Storage experiment: 6 months at 40C and a warning on heated storage

// Published January 25, 2019 by mgualt

The storage experiments continue: today we have two samples taken from my favourite cake of young sheng: the 2015 Biyun Hao Lishan Gongcha. The sample on the left was kept at room temperature (23C) over the past 6 months, while the sample on the right was kept in a hotbox at the quite high temperature of 40C. Both samples were kept in small mylar ziploc bags.

At this point, I need to give an important summary of and warning about heated storage for people who are thinking of trying it:

Why heat or humidify Puerh tea in storage?

The reason is that the transformation of the tea from green to brown is done by enzymes, and these enzymes (whether they come from the plant or microbes) work best in a certain humidity and temperature range. An air-conditioned or heated home is, typically, not in the ideal range. My experiments have been focusing on raising the temperature to 32C (or, for today, 40C), and the relative humidity to the range 60-70%.

So, what’s the warning?

A puerh cake is like a slightly damp sponge, and we want to keep it damp. If we raise the temperature, the moisture will evaporate faster. So, either we need to add humidity to the environment, or we need to protect it in a container so that the humidity does not escape. If the ambient humidity is, for example, below 60%, the tea will eventually get too dry, and if you heat it, it will get drier faster.

Those storing tea in mini-fridges or bins with loose-fitting lids know that these containers are not air-tight. Without added humidity, tea stored this way will eventually dry out; if you heat it, it will dry out faster. The solution in these cases is to add humidity.

Now back to the Lishan Gongcha. Sorry about the glare on the photos. On the Left is the room temp stored, on the Right is the 40C stored. I will only indicate differences instead of reviewing the tea itself.

  1. Colour is slightly darker on the left. and there is more empty cup aroma on left. On the right, tea is softer, sweeter and significantly thicker.
  2. Right is again thicker and sweeter.
  3. Right is thicker, sweeter, and specifically has good sweet huigan. On the left, it is greener in comparison. Has more of the classic young sheng brashness and astringency.
  4. Right is thicker, smoother, good huigan is noticeable again.
  5. Left is noticeably greener and not as creamy. On the Right, the experience goes in distinct stages: thickness first, then sweetness, astringency, and ending with resinous aftertaste.
  6. Greener on the Left. Right is now slightly darker in colour. No question it is sweeter and smoother.
  7. Longer steep now, right is even more noticeably oily and thick in comparison to the Left.
  8. Very long steep – 5 mins. Left is initially more bitter whereas on Right there is a contrasting honey sweetness.

Of course – these two samples are extremely close together, with only 6 months of heating they are not far apart. However, it was clear to me and my drinking partner that the main difference after 6 months of 40C heating is an increase of thickness/creaminess/oiliness of the liquor, a lessening of the green harshness, and an increase in the sweet qualities of the tea. Also, since this is a higher temperature than is usually recommended, I was looking out for any negative characteristics and I could not find any. It seems as if the tea is doing very well at 40C, and we’ll see what happens at the 1-year mark.


  1. Emil
    January 25, 2019 @ 10:52 pm

    Cool post and thanks for the warning! So next experiment is to add temp and humidity at the same time? (thinking that adding humidity would counteract the drying out effect of increasing the temp)

    • mgualt
      January 25, 2019 @ 10:59 pm

      Hi Emil, please see previous posts. The storage experiment is to bring cakes up to a fixed generated humidity and keep them at 32C or 40C. I am including this warning because I noticed some people are heating their tea without protecting or humidifying it. The warning is not a result of the experiment, its a PSA.

  2. Alex D
    January 27, 2019 @ 3:38 pm

    Hi Marco,

    I apologise for sticking my nose in like this, but imho, this post could do with a warning for the warning.

    The pre-hydrated, mylar-bagged hotbox setup is a fantastic way to maintain humidity under elevated temperature without (it would appear thus far) risking mold.

    This has not been my experience with concurrent (to the heating) salt-pack humidification.

    The natural conclusion one might come to after reading the article is that if one doesn’t want to or can’t emulate your experimental storage, that they can heat up a pumidor and just be sure to humidify it sufficiently. I’d humbly suggest that the risks of such an approach are best mentioned.

