Thermoses are great at keeping things hot and they are also great a keeping food and drinks cold.
If you're going out of the house and want to have cold milk with you then you might wonder how long can you keep cold milk in a thermos?
This is a good question as we all know warm milk can spoil easily and become potentially dangerous to drink. While there is no hard and fast rule there are some guidelines to help you safely keep milk in a thermos for hours at a time.
So how long can you keep cold milk in a thermos? The answer depends on a variety of factors:
You can keep cold milk in a thermos for 2-6+ hours without it spoiling assuming it stays below 40ºF (4ºC). Once it rises above this temperature you should drink it within 2 hours or throw it out. Milk will last longer in a thermos if it starts at near freezing temperature or if you use a reusable ice pack like the myColdCup ice stick.
But before you go pouring some milk into your thermos and thinking you're fine there are a few things you should know about how to safely keep cold milk in a thermos so it doesn't go off and so you don't get sick.
How To Safely Keep Cold Milk in a Thermos
Keeping milk safely in a thermos all comes down to what the USDA classifies as the “danger zone” for food and drink and for milk in particular.
Once food is between 40ºF (4ºC) and 140ºF (60ºC) bacteria can grow which can cause milk to go bad and can cause food poisoning.
Milk contains lactobacilli which can sometimes survive the pasteurization process and multiply and grow in your milk.
Your fridge keeps milk at that 40ºF temperature which helps keep the milk for longer, but as soon as it's out of the fridge and starts to warm up you need to drink it quickly otherwise it will go off.
Recommendations are to drink milk within 1-2 hours of it being in the “danger zone” in terms of temperatures. Obviously the warmer the milk the faster the bacteria can grow.
So you've got that 1-2 hour window once it's in that zone.
So if you want to keep cold milk for a long period of time in a thermos then your goal is to keep it at or below 40ºF (4ºC) for as long as possible. The longer you can keep it below the temperature the longer it will stay good to drink.
If you're just taking milk straight out of the fridge it's right on the edge of that danger zone. You might get 1-2 hours where it's at or below 40ºF but it'll quickly warm up just enough for bacteria to grow.
It'll still be cold to drink, but it may go off or not be safe to drink.
So how can we keep milk cold and safe for longer in our thermos? Well there are a few ways to do it.
Start With Your Milk At A Cold 32-36ºF (0-2ºC)
A major tip for keeping milk longer in a thermos is to start with colder milk.
Your fridge will usually be right around that 40ºF mark, just outside the danger zone. So you'll want to cool your milk down a bit more before putting it in your thermos.
Put it in the freezer for a little while or put it at the back of the fridge right near the cooling fan to bring it down to just above freezing temperature. Then put it in your thermos.
Because it's starting out colder it's going to last longer until it enters that danger zone and your 1-2 hour timer begins.
Use An Ice Pack To Keep The Milk Cold
Adding in a frozen ice pack to your thermos will do wonders to keep the milk cold and will extend the life of your milk to 6-12 hours or even longer depending on the conditions.
As long as the ice pack stays frozen and keeps your milk below 40ºF the milk should be fine.
You don't want to use loose ice and this will melt and dilute your milk. Instead get an ice pack specifically designed for bottles and thermoses.
If you're in a pinch and don't have an ice pack that will fit then you can put some ice from your freezer in a small zip lock bag and seal it up. Then place the zip lock bag and ice into your thermos to keep your milk cold.
As the ice melts it'll stay inside the bag and it'll keep your milk cold much longer than no ice at all.
Use UHT/Long Life Milk Instead
UHT (Ultra High Temperature) milk, also known as long-life milk, has been exposed to a much higher temperature that pasteurized milk.
This is much harder for bacteria to survive and it kills almost all bacteria.
Long life milk can last for months or even years without going bad as long as you don't open it.
You can take long life milk with you in the packing it came in (skipping the thermos altogether), or you can put it in your thermos and follow the steps up above to keep it cold.
You'll still want to keep it out of the danger zone as much as possible, however if it does warm up a bit it's going to take much longer to go off than regular milk would.
Can You Keep Hot Milk In a Thermos
The same principles apply when looking to keep hot milk for a long time in a thermos. This is what's important to know:
If you want to keep hot milk in a thermos you need to keep it above 140ºC (60ºC) for as long as possible. Heating the milk to near boiling is the best strategy to do this and should keep it safe for 4-6 hours. However, being kept at high temperatures for a long time can denature the proteins and ruin the taste and texture of the milk.
If you keep milk warm (but not above 140ºF) then you are creating a breeding ground for bacteria to grow. If you do this the milk will spoil very quickly, sometimes even within an hour, and if left too long it could make you sick.
To keep hot milk in a thermos make sure you preheat the inside of the thermos by filling it up with boiling water and leaving it for 2-5 minutes before discarding the boiling water.
Heat you milk up to a hot temperature and pour quickly into your thermos and close the lid quickly.
Different thermoses perform differently so the quality of your thermos and the starting temperature of the milk will determine how long you can safely keep hot milk for.
Ideally when you open your thermos and want to use your hot milk it should still be above that 140ºF temperature.