Insulated Thermos flasks are a great way to keep soup hot for lunch on a cold day. But how long does a thermos keep soup hot and is it safe to keep soup hot all day or can it go off?
A thermos can keep soup hot for around 4-7 hours or more. Make sure to bring your soup up to near boiling temperature before putting in your thermos so it can stay hot for longer.
A thermos won't actively heat up your soup it'll just keep it hot and slow down the cooling process. So the hotter the temperature the soup starts out the longer it will stay hot throughout the day.
How To Keep Soup Hot In a Thermos All Day Long
The right preparation and a couple of simple steps can help you to keep soup hot all day in a thermos and stop you from having lukewarm or cold soup by the time you want to drink it.
Make The Soup Extra Hot Before Pouring Into Your Thermos
Remember, you're making the soup to be a good temperature to eat later in the day, not right now. So you'll want to make your soup as hot as possible before putting it in your thermos.
It's advised to bring your soup up to boiling point, or nearly boiling point, then quickly pouring it into your Thermos and putting on the lid so no heat escapes.
This will keep your soup hotter for longer and it'll also mean that your soup is going to spend less time in “the danger zone” of 40-140ºF (4.4-60ºC) which is where bacteria can multiply and grow and cause your soup to go bad.
Ideally you want your soup to be above 140ºF (60ºC) when it comes time to drink it. This not only keeps your soup out of the danger zone where bacteria can grow but colder than this and your soup will start to feel not hot enough.
Pre-Heat The Inside Of Your Thermos
Pouring hot soup into a cold or room temperature Thermos will cause the Thermos to suck some of the heat out of your soup.
It isn't a huge amount but if you want to keep your soup hotter for longer then it's a good idea to pre-heat the inside of your Thermos first.
You can do this by filling it up with boiling water and leaving it for 2-5 minutes so the stainless steel gets really hot.
Then discard the water just before adding the soup and the walls of your Thermos won't steal heat from your soup so it'll stay hot longer.
More Soup = Hot For Longer
If you only have a small amount of soup you have less total heat energy and it'll cool down faster, especially if your Thermos isn't completely filled and there is air inside your Thermos.
The larger the volume of hot soup you start with the longer your soup will stay hot for.
Use The Right Size Thermos
You want to make sure you use the right size Thermos for the amount of soup you have.
If there is lots of air in your Thermos this will work to cool down your soup faster.
Ideally you want your Thermos to be completely full (or as close to full as possible) and have little to no air inside your Thermos.
Wrap It In a Towel and Keep It Away From The Cold
Cold outside air and exposure to anything cold will cause your soup to cool down faster in your Thermos.
Wrapping your Thermos in a towel can help to insulate it from the cold outside air and keep your soup hot for longer.
Wrap It In Aluminum Foil
The vacuum insulation in a thermos flask is great at stopping 2 types of heat transfer (convection and conduction) but it isn't great at stopping heat loss through radiant or infrared heat.
Aluminum foil is an excellent reflector of heat radiation. So by wrapping your thermos in aluminum foil you can insulate against all 3 types of heat and keep your soup hotter for a longer period of time.
Why Isn't Your Soup Staying Hot In Your Thermos?
If you're using a thermos and you're find that your soup isn't stay hot for a very long period of time then it could be the way you're preparing the soup and thermos or it could be an issue with the thermos itself.
The main reason your soup isn't staying hot in your thermos is either that you didn't make your soup hot enough, didn't pre-heat your thermos or the insulation in your thermos is broken.
Here are some reasons your soup might not be staying hot very long in your thermos.
You're Not Making Your Soup Hot Enough
Remember, a Thermos isn't perfect and your soup will slowly get colder over time.
So the hotter your soup starts out the hotter it will be when it comes time to drink it.
Ideally you want your soup to be at or near boiling temperature when you pour it into your Thermos.
If your soup isn't staying hot theres a chance you're not heating it up enough.
You're Not Pre-Heating Your Thermos
While this doesn't have a huge effect on heat retention if you fail to pre-heat the inside of your thermos then the stainless steel wall will strip some heat from your soup making it get colder faster.
You Vacuum Insulation is Broken
Thermoses are able to keep soup hot for so long because they have double walled vacuum insulation.
It's the vacuum that does most of the insulating as heat struggles to pass through it.
However, sometimes the vacuum seal can break and your vacuum fills up with air. When this happens your thermos will not longer insulated as well and will need to be replaced.
This may be covered under warranty or you may need to buy a new thermos altogether.
Can Soup Go Bad In a Thermos?
Like all foods it is definitely possible for soup to go bad in a thermos. But how can soup go bad in a thermos and how can you avoid having your soup go off?
Soup can go bad in a Thermos if the temperature range falls below 140ºF (60ºC). Below this temperature bacteria can grow and multiple. Seafood, chicken and meat soups tend to spoil faster than vegetarian soups.
To avoid your soup going bad you'll want to heat it up to a very hot temperature before you pour it into the thermos.
This hot temperature will kill off most bacteria and the hotter your soup starts out the longer it will stay above 140ºF (60ºC) where bacteria struggles to grow.
Once your soup drops below this temperature you ideally want to drink it within 1-2 hours to ensure it hasn't gone bad.
Generally speaking, soup that starts out at boiling temperature will stay above 140ºF (60ºC) for about 3-7 hours in a Thermos. This varies based on the size and quality of your thermos and the type of soup.
You'll then have another 1-2 hours to drink your soup before you have to worry about whether or not it's gone off.
This gives you somewhere in the range of 4-9 hours to drink your soup before it has any chance of going bad.