the microwave. Be careful; water microwaved for too long can become overheated and could explode. Sticking a chopstick or wooden skewer in the cup can prevent this. It conducts excess heat away.
Like eggs in their shells, any foodstuffs with unbroken skin or covering (like sausages) can explode due to pressure building.
Always prick them, so any heated vapor has an escape route. The same goes for packaged foods with plastic covering. Prick them with a fork before microwaving.
Non-food does and don’ts
Let’s just get off the subject of food for a moment and take a look at some of the other does and don’ts, starting with the does.
- You can microwave a damp sponge to disinfect it. Just squeeze any excess moisture out first.
- You can sterilize soil to get it ready for seeding.
- Put dish towels into a zip lock bag (unzipped while microwaving) and get yourself a makeshift hot water bottle.
- Warming bag. Fill a bag or sock with beans or rice and heat it up.
- You can heat certain beauty products such as hot oil conditioners, hot wax for exfoliating, and face “mud” masks, etc.
- Water with lemon in it. This actually cleans the microwave.
I think you’ll agree there are some surprises there. Now for the don’ts
- A dry sponge will easily catch fire if microwaved.
- There is an “Is it a good idea to microwave this?” show on YouTube, which points out many items that are unwise to put in your microwave. Take a peek. It’s very informative, if not a little obvious too.
Well, that just about concludes our excursion into microwaving cardboard and more, so what have we learned?
As you can see, there are many uses you can put your microwave to, but heating food is likely to be the most popular.
Just bear in mind that microwaving cardboard depletes its water content dramatically, especially when you put it in the microwave in isolation. It is likely to catch fire.
But simply reheating foodstuffs in pure cardboard boxes should be okay, providing it doesn’t contain any of the prohibited items mentioned above. The bottom line is always to read the labeling on the package.