Thermal bags are designed to either keep food hot or cold, but how long exactly do thermal bags keep food hot and do they actually work?
Generally speaking, thermal bags will keep food hot for 2-3 hours without an extra heat source. But it depends on the food. Denser foods like soups, stews and meat dishes have more overall heat energy and will stay hotter than lighter foods like bread, pasta and rice will cool down quicker.
Thermal bags do work and they do keep food hot longer than if you just put food in a regular bag. This is because they are designed with insulating properties that both traps heat and reflects heat back onto the food.
How Do Insulated Thermal Bags Work To Keep Food Hot?
Thermal bags are clever devices that are small and use the natural insulating properties of the material they are made from to trap heat inside the bag (or keep heat out if you want to keep something cold).
Thermal bags use a couple of different methods to keep food hot.
Thermal bags work by using a reflective material (often aluminum) that reflects heat radiation, stopping it from escaping. They also use an insulating material, usually plastic, paper or foam, to stop heat escaping through conduction. Combined they trap a lot of heat allowing your food to stay hotter for longer.
Because thermal bags are airtight they also trap steam and stop it escaping from your food. As steam leaves your food it actually takes with it a lot of heat energy and this cools down your food faster. So trapping the steam keeps the food hotter than it would otherwise.
Because the insulation is usually quite thin thermal bags are only ok at keeping things hot. If you want them to keep food hot for longer then wrapping them in a towel or adding other insulating layers can add to their effectiveness.
Do Insulated Thermal Bags Actually Work?
Yes, thermal bags do actually work to keep food either hot of cold.
If you were to put hot food in a thermal bag and compare that to hot food left out or hot food put in a plastic or paper bag then you would find that the food in the thermal bag will stay hotter for longer.
This is because the foil reflects heat back onto the food as the plastic/paper layer of the thermal bag also insulates whatever is inside the bag.
Thermal bags don't have any active heating element they simply do their best to trap whatever heat is in your food and stop it from escaping
How To Use a Thermal Bag To Keep Food Hot
To use a thermal bag to keep things hot you don't have to do anything special.
Simply heat up your food and quickly put in your thermal bag sealing the thermal bag shut. You'll then want to keep the thermal bag away from anything cold and wrap the thermal bag in a tea towel or paper towel in order to keep food hotter for longer.
There is no need it “activate” a thermal bag and the thermal bag does no active heating of its own. It doesn't have an heating element or any heat source to keep your food warm.
It only keeps the food warm by trapping the food's own heat and stopping it from escaping.
This is why the more heat energy you have in a thermal bag the longer it will stay hot for. Sometimes adding something like a small heat pack into the bag with your food can keep it hot significantly longer.
How To Make a Thermal Bag Keep Food Hot For Longer
While thermal bags work fairly well to keep food hot for a couple of hours you can actually keep food hotter for longer in a thermal bag with a few simple steps.
Wrap Your Food In Extra Aluminum Foil
The foil in thermal bags will reflect back a fair amount of heat energy, trapping it in the bag keeping your food warm.
But first wrapping your food in aluminum foil can help to trap even more heat energy.
It also helps to keep the heat and the steam closer to the food keeping it warmer.
Wrap It In a Paper Towel
Thermal bags insulate mostly through the foil reflecting back heat radiation. However, they only have a thin layer of plastic, paper or foam to stop heat loss through convection.
Paper is actually a surprisingly good insulator and you can used to beef up the insulating properties of the thermal bag.
Heat up your food and then wrap it in a paper towel before putting it in a thermal bag.
The paper towel will both insulate the food and trap moisture stopping your food from drying out. Just like how people use paper towels in the microwave to stop food from drying out.
Make Food Extra Hot To Begin With
Remember that your food is still going to cool down over time when it's in a thermal bag.
If you make the food the perfect eating temperature to begin with it'll cool down and be lukewarm or cold when it comes time to eat it.
Instead you want to heat your food as hot as possible so it has more overall heat energy and can lose more heat before it becomes lukewarm or cold.
This way your food will slowly cool down and when it comes time to eat it then it should still be warm and pleasant to eat.
Supplement With A Heat Pack or Hot Water Bottle
Rather than just relying on the heat inside your food to keep everything hot adding in an external heat source can make your food stay hotter dramatically longer than it would on it's own.
Thermal bags are usually quite small and so trying to add a hot water bottle or hot brick like you would if you were keeping food warm for transport just won't work.
However, small instant heat packs can be a great little packet to add into your thermal bag.
You can get disposable ones or reusable ones. I prefer the reusable ones and they are in plastic (not paper) and you can use them multiple times.
Activate the heat pack and then wrap it in aluminum foil and put in the thermal bag with your food.
What Are The Best Thermal Bags To Keep Food Hot The Longest?
When looking for a thermal bag there are generally two main types of thermal bags.
Small Ziploc thermal bags or small paper thermal bags that will fit inside your backpack or other bag. Or reusable thermal bags that have a canvas exterior – I would categorize these as soft coolers with a foil lining.
Honestly, while I can easily make a list of the best soft coolers on the market based off quality, ice/heat retention, styles and price I struggle to make a similar list of the best thermal bags.
Most thermal bags are cheap “me too” products with a foil lining, small layer of thin foam insulation. There is nothing that really stands out as the best thermal bag.
Most of them perform similar to each other and I would just recommend getting the one that suits you based on size, style, durability and price.
Performance is going to be pretty similar across the board.
What Is The Best Container To Keep Food Hot?
If you're looking for the absolute best container for keeping food hot then a thermal bag isn't it.
A thermal bag is decent at trapping heat and works better than a regular bag or nothing at all but there are many better options out there.
The best container to keep food hot is a vacuum insulated flask. Vacuum is the best insulator and traps the most heat and can keep food hot for 4-6+ hours or even longer. Otherwise a good cooler can work really well and thermal bags and aluminum foil also trap in heat.
While there are a few options nothing beats a vacuum insulated flask.
Thermos is the original brand and a lot of people know about them but now there are lots of other big name companies like Hydro Flask as well as smaller no-name brands making insulated food flasks that work just as good.
I personally recommend the Hydro Flask Food Flask or the thermos food flask if you want to buy one yourself.