    • mgualt
      January 27, 2019 @ 3:51 pm

      Yes of course I agree, adding humidity involves risks I wanted to avoid in my list at the beginning of the storage posts. I am making no recommendations on storage at all, just explaining that readers should not simply take my posts as advice to heat their storage.

      • Alex D
        January 27, 2019 @ 3:57 pm

        For sure, for sure!

        I only bring this up as there’s a fair bit of hope/excitement surrounding your idea/setup (from myself, included).

        I really should dip into mine and if a difference has developed yet.

        Looking forward to future updates!

        • mgualt
          January 27, 2019 @ 4:00 pm

          Ok good to hear of the interest, I’ll keep posting about it. I want someone to try to simulate HK Trad storage at home… When will it happen!?

          • Alex D
            January 27, 2019 @ 4:27 pm

            Perhaps when I graduate and have money to blow on the casualties, lol.

  3. Teaboy
    March 3, 2019 @ 9:21 pm

    Where do you get your mylar bags, both cake-size and sample-size? I find most mylar bags have a lot of odor, but some don’t.

    • mgualt
      March 3, 2019 @ 9:26 pm

      I go for 5 mil bags manufactured in the USA. These are available from eBay or from a bunch of places online. The problem is that when Mylar bags are newly manufactured, they can have a smell. Sometimes you get unlucky and get a new batch and they need to be aired out for a while. Opening them up and keeping them somewhere warm seems to speed up the process.

      • Matt
        April 17, 2019 @ 9:04 pm

        What dimensions do you use to fit standard cakes? Mind sharing a link to the bags? Thanks!

        • mgualt
          April 18, 2019 @ 3:10 am

          The 1 gal 5 mil ziplock Mylar bags fit two cakes, that’s the majority of what I use. Best wishes

  4. Toby
    June 18, 2019 @ 4:35 am

    I’ve decided to take the plunge into a set up like yours for my modest pu-erh collection (about 9kg in storage, average value pretty low, maybe 15c a gram). For two primary reasons related to my local (indoor) conditions, which are both too humid and too cold a lot of the year, and then quite hot and very dry in the summer.

    I also have a reasonably diverse set of teas I want to store (lots of young stuff, but also some trad storage, some taiwan stored, some Liu An, some Shoumei) so the sealed storage means I can store all these together without worrying about smell/taste crossover.

    I’m going to aim for 25c at 65% RH, which I take to be numbers that are relatively safe; quite unlikely to mould, but not too low to stifle ageing too much (higher on both would probably be better for ageing, but also more dangerous).

    My main concern/question is that if you condition a cake to 65% RH at 25c and then seal it in a mylar bag (without anything inside the bag to control humidity) and the temperature drops significantly (for example if a power outage stops the heating) then the RH will presumably shoot up and you are quite likely to get mould.

    I’m wondering if you have any thoughts on this as a problem? In particular I was wondering if you’ve done any more experiments (like the conditioning post) on what happens to the humidity of a cake sealed in a mylar bag as temperature changes? I’m hopeful that the cakes themselves might act as a RH buffer, but I wouldn’t want to bet on it without more testing.

    Of course power outages are not that common where I live, but it is a risk. I am thinking of getting an Uninterruptible Power Supply just in case; that will keep the heat on for at least a few hours if the power goes out. But heaters can fail, inside temperatures can drop for other reasons, etc. etc.

    • mgualt
      June 18, 2019 @ 5:16 am

      If there is no added humidity and if the temp decrease is slow (that’s why I use a well insulated container like a cooler or styrofoam box) then you can avoid condensation. I have not had condensation from lowering from 32C to 25C when my cakes are in the mid 60s and storing in a well insulated Coleman cooler.

      Note also that 25C is still kinda cold, it’s room temp for me. Aren’t you heating your house to around this temp already?

      • Toby
        June 18, 2019 @ 10:46 pm

        That’s good to know. I have a reasonably well insulated cooler, I’d only expect the temperature to change dramatically if the heater fails, which is unlikely. Or when I’m taking cakes out to drink them, which is not often (I take out at least 100 grams at a time, I don’t like chipping off just enough for one session at a time), but then I’d be opening the bags which should allow me to avoid condensation.

