It’s normal to want to keep food cold in your lunch box. But what about keeping food completely frozen?
Whether it be ice cream, meat or medicine if you want to go on a longer trip or won’t be around a fridge for a while there are some things you MUST DO if you want your food to stay frozen (not just cold) in a lunch box.
To keep food frozen in a lunch box use salt water ice packs or ice packs designed to stay well below 32ºF/0ºC for hours. This will ensure food stays frozen (not cold). Using a thermos or thermal cooker or using dry ice are also good ways to keep food frozen for longer. If you don't have those make sure to heavily insulate your items and lunch box and keep away from sunlight or warm food.
If you need to keep food completely frozen in your lunch box you can't just throw it in the lunch box with some ice and expect it to stay frozen. You need to use some specific techniques and we’re here to help you know exactly what to do.
Learn how to keep your food completely frozen in your lunch box below.
1. Pre-Freeze Your Items Beforehand and Make Them As Cold As Possible
The easiest way to keep things frozen is to start them off frozen!
Turn your freezer as cold as it can go and put your food in at least 24 hours before you plan to pack it up. It usually takes items about 24 hours to get down to the temperature of the freezer (which is usually 0ºF/-18ºC). You'll also want to put the food at the BACK of the freezer so it gets as cold as possible,
If you have a deep freezer, that can make the food even colder, so use that instead of a standard kitchen freezer if possible.
2. Insulate It As Best As Possible
A lunch box doesn't actively keep food cold like a fridge or freezer does. It just slows down the warming up of your frozen goods. Given enough time all the food in your lunch box will warm up to room temperature.
Keeping your food frozen for longer is a matter of maximum insulation.
A well-insulated lunch bag, like the Yeti Day Trip, will help keep your food frozen for longer. If you don’t have a high-quality insulated lunch bag, you can use towels, sweaters, and even aluminum foil to wrap your frozen items.
Aluminum foil is an excellent option because it reflects heat and prevents evaporation. Just make sure not to put it next to anything warm. Otherwise, it’ll heat your cold food.
The better the lunch bag you use or the more insulation you cover your frozen item in the longer it'll stay frozen. Don't rely on your insulated lunch bag alone, always add extra insulation where possible.
3. Use Salt Water Ice Packs or Ice Packs Designed To Stay BELOW Freezing Temperature
Saltwater ice packs are better than regular ice for helping keep food frozen. These packs stay between 15ºF/-9ºC and 28ºF/-2℃ so they’ll help keep your frozen foods nice and frozen.
Regular ice or blocks of ice, on the other hand, will keep food at exactly 32ºF/0ºC. Because of some outside heat in reality food ends up above this temperature which is why regular ice doesn't work to keep food frozen.
Some ice pack, like the Engel 20 Ice Pack, start melting at around 20℉/-7ºC and they will stay at this temperature which is cold enough to keep your food frozen. Read more above why salt water ice is colder than regular ice and the science behind it if you want to learn more.
Ice packs like this and will pull residual heat out of your food, so it stays completely frozen and this even works to keep ice cream frozen in a cooler or lunch pack.
If you’re resourceful, you can make salt water ice packs at home. For just a few dollars, you can have a tremendous reusable ice pack that you can customize to the size of your cooler or lunch box. Plus you can add more salt or less salt and vary your desired temperature.
Alternatively check out my list of the best ice packs for coolers and lunch boxes. But really for keeping food completely frozen you can't go wrong with the Engel 20 ice pack.
4. Use An Insulated Thermos
When we use a thermos, we usually talk about keeping hot foods hot. We keep our coffee, our tea, and our soup in thermoses.
But did you know that insulated thermoses, which use vacuum insulation, are also incredible at keeping your cold or frozen foods frozen?
Vacuum insulation is the most effective insulation, whether you want to keep heat in or out and this means they are some of the best ways to keep food completely frozen in your lunch box.
See insulated bags or lunch boxes use foam and reflective insulation to keep food cold, but because they aren't very thick they don't really work that well and food will stop being frozen in a couple of hours.
Insulated thermoses or water bottles, on the other hand, can keep things frozen for DAYS at a time. This speaks to the power of the vacuum insulation. It's one of the best ways to keep ice cream frozen without a freezer.
If you have drinks you want to keep cold or soup you want to freeze so it will last, throw it in a thermos. Once you’ve frozen that thermos, it can keep your items frozen for days.
You just need to be a bit careful about putting a thermos in the freezer as it does have the potential to ruin the vacuum insulation – but placing already frozen things in a thermos is almost always fine.
5. Use A Thermal Cooker To Keeper Larger Items Frozen
This idea is a thermos but on a giant scale. Thermal cookers are great for cooking items and keeping them hot. However, they’re also incredible at keeping food cold!
Like a thermos, a thermal cooker uses vacuum insulation to prevent heat transfer. They are just much bigger than a regular thermos which means you can use them like a mini cooler or lunch box, but with the best insulation possible.
So, for instance, you could make ice cream and keep it frozen using the thermal cooker. Instead of “cooking,” the thermal cooker will keep your ice cream insulated, so it’ll still be frozen instead of turning into soup!
You could also just put frozen items like meat, drinks or medicine inside the thermal cooker WITH ice packs that are designed to melt and stay colder than 32ºF/0ºC and these ice packs combined with the great insulation will keep your items frozen for longer than a regular lunch box or bag would.
6. Use Dry Ice
If you have access to dry ice, it can be an effective option for keeping your food frozen.
Dry ice is frozen carbon dioxide and sits at -109.2ºF/-78.5ºC and stays that temperature until it completely disappears.
Because it's so incredibly cold dry ice can be used to keep a variety of items completely frozen for a long period of time. Dry ice will last for days in a cooler, keeping food frozen the entire time and dry ice lasts a few days for shipping too if it's used correctly and insulated with styrofoam.
While it’s great for keeping things cold, you want to use it properly and safely. Make sure you wear gloves when handling the dry ice, and wrap it in a newspaper or towel, so it’s not directly touching your food.
7. Keep Your Cold Items At The Bottom Of Your Insulated Bag and Away From Any Warm Items
If you don't have access to dry ice or ice packs that stay colder than 32ºF/0ºC then you'll want to keep your frozen items at the very bottom of your insulated bag and completely surrounded by ice.
Just remember – don’t put them next to something warm. They will conduct the heat, and your frozen item will defrost faster.
Ideally keep frozen items and room temperature/warm items in completely different bags and away from each other.
8. Minimize Empty Air Space In Your Lunch Bag
The tighter you pack your lunch bag, the longer your items will stay cold! Empty air space in your cooler will heat up faster, so the less air space, the better.
If you don’t have enough food to fill your lunch bag, consider using the insulation tricks discussed above. Wrap your food containers in towels or aluminum foil and fill gaps between your containers with paper towels so that the air has nowhere to circulate.
It’s not hard to keep food completely frozen in your lunch box. Whether you need to keep it cool for a few hours or a whole day, there are lots of methods you can use:
- Pre-freeze your food and your lunch box
- Use dry ice or salt water ice packs
- Insulate your food with towels and aluminum foil
- Consider thermoses or thermal cookers
- Separate your warm and cold foods
Using these tips, you should be able to keep your food frozen in your insulated lunch box for a day or more!