It's often advised that you shouldn't put milk in a thermos or insulated flask but it is rarely explained why. The fact is you can't put milk in a thermos but there are some real risks associated with it and precautions you need to take.
So why do people and companies say you can't put milk in a thermos?
You actually can put milk in a thermos but it needs to be below 40ºF (4.4ºC) or above 140ºF (60ºC) otherwise harmful bacteria can grow causing your milk to spoil and become dangerous to drink.
It's difficult to monitor exact temperatures so companies advise against putting milk in altogether.
What Are The Risks Of Putting Milk in a Thermos?
The major risk when keeping milk in a thermos is the risk of bacteria growth and the milk going bad.
Even when pasteurized milk contains leftover bacteria like lactobacilli which, given the right temperatures, will rapidly multiply and spoil your milk making it dangerous to drink.
Temperatures below 40ºF (4.4ºC) are considered safe to store milk as the lactobacilli struggles to grow at such a low temperature. It still grows and multiplies just slowly.
The problem with this is that your fridge is usually at or around this temperature so as soon as you put cold milk in a thermos it will warm up above this. So you need an ice pack or something to keep the milk cold all day long.
Temperatures above 140ºF (60ºC) are also considered safe to store milk and these high temperatures kill off most lactobacilli so they can't grow effectively.
If you're putting milk in coffee or tea it's likely that the milk is sitting at or above this temperature and should be safe.
But once your tea or coffee starts to cool down then the lactobacilli and other bacteria can grow and spoil your milk.
A general rule of thumb is that once your drink with milk in it enters the “danger zone” of 40-140ºF (4.4-60ºC) you have approximately 1-2 hours to drink it before it spoils.
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Can Milk Go Off In a Thermos Flask?
Just because milk is being kept in a thermos flask doesn't mean the milk can't go off.
Milk can definitely go off in a thermos flask. This happens fastest when the temperature of the drink with milk in it is between 40-140ºF (4-60ºC), also known as “the danger zone”.
However, just like milk will eventually go off in a number of days when kept in your fridge and about 6 or so months when kept in the freezer so to will your milk go off in a thermos, even if kept at very low temperatures.
How Long Does Hot Milk Last In a Thermos?
Despite what some people may say you can safely keep hot milk in a Thermos for an extended period of time without it going bad.
As long as your milk is above 140ºF (60ºC) it will stay safe to drink all day as this temperature is too hot for bacteria to successfully grow.
However, once the temperature drops below 140ºF (60ºC) you'll want to drink your coffee, tea or any drink with milk in it within about 30-60 minutes. Otherwise bacteria can grow making it unsafe to drink.
Having hot milk in a Thermos for extended periods of time, especially when mixed with coffee or tea can cause the milk to curdle. This doesn't necessarily mean it has gone off, but it will ruin the look and texture of your drink.
Learn more about why milk curdles in a Thermos.
How Long Does Cold Milk Last In a Thermos?
You can keep cold milk in a thermos but ideally you want to have a way to keep it cold (below 40ºF/4.4ºC) for the entire time you're storing it.
You can generally keep cold milk from the fridge in a thermos for 2-6+ hours without it spoiling assuming that it stays cold that entire time.
Once it rises above 40ºF (4.4ºC) you should consume the milk within 1-2 hours to ensure that it hasn't spoiled.
It can be difficult to predict how long your milk has been in the “danger zone” for and if it's still good to drink.
The sniff test or looking for signs of curdling are good ways to check if the milk is still good to drink. You can also use reusable ice packs like the myColdCup ice stick that won't dilute your milk but will keep it at a safe drinking temperature all day long.
I wrote an entire article on how long you can keep cold milk safely in a Thermos. So check that out for more details and tips and tricks.
Can You Keep UHT/Long Life Milk In a Thermos?
Long life milk (also known Ultra High Temperature or UTH Milk) has been exposed to a much higher temperature than regular milk which kills off almost all bacteria.
Long life milk can last for months or even years in the packaging it came in without spoiling.
However, once you open up long life milk and put it in your thermos the rules of the “danger zone” still apply.
Because the milk is now exposed to the outside air and the bacteria in your water, coffee or tea and bottle bacteria can still grow it in.
Long life (UHT) milk is unlikely to spoil as quickly as regular milk, but it can still go off over time and it's best to be on the safe side.
If you're using a thermos to store UHT milk then try to keep it below 40ºF (4.4ºC) or above 140ºF (60ºC) and once it enters the danger zone temperatures you should drink it within about 2 hours in order to be on the safe side.
Can Milk Damage a Thermos Flask?
Some people may wonder that Thermos and other brands advise against putting milk in their flasks, cups or bottles because it might somehow damage the product itself.
This is not true.
Thermos flasks (as well as vacuum insulated bottles from other brands) are generally made out of kitchen grade 18/8 stainless steel.
This type of steel does not react with milk and milk is not acidic enough to corrode the stainless steel or cause any issues.
Both cold and hot milk are fine to be kept in thermos flasks and other vacuum insulated stainless steel flasks. They won't damage the product or cause any issues.
The exception to this is glass lined thermos flasks. While milk won't damage the glass cleaning the inside of these can be difficult as the glass is fragile.
Using milk products in these can lead to you being unable to clean them properly. You make break the glass during the cleaning process or not be able to clean it properly leading to mold growth.
But when it's made from stainless steel it should be fine. I written previously that it's fine to put milk in a Yeti cup and it's also fine to put milk in a Hydro Flask if you are careful about temperatures.
Milk Can Cause Your Thermos Lid To Get Stuck
If you've had a drink with milk in it in your Thermos and you've left it for a while it can go bad.
As the bacteria in the milk grow and multiply they release gases. These gases can build up inside your Thermos or insulated cup or bottle and put a lot of upwards pressure on the lid of your thermos.
If your lid screws on this can put so much pressure on the lid that it can become stuck and you'll be unable to open it.
I've written about how to open a stuck Yeti bottle lid and you can use the same tips if your Thermos lid gets stuck.