How to Keep Water Hot for Longer in a Thermos

Mate is not like tea or coffee — a single gourdful of yerba mate can sometimes last you a whole day, depending on the size of the gourd and your drinking cadence. And if with tereré we can use ice cubes to keep water as cool as possible, with hot mate we have to rely solely on insulating properties of your vacuum flask. If you want to maintain the desired temperature of the water for your mate for the longest time possible, it is crucial to know how to maximize the efficiency of your thermos.

How long does a mate session last?

Some time ago, I ran a quick poll on r/yerbamate to see how many hours on average does one’s mate ceremony last. Surprisingly, a lot of people are managing to finish their mate in under an hour, but, predictably, for the majority of materos it lasts longer than that.

Personally, I find myself drinking mate for about 4 hours on average, often taking a pause in the middle of my session and finishing it later in the day. I am a pretty slow drinker compared to many other people and even my smallest gourds can last me the whole day.

I was always frustrated by the fact that water in my vacuum bottle is getting too cold for my taste too quickly, before I am able to finish my mate, to the point where I seriously considered to change my 1 liter thermos for a smaller one, hoping that it will hold temperature better at the expense of the volume and convenience of not having to constantly prepare more water.

Getting the most out of your vacuum flask

Luckily, you don’t have to buy a new thermos if you find yourself in a similar situation. The trick that I’m about to show you is well known among the backpackers, survivalists and camping enthusiasts who frequently rely on their vacuum containers in their journeys for storing hot food and beverages on the go.

Get the hottest water you can, ideally boiling, fill your thermos to the brim, close the lid and leave it for at least 30 seconds. After that, discard that water and immediately fill it with your drinking water of a desired temperature for mate. That’s it!

Stanley Classic Legendary 1-liter Vacuum Bottle

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I’ve seen some people recommend preheating it that way for 5 or even 10 minutes, but in my experience heating a vacuum bottle for at least 30 seconds is enough for it to reach its maximum efficiency. In a real world scenario, I usually fill up my thermos with the hottest water from my boiler and leave until my drinking water is heated to an intended temperature and my mate is prepared and ready to be enjoyed, which gives my vacuum flask a solid few minutes to preheat.

Most of the water coolers have a hot water valve that can be used as a quick source of super hot water. You can also simply boil some amount of water alongside your actual drinking water and use it to preheat your thermos, or use the hottest tap water if you feel lazy — it won’t be as efficient, but it’s better than nothing.

The difference

I did two experiments to see how much difference this simple trick makes. For both experiments, I filled 2/3 of my 1 liter vacuum bottle with 80°C / 175°F water and then measured the temperature of this water every hour for 8 hours. In the first experiment I did nothing with the thermos, and in the second I preheated it for 30 seconds with boiling water.

As you can see on the chart above, the difference between 30 seconds preheating and no preheating is not dramatical, but still pretty significant. I would argue that even for fast drinkers, who finish their mate under 1 hour, this trick still can be useful, as you lose 4°C / 7°F more in that first hour if you don’t preheat your vacuum flask, which can play a big role for certain yerba mates, like Rosamonte Tradicional or La Merced De Campo.

The water in preheated thermos stayed above 60°C / 140°F for the entire duration of experiment, while in non preheated thermos it dropped below already after the 6 hour mark, which to me rendered such water too cold and useless for hot mate.

On the practical level it means that by preheating your vacuum bottle you will get a much more balanced mate for the first few hours of drinking, but also it gives you convenience of not having to reheat your water if your mate ceremony dragged on for an entire day. Preheating thermos also gives you a peace of mind, knowing that you can pause your mate session at any time and not lose too much heat when you resume drinking later in the day.

Pico Cebador Thermos

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Next steps

Now that you’ve learned this simple and easy trick, hopefully your future mates will bring you even more joy than before! Water is often overlooked when it comes to beverages such as mate, tea or coffee, even though it is the largest by volume component of any infusion! Correctly preparing water for mate can be a gamechanger, but there is so much more than simply heating it up and keeping it at a certain temperature. If you’re interested in taking your mate to a whole new level, check out my guide to learn everything you need to know about water for mate.

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I would love to learn about how you drink mate. How long does your average mate session last? Do you have any tips & tricks related to vacuum flasks and water for mate? Share it down below!