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Cardboard boxes with food are just like peanut butter with jelly — the ultimate pair. From our favorite Chinese food to pizza, these boxes are our go-to for any takeout food.
But why just store food in cardboard food containers? Why not save time, and heat the leftovers in the same box?
While the convenience of cardboard can make you believe that you can heat it with almost anything inside it, things aren’t that simple — especially when it comes to putting cardboard in the oven or microwaving cardboard.
There are actually many do’s and don’ts about microwaving your food in cardboard boxes. Let’s find out can you microwave cardboard?
Can you put cardboard in the microwave?
Well, there is no yes or no answer to this question. The cardboard itself is not dangerous in any way when microwaved. The problem is whether it has a film on it. If your container is made of pure cardboard, there is no problem microwaving it. However, if your cardboard has a plastic coating or any other material on top, do not microwave it. These can cause contamination in your food.
Apart from the contamination, the cardboard catching fire is something people fear. Since the microwave does not work on real flames, this is unlikely.
However, the radiation in the microwave can cause a fire if it is left for long periods. But if you hurry it up, there is no cause for concern.
Is it safe to microwave cardboard?
Yes, it is absolutely possible to safely microwave cardboard safely. If your container is made of pure cardboard, you can put it in the microwave safely.
However, this is only true for plain cardboard. Most types of cardboard boxes, especially those of low quality used for food takeout, have a coating inside. This coating prevents any liquid, like sauce and oil in the food, from getting absorbed in the cardboard.
This coating can be quite dangerous when microwaved. Here are some coatings to look for when microwaving cardboard:
Plastic coatings are common in cardboard. This plastic containers can burn in the high temperature created inside the microwave to release dioxins which can cause cancer when ingested.
In fact, the Food and Drug Association USA recommends that plastic should not be in direct contact with the food when heated.
Since the plastic lining is on the inside of the box, preventing contact with food is impossible to avoid when heating food in the microwave.
PRO TIP: You can also prevent the release of these dioxins by reducing the temperature at which the food is heated.
Unfortunately, you do not get this option in most microwaves. Microwaves operate at a temperature of 212F, while plastic should not be heated above 150 degrees Fahrenheit. So stay away from plastic-lined cardboard at all costs.
Wax is another material used to line the cardboard boxes. Wax paper, especially the type bleached with chlorine, can transfer dioxins in the food.
Although the dioxins this releases is very little in amount, the World Health Organization says the dioxins cling very strongly to fat. Since wax has a fatty base, it will retain a lot of these dioxins. This means it is best to avoid cardboard with a wax coating.
Another concern is the waxy coating catching fire in the microwave. Wax is okay for a small amount of time in the microwave. However, if you're heating something for long periods, the wax can catch fire.
Cardboard does not catch fire in the microwave because the temperature isn't high enough. Nonetheless, if the wax catches fire, the cardboard too will catch fire very fast causing a potential fire hazard.
Metal bits in the cardboard, like staples to hold the box together or metal handles, can also cause fires. When the food heats up, the water inside the food evaporates and releases energy.
However, when the metal heats up, the particles in the metal have nowhere to go. You'll most likely see sparks in your microwave. While you might not cause a major fire, your microwave will be ruined.
Smooth metal pieces like pins cause another problem. These smooth surfaces reflect the radiation from the microwave instead of absorbing them.
What happens then? The radiation bounces around the inside of the microwave until the microwave overheats. An overheated microwave is at risk of exploding.
Some glues are food safe; however, the problem is you do not know which are not. Glue is generally safe to microwave if it can withstand high heat. Epoxy glue is a very good example of this.
However, other glues will melt when placed in the microwave. Once the glue melts, it will stick to the food.
If the cardboard is coated with glues used for crafts, they are not safe to ingest. They can be toxic and should be avoided at all costs.
Other cheaper qualities of glue may even break down entirely in the microwave, releasing toxic fumes and vapors.
When ink is microwaved, there is a risk of it bleeding into the food. The ink contains glycol or glycol ether which means the ink can be poisonous when ingested.
However, unless the ink is on the inside of the container, this is not harmful. So most cardboards containers with prints on them should be safe.
How long can you microwave cardboard safely?
Microwaves can be safely microwaved from anywhere to 60 to 90 seconds.
During this period, any material in the cardboard will be unable to gain enough time to break down. The cardboard itself will not be able to catch fire either.
You can slightly increase the heating time if you are heating a liquid. This is because liquids heat up slower than solids since solids are better conductors of heat. It is safe to heat cardboards with liquids for 120 seconds.
A Word of Caution: When microwaving cardboard, remove any metal pins in the box beforehand.
Can you microwave a cardboard box safely?
Generally, you can safely microwave a cardboard pizza box. Since pizza does not have liquid, there is no plastic or wax film on the inside of the box. This means it is microwave safe.
However, be sure you do not microwave the box for more than 1-1.5 minutes. Even though the box itself is safe, too much heat can cause it to catch fire.
Recycled cardboard can be dangerous when burned because it can contain toxic chemicals. Nevertheless, if the cardboard doesn’t catch fire, there is no cause for concern.
How do you know if a cardboard is microwave safe or not?
If you're wondering whether your cardboard box is oven safe, the first thing you should do is check for any labels on the box. If the box is tested to be microwave safe, the manufacturer will have mentioned it on the container.
If there is no label, you can run several tests yourself:
- Hold both the inside and outside of the cardboard under the light and see if there is a reflection. If there is a plastic coating on the cardboard, it will reflect light.
- You can also run your hand on the surface to see if it is smooth. Pure cardboard should feel rough and dry since it has very little water content.
- If you're still unsure, you can run a small piece of cardboard underwater to see if there is absorption. Both wax and plastic are waterproof, but cardboard absorbs water. If water gets absorbed, you can use it. Otherwise, avoid putting it in the microwave.
- Check your cardboard for metal bits, and check if the ends have been glued together. If either of these things is present, do not microwave the cardboard.
Tips when microwaving cardboard
- Check for any plastic, wax, or other contaminants.
- Heat the cardboard in a series of small times to prevent it from catching fire.
- Keep a careful eye on the cardboard while it is being microwaved. If the cardboard catches fire, switch the microwave off immediately. Put out the fire, and dispose of the food. Normal cardboard is safe when burned, but recycled cardboard can be toxic after burning.
- Do not keep the cardboard empty in the microwave. It will lose water and burn once dry.
- If you need to microwave a lot of food, do so in small batches so they are done faster. Do not microwave cardboard for more than 90 seconds.
- Check if your cardboard is made of pure cellulose. This type of cardboard has zero health concerns and is the best for microwaving.
- Separate the plastic or metal parts from the cardboard. For example, remove the plastic caps from coffee cups. Remove the plastic pizza savers from the cardboard pizza box too. Be sure to remove the metal handle from Chinese takeout containers.
Undeniably, cardboard is a staple when it comes to takeout food. This is why it is so incredibly convenient to be able to heat the leftovers in the same box. But the question remains; Can you microwave cardboard safely?
Well, you don't have to worry any longer. The bottom line, as long as you check the cardboard for any contaminants you can microwave it safely. Plus, keeping an eye on the cardboard while the food heats doesn’t hurt.
What’s more, do not overheat the cardboard to prevent any accidents. And viola! That’s how you can safely microwave your favorite leftovers in cardboard boxes.