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


Leave a Reply

Your email address will not be published. Required fields are marked *