        The lack of heating is a big part of the reason I’m doing the set up. Central/constant heating is rare in homes in Australia (we generally use heaters in the rooms we are actually using and only during the morning and evening, not the day time or overnight), which makes sense for most of the country where it is warm (for example Sydney almost never gets below 7 or 8c, and definitely never under freezing), but not for Canberra (where I live) where it regularly gets below freezing for about 4 or 5 months of the year. The inside temperature doesn’t usually drop below 10c, but it might get less than that if I am away for a few days and so I don’t turn any of my heaters on. I want to heat to 25c so to avoid the dangers of low temperatures really stalling ageing, rather than to increase the rate of ageing substantially.

  5. mrmopar
    May 11, 2020 @ 7:26 am

    Great post. I am wondering do you think the aging process is aerobic on anaerobic? I wonder is totally sealing the bags would stop the process if oxygen is used up in the bags.

    I am aware of some vendors sealing tea in shrink wrap plastic. I would think for the process some oxygen would be necessary. Maybe opening the bags on occasion to allow fresh air in.

    Great post and results with the experiments.

    • mgualt
      May 11, 2020 @ 10:05 am

      I am interested in, and have been testing, the oxygen issue. I can make some comments about what you said:

      1. The enzymes use oxygen to do their work. So, some oxygen is needed. The question is how much. I’ll comment more on this in the future when I have results, but I estimate that there is enough oxygen in the sealed mylar to support at least five years of aging, if not more. If you vacuum seal, this is a different story.

      2. There are many kinds of shrink wrap and all the types I have seen are extremely porous, so much so that from the point of view of oxygen it is completely irrelevant. Water vapor can pass very easily, and so if the shrink wrapped cake is kept out of proper storage it will dry out. Shrink wrapped display cakes, for example, might be very dry, and need rehumidification.

      • Mrmopar
        May 12, 2020 @ 9:14 am

        Awesome. I had wonders about the oxygen level as well. I try to open my cakes at least once a year to refresh the air.

        On the shrink wrap, I think it is common practice in China with the display cakes. I have seen some vendors ship this way too. Not sure why but whatever floats their boat.

        Thanks for the blog and posts. Seems you are doing some of the things I do as well. Looking forward to your notes as time goes by.


      • Karma
        June 19, 2020 @ 5:51 pm

        A point of note here, is that mylar is not impermeable to O2, and 5mil mylar should let it around 1/ml of O2 a day. That should likely be plenty.

        • mgualt
          June 19, 2020 @ 7:38 pm

          The spec sheet you linked to is for Mylar film, which is only the plastic part of the Mylar bags I use — these are metallized. This means the gas and vapour transmission is significantly less than the plastic alone. I estimate it’s around 0.1ml-0.3ml of O2 per day for a bag with about 0.25m^2 of surface area, and could be significantly more if the bag is zip sealed. The net result of this is that the amount of air already in the bag, and which gets into the bag when they are opened every few months, is more than the amount that gets in through diffusion. As for what is “plenty”, I estimate that the enzymes in half a kilo of puerh would use at most around a liter of O2 in a year, although this estimate is based on what is known about C.Sinensis Sinensis enzymes. I assume it’s not much different for Assamica. The outcome of all this is that every estimate I’ve been able to make points to the fact that there is enough oxygen to keep the enzymes going strong, which is no surprise given my results.

  6. PuPuPlatter
    June 11, 2021 @ 12:13 pm

    i enjoy this post and liek the logical approach! I’d like to pose a question. Do you think dried out cakes say a few years in 20-40% humidity could revive and age into something that is pleasing or do you believe they stay flat and meh? I wonder if a tea that had the markings of a good aging candidate, ie. mouthfeel, astringency, bitterness would end up aging out converting those aspects regardless of whether it was abused in the first few years. This obviously has the markings of a long term longitudinal experiment. Thoughts?

    • mgualt
      June 11, 2021 @ 12:42 pm

      Yes it is possible to rehab sheng puerh cakes using rehumidifying and heating, depending on the conditions they were stored in. often however people who keep their tea unprotected from cold and dry also keep it unprotected from smells and/or mold, and there is not much you can do about that.


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