Tea in the Thermos

Tea in the thermos

My thermos. My favourite one is the second one to the left, an 800 ml from Tiger. The 500ml blue one from Zojirushi is kind of okay too for a brief outing. Both are Japanese brands.

The thermos flask (vacuum flask) was my first teapot.

It was not those nice looking tiny stainless steel ones that are popular today, but those half gallon (2-litre) double-wall glass vacuum flasks with big clumsy aluminium outers and a cork stopper. It was tea for the family throughout the day. We put a bit of tealeaves in it in the morning and continued to fill it with boiling water before the tea inside reached the bottom. When it got too stale or too pale, we discard the old ones and put in some new. Or we got lazy and just put in some new ones over the old. Or we got even lazier by withstanding whatever taste it was. We were poor and had nothing much anyway. Anything with tastes was better than no taste. That was some forty years ago.

After I got serious with tea, I had made some attempts to make tea with the contemporary kinds of thermos with bad results. The insulated bottles were then left to use only for bringing soup to my mom or hot water in outings when it was cold.

Rust in the thermos

Stainless steel is a preferred material for flasks that I would be carrying around. Except when its production is poor quality. See the rusts at the joints? I have used this one for only a couple of times and stop using it since. From a spin off brand by Buffalo.

Inspired by the British

That was until I read about the role of tea during the wars from a book by Alan Macfarlane some years ago (1). He wrote that the British soldiers were rationed weak tea for filling their canteens during battles and argued that was one reason the soldiers won the wars.

I was less interested in finding out about the physiological effects of weak tea for winning wars but got reminded that a large proportion of tea drinkers, including those that are like us when we were young, drink weak tea. Weak tea that can be kept in the canteen through the day because the change of the less stable tea substances in it is proportionally much lower. It also means less concentration of tea substances that further change upon storage for that few hours, so the taste could be a bit more stable, albeit less of it.

Tea on the go vs Ready-to-Drink products

So how weak could I make a tea that keeps acceptably in a thermos but not as stale and pale in a ready-to-drink product? If I succeeded, I could have tea on the go. In all my hiking and city excursions (I consider going to town an excursion, the canyons of concrete, the thick rapids of human flows, the extreme environments of poor air quality and huge temperature differences…), I could have tea whenever and wherever!

That idea jump started my experiments with tea in the thermos.

Previous experience tells me that the effective heat retention character of the thermos would mean chemical changes for certain tea substances which would lead to bad taste. So I cannot infuse tea within the thermos. I can see the thermos only as a container.

Preparing the tea separately for filling the thermos

I prepare the tea in a separate pot while preheating the thermos with warm water. Since I know heat would kill the taste and health contributive substances in tea (2), and I do not want to be scalded when drinking, I preheat the thermos to only about 70°C, with 20 something degrees for heat loss during the trip.

tea stain in the thermos

All is fine with the convenience of the modern thermos. Except for a few issues: I am very sceptical about the safety of the plastic coating in some of the products, which I think is not that safe on prolonged storage of hot beverages. The tea stain inside the crevices always bother me. A reader once suggested that I use a denture tablet. It works when the stain is light and quite fresh.

I began with a few Minhongs, which I suspected would be similar to some of those used in earlier continental wars fought by the British soldiers. It was okay after a few tries and adjustment. Kept in the thermos for a few hours and still bearable. Nothing like the tea when properly prepared and drunk though, but some characters of the tea were still notable. A lot more like tea than any bottled products in the supermarket.

tea preparation tips

And then it was pu’er, the tea that I knew would keep the best, since that was the same category of tea I had when I was small. Naturally, the shu type maintained its profile better than the sheng type.

It has now been four years that I bring tea in a thermos sometimes even for a trip to the cinema. I have tested most teas that I would personally drink and here is my personal conclusion:

Tealeaves to water ratio

0.5~0.3g per 100ml water, especially for greener teas. The longer the tea is to stay in the flask before drinking, the less leaves you should use

Tea varieties

In terms of maintaining the best resemblance to a freshly prepared cup of tea after a few hours, in descending order:

Shu cha types of Pu’ers > machine-made black tea > matured shengcha pu’er > browned styled Wuyi oolong/Tieguanyin > full leaf black tea > white tea > higher-fired green tea > baked/roasted green tea > classic style Phoenix oolongs > greener oolongs > steamed green tea

Note also that this descending list defines only the amount of taste change or loss, but selections with more taste in the beginning may still taste better even if they are lower in the list. One example is  classic style Phoenix oolongs.

not much quality in bottled tea

A researcher of the Tea Research Institute in Hangzhou once told me that her husband, also a tea researcher but for a most popular F&B brand, gave up using better quality tea leaves because the tea liquor lose almost everything once bottled. The money for the quality is wasted, he said.

I think there is a fine line between tea kept in a thermos for a few hours and ready-to-drink products. Health content asides, the taste, the temperature, and the independence from marketing influence to decide what tea you can drink, are to me strong enough reasons for carrying that bit of extra weight for 800ml of a quality tea of my preference.

1. Alan Macfarlane, Green Gold : the Empire of Tea, Ebury, 2003
2. ZY Chen et al, Degradation of Green Tea Catechins in Tea Drinks, Journal of Agricultural and Food Chemistry 2001, 49, 477-482

Related reading:

  1. mark says:

    when i prepare tea in a thermos for the night it acquires a strange taste like a detergent taste
    thank you

    why do you think that is happening?

    • @mark, sorry for late reply. We have noticed that same strange smell happening in certain situations too but not sure about the reason. It is more likely to happen with certain kinds of green and black tea; also when temperature is too high or when covered for too long.

      Too many elements are in play now and we want to do better study first before we conclude what really happens.

      We have been quite successful in avoiding that by first infusing the tea in a separate vessel first before pouring slowly into the thermos. Prepare tea the night before is not a very good idea. The taste simply turns too strange for many tea selections after a night in the thermos.

      Shu cha puer is the tea that changes the least upon long keep in the thermos. Then there are the baked and medium baked oolongs. Green and black teas are not as reliable.

  2. That’s a very difficult question to answer. Usually the bottom portion of the tea in a thermos would get quite cold towards the end of the day. To solve that problem, maybe you can bring two thermos, each for one half of the day. For the one that is for the second half of the day, fill it with a tea that sustains better, such as a shu cha pu’er.

  3. John says:

    What do you recommend If I’m at the office 8-10 hours, and I can’t prepare me tea there?

    • K says:

      Just buy a genuine Thermos 1.2L or 2L flask. If I start with 90 degree water, after 12 hours it only drops to 70 degrees (Celsius obviously). After 24 hours it’s still at 60 degrees.

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