A Thermos is a great way to keep food hot for a long period of time so you can eat it later. But will hot food spoil in a thermos?
What's to stop the bacteria from growing and spoiling your food, potentially making you sick and how can you know if you food is still good to eat?
Hot food won't spoil in a Thermos as long as it stays above 140ºF (60ºC) or 165ºF (74ºC) for meat or leftovers. Hot food will usually last about 4-6 hours before dropping below this temperature. Once it does harmful bacteria can grow making your food unsafe to eat.
So keeping food safe to eat in your Thermos is all about managing the temperature of your food and either keeping that food very cold so bacteria can't grow or keeping it very hot to kill off bacteria and stop them from growing.
The “Danger Zone” For Keeping Food
Keeping food in a Thermos so it won't spoil is all about keeping your food outside the danger zone where bacteria can easily grow.
This danger zone is between 40-140ºF (4.4-60ºC).
The USDA Food and Safety and Inspection Service state:
Bacteria grow most rapidly in the range of temperatures between 40 °F and 140 °F, doubling in number in as little as 20 minutes. This range of temperatures is often called the “Danger Zone.”
Looking at the graph below you can see that it's advised that some meats and leftovers be kept above an even higher temperature than 140ºF (60ºC).
Some Foods Spoil Quicker Then Other Foods
Not all food are created equal and some food spoil much faster than others when kept in a Thermos.
Rice is bad to keep in a Thermos and can spoil really easily and milk in a Thermos can also be difficult to keep safely.
Chicken, fish and other meats also don't last as long and can spoil faster than vegetables. Vegetable soup can stay hot for hours in a Thermos and generally stays out of the danger zone and is one of the easiest things to prepare and eat from a Thermos.
So depending on what food you are taking with
How To Stop Food From Spoiling in a Thermos
Without the proper preparation and care your food can spoil in your Thermos.
At best this leads to you not being able to eat your food and being hungry, at worst you could get sick.
But there are a few simple things you can do to stop your food from spoiling in your Thermos.
Night Before Prep Is Extremely Important
If you plan to take leftovers the next day then properly prepping your food the night before will reduce the chances of your food going off the next day.
After dinner is cooked (unless you keep it above 140ºF) then bacteria will start to grow and multiple in your food immediately.
Ideally, straight after cooking you want to put the leftovers in a shallow tray or container and put it in the fridge. The faster it cools down to below 40ºF (4.4ºC) the better and the safer it will be to eat the next day.
Make Your Food REALLY HOT Before Putting In Your Thermos
Remember, you're warming up your food to eat later in the day not to eat right now.
So you want to heat up your food to a very hot temperature. Above 165ºF (74ºC) is a bare minimum but if possible you want to heat food as close to boiling temperature as possible.
This will both kill off a lot of the bacteria that has grown, but also it'll mean that your food will stay above 140-165ºF (60-74ºC) for longer in your Thermos.
And the longer it stays out of the danger zone the better.
Pre-Heat Your Thermos
It's a good idea to pre-heat the inside of your Thermos before putting your food in.
To do this fill it up with boiling water and leave for 2-5 minutes before discarding the water and putting your food in.
This stops the cold stainless steel from sapping heat out of your food, again keeping it out of the danger zone for longer.
This doesn't have a huge effect, so if you don't have time to do it then it's ok, but if you can do it then it helps.
How To Know If Your Food Is Safe To Eat In a Thermos?
Knowing whether or not food has entered the danger zone and for how long it has been in this temperature range can be difficult.
If you open your lunch to find it's lukewarm, how long has it been lukewarm for and is it still safe to eat?
Unfortunately there is no hard and fast rule to know whether or not food may have spoiled in your thermos and would be unsafe to consume.
The first thing you can do is smell your food and see if it smells alright.
If it smells a bit off or a bit funky then don't eat it.
Estimation Of Time in Danger Zone
It's hard to estimate how long your food has been in the danger zone, but over time you'll get a feel for how long certain types of food stay hot enough to be safe.
140ºF (60ºC) is generally the temperature people like to drink their coffee. So I use this as a gauge to work out what temperature the food in my Thermos is at and how long it has likely been below that temperature.
Health organizations recommend you eat food within 1-2 hours of it being in the danger zone, but this obviously varies from food to food.
You can also do a little taste test of your food and not eat a big chunk of it.
If it tastes off then discard it.