I love a hot coffee and a thermos is a great way to keep your coffee hot all day long. But can you put coffee with milk in a thermos?
Or will the milk go bad and potentially be dangerous to drink?
I always make my coffees with milk and I love taking them on my adventures through the national park. So I wanted to work out if I could safely take hot milk coffee in my thermos and how long it would last. Here's what I found:
You can safely keep coffee with milk in a thermos as long as it stays above 140ºF (60ºC). Once it goes below this temperature you should drink your coffee within 1-2 hours otherwise bacteria can grow to harmful levels.
If you're making coffee with near boiling water and just adding a bit of milk this should be fine. If you're steaming your milk at home (like I do) then your coffee will start around that 140ºF temperature so you need to be careful.
Between 40-140ºF (4.4-60ºC) is considered the “danger zone” for food and drink and especially milk. In this temperature range bacteria like lactobacilli can grow and multiply, spoiling your milk and potentially making you sick.
This is why it's advised once your milky coffee hits the danger one you drink it within 1-2 hours to avoid it spoiling and going off.
Using UHT or long life milk extends the lifespan of your coffee because the milk has been heated at much higher temperatures killing off almost all bacteria. However, too long in the danger zone once opened and even this milk can go off.
Is It Safe To Store Coffee With Milk in a Thermos?
Storing milk in a thermos can be safe if you do it properly, but it can be risky if you don't store it at the correct temperature. This is why lots of companies say you can't put milk in a thermos.
Storing milk successfully for long periods of time in a thermos is all about managing and monitoring the temperature.
Temperatures above 140ºF (or 60ºC) is too hot for most bacteria in milk, including the common lactobacilli which often causes milk to go bad. These temperatures kill off most of the bacteria, but some can still survive.
Pasteurization of milk is where they heat milk to 145ºF (63ºC) and keep it there for 30 minutes to kill off most bacteria. Or they heat it to 162ºF (72ºC) and keep it there for 15 seconds.
So putting milk in hot coffee (especially coffee near boiling temperature) in a thermos is kind of like pasteurizing it and the high heat should kill off most bacteria making it safe to drink.
140ºF (60ºC) is actually the temperature most people like to drink their coffee at. So only when you notice that your coffee seems a bit cold to drink do you need to start worrying about it not being safe to drink anymore.
Once it goes below that temperature you should try to drink your coffee within 1-2 hours to ensure it's still safe to drink.
If it starts tasting or smelling funky and you think the milk may have gone bad then tip it out and don't drink it.
Will The Milk in Coffee Go Bad In My Thermos?
The milk in your coffee is unlikely to go bad in your thermos as long as your coffee remains hot and above 140ºF (60ºC).
The hot temperature of coffee kills off most of the bacteria in milk and ideally you should be putting in milk that is safe to drink anyway.
However, if you leave you coffee long enough or don't heat it up enough and it drops below 140ºF (60ºC) then bacteria can more easily start to grow and multiply.
Once your coffee starts to get cold you have about 1-2 hours to drink it before you need to start worrying about the milk going bad.
How Long Does Milk Coffee Last In a Thermos?
A thermos has the ability to keep drinks hot for a very long period of time. But how long does milk coffee last in a thermos before it starts to go bad?
This all depends on how hot your coffee is to begin with and what type of milk you use.
Coffee made with boiling water with a bit of milk added can last 4-6+ hours in a thermos and still be safe. Barista made coffee will last about 2-4 hours before the milk can become questionable.
Coffee made with UHT or long life milk can last even longer than this before it starts to go bad.
It all comes down to the “danger zone.” A range of temperatures where bacteria in milk (and other food and drinks) can easily grow.
Between 40-140ºF (4.4-60ºC) is considered the “danger zone”. Once your coffee with milk in it falls into the range it should be drunk within 1-2 hours.
Boiling hot coffee that is put in a thermos with a bit of milk added will likely stay above 140ºF for anywhere between 3-6 hours or maybe even longer.
140ºF is the temperature most people like to drink their coffee. So if you open your thermos and your coffee still feels too hot drink you know the milk should still be safe as it hasn't entered the danger zone.
Barista made coffee like a cappuccino, latte or flat white are made almost entirely of milk. This milk is generally heated to 140-145ºF (60-63ºC) or up to 158ºF (70ºC) for those who request extra hot.
This temperature is just above the danger zone and a thermos will likely only keep it this hot for around 1-2 hours. You then have a further 1-2 hours to drink it once it hits the danger zone.
So all up a barista made coffee will likely last around 2-4 hours in your thermos.
Will Milk Curdle In My Coffee In My Thermos?
One of the downsides of keeping coffee with milk it in in your thermos is that the acidity of the coffee mixed with the long exposure to heat can cause the milk to curdle or clump together.
If your coffee is still hot this generally isn't an indicator that the milk has gone off. It'll still be fine to drink it'll just make your coffee look gross and the texture and taste of your coffee might not be the best.
This happens because milk is a colloid where things like fats and proteins are all floating around together in water.
Naturally they repel each other and don't clump together, but when the milk gets too acidic this changes and the proteins can clump together.
Heat accelerates this process which is why milk can curdle in your coffee when you keep it in a thermos for long time. I've written more about this and how to stop curdling from happening in my article on why does milk curdle in a thermos flask?
The best ways to avoid this are to always use fresh milk, use less acidic coffee, let the coffee cool down a bit before pouring in the milk and you can also using a tiny bit baking soda (soda bicarbonate) to make the coffee alkaline so the milk doesn't curdle.