Whether you want to reheat some food in a takeout box or warm up pizza, you may ask, “Can you microwave cardboard?” Obviously, it would be more convenient to reheat the food in its cardboard container instead of dirtying another dish or plate. However, it can feel risky to do so with a microwave.
This article will answer any questions you have concerning the safety of microwaving cardboard. We’ll let you know all the issues you should watch out for when heating food in a cardboard container.
Is it safe to microwave cardboard?
Typically, it is safe to microwave cardboard. However, you should always stay nearby and keep an eye on the food as it warms up to ensure there are no potential hazards.
You may think the cardboard would quickly catch fire in the microwave. After all, people often use paper and cardboard to start fires because they light fast and burn easily. So it seems logical that this would make microwaving cardboard hazardous.
However, based on how microwaves work, the fire hazard is not as immediate as it may seem. When you use a microwave to heat foods or liquids, the water molecules start to move at a high frequency. As they vibrate faster and fast, friction and heat are created. This is how the food heats up.
However, microwave ovens are different from other heating methods. Microwaves use electromagnetic waves between high-frequency radio waves and infrared radiation. These waves are easily absorbed by the water, sugar, and fat molecules (1).
Cardboard does not typically contain high amounts of moisture content, so the cardboard does not warm up as fast as food or liquid does. Even if moisture from the food transfers to the cardboard, it will generally not be enough to change the cardboard drastically. This is why it is generally safe to microwave cardboard.
However, cardboard can catch on fire. Of course, anything can catch on fire in the right circumstances.
Different types of cardboard
Generally, the cardboard containers used to store food are paperboard or corrugated fiberboard. However, these two types can be used to make a wide variety of food transportation items, such as pizza boxes, takeout containers, eco-friendly food trays, disposable cups, and recyclable bowls, to name a few.
Tips for microwaving cardboard
There are numerous situations in which we would not recommend microwaving cardboard. However, if you opt to proceed to microwave cardboard anyway, be sure to follow these easy and practical tips.
Check the label
Be sure to check the label on the container. If it is marked microwave safe, you are good to go. However, if it says not to microwave the container, follow that guideline to avoid hazards. As a best practice, only microwave an item that says it is microwave safe.
The most common microwave-safe symbol is three wavy lines. Sometimes the lines are within a rectangle which represents the microwave oven.
Alternatively, some manufacturers opt to use words instead of a symbol. In this case, look for a message that says “Microwave safe,” “Do not microwave,” or something similar to guide whether microwaving food in the container is deemed safe.
It is common for cardboard to be made of recycled materials. The danger is that recycled cardboard may have small amounts of materials unsafe to microwave inside it, such as metal. This is another key reason to check the label for safety instructions on using the product.
Related: Are Yetis Microwave Safe?
Heat food in short periods
If you are reheating food in a cardboard container, only heat it for relatively short periods and avoid high heat levels. This reduces the risk of the cardboard catching on fire. Instead of leaving the microwave running, program shorter cooking times so that you can stop the microwave and stir the food.
Short heating times also allow you to check the integrity of the cardboard container and catch problems early. Using a low-power setting can also help reduce the risk that the cardboard container catches fire.
Beware of metal
You should never put metal in the microwave. It can damage your microwave and potentially start a fire. For example, if you have a cardboard takeout box with metal handles, it should not be placed in the microwave. The most common example of this is a Chinese takeout box.
Examine the construction of the container
Not all cardboard containers are made the same. For example, many cardboard containers, such as soup containers, have a waxy coating.
When the container heats up, the wax coating may leach chemicals into your food or emit toxic fumes (2). The same is true for plastic coatings that sometimes cover the container. You do not want plastic or wax to drip into your food.
Some containers are held together by glue or have ink printed onto them. Unfortunately, these elements also risk dripping or leaching into the food you are warming up.
Not only can these chemicals have health implications over the long term, but they can also tarnish the taste of the food you are reheating.
The whole goal of reheating the food in a cardboard container was to simplify your life and cut down on the number of dishes you have to clean. You do not want to make things more difficult on yourself by throwing out the tainted food and then scrambling to figure out what you will make for dinner instead.
Consider the elements used to make cardboard boxes
Pure pulp and wood fiber are often used to make cardboard boxes. Often, cardboard boxes contain a moisture level up to 12%. When they are heated, the water level starts to drop. If the pulp or fiber dries out too much or gets completely dry, it will be easier for the cardboard to catch on fire.
However, this is not a common problem if you microwave food for short durations of time and have a sufficient amount of food or drink in the container. The food will absorb most of the microwave’s energy. This will keep the cardboard from drying out too much.
Beware of the type of food in the container
You should avoid reheating food with high water content in a closed cardboard box. When the box is closed, and heat is generated, it will heat up slower. Also, if the box is closed, the pressure will try to escape, which can lead to an explosion.
Additionally, the liquid inside could harm the integrity of the cardboard, causing it to get soggy and break down. All of these potential hazards are putting your safety, your microwave, and the food itself at risk.
Likewise, you should always ensure food or liquid is inside the cardboard container. For example, you should never microwave an empty cardboard container. Doing so poses a serious fire hazard as no food is inside to absorb the microwave’s rays and keep the cardboard from drying out.
Microwaving pizza boxes
If you want to reheat some leftover pizza, you may wonder if cardboard pizza boxes are safe to put in the microwave. While the whole box likely will not fit in your microwave, you could tear off a piece of the box and use it as a plate. Alternatively, if you bought pizza by the slice or a personal pan size, you may be able to fit the entire box in your microwave.
Typically, cardboard pizza boxes do not pose the danger of putting plastic or wax paper in the microwave on them or metal attachments. So, they are generally safe to place in the microwave to reheat food. As a bonus, a cardboard pizza box is often porous. Therefore, it will not get as hot as a regular dish or plate would.
This is helpful when reheating pizza as it helps ensure that you do not burn yourself on the plate but have cold pizza.
Microwaving cardboard paper plates
When it comes to cardboard paper plates, you can microwave them as long as they do not have plastic or waxy coatings. The cardboard plate’s low moisture content means it does not warm up as fast as the food does. This helps keep the plate from overheating or catching fire in the microwave when exposed to the microwave’s energy.
Related: Can You Microwave Plastic?
Microwaving burger boxes
If you go to a fast food restaurant or a local diner and order a burger, you may be unable to finish it. If you take it home and want to reheat it later before enjoying the remainder of the burger, you may be wondering if the burger box is microwavable.
Typically, these cardboard boxes are made without metal, plastic, or wax, so they are considered safe to reheat in the microwave. However, to ensure the box’s integrity, do not microwave it for long periods or expose it to high heat levels.
While it is generally safe to microwave food in cardboard containers, problems may arise from doing so. Generally speaking, if you need to warm up food for a long time, it is better to use a microwave-safe dish or plate.
If you plan to put cardboard in the microwave, ensure that you only microwave it once. Do not heat up cardboard in the microwave more than one time.
To microwave cardboard safely, use a low heat setting and a short time. Keep an eye on the food you are reheating and the container’s integrity. If it shows signs of burning or wear, stop the microwave and transfer the items to another microwave-safe container to finish heating it.
Can you microwave cardboard FAQ
Is all cardboard microwave safe?
No, not all cardboard is safe to microwave.
Do food experts recommend reheating food in a container made of cardboard in the microwave?
No. While it is typically safe, the reheating process may differ, and the taste of the food can be affected by the chemicals in the cardboard container. Using a microwave-safe dish or plate is a better option.
What practical tips can be followed to avoid having a cardboard container catch fire in the microwave?
Be sure to examine the container to ensure it does not contain metal handles or plastic or wax coatings. If you opt to reheat food in the microwave, avoid high heat levels and do not leave the container in the microwave for too long. Be sure to keep an eye on the microwave while you are warming up the food to ensure that if any hazards arise, you can address them quickly.