Conditioning experiment

// Published December 14, 2017 by mgualt

A puerh cake will generate a certain relative humidity (RH) when sealed in a close-fitting mylar bag.  Cakes which have just arrived from Taiwan natural storage, for example, will typically generate between 55-65% RH at 24C (Room temperature).

If you heat them to 32C, they will typically generate a higher RH, ranging from approximately 60-70%.  Here are some examples that I’ve measured:

RH at 24C RH at 32C
1701-7542 66% 70%
801-8582 60% 63%
XG Love Forever 58% 63%
2006 YQH Qixiang 58% 63%
69% Boveda 68% 70%
Empty 50% 45%

Once I understood how this worked, I tested a problematic cake I own. The cake stands out as quite strongly musty, geosmin-like, earthy.  Some people definitely are fans of this profile but I am not, and since the cake was advertised to me as natural Taiwan storage, I feel I didn’t get what I ordered.  The cake is a famous Taiwanese production from the mid-2000s and is regarded as one of the best from this particular maker.

The surprising result was that the cake was unusually dry. I measured it at 47% RH at 24C!  Then at 32C it rose up to 50%!  This is a dry cake for sure.  I don’t know if the dryness caused the mustiness, but in any case that was what I found.

Next I decided to “condition” or re-hydrate the cake, using a Boveda 69% pack. Here is the result after one month of conditioning at 32C-34C:

Ignore the blip in the middle: I accidentally moved the heater for a few hours.

What we see here is that it took a full month for the RH to rise from around 51-53  to 61-63, i.e. about 10 percentage points.  I was surprised by how long this took.  The cake is obviously still absorbing what the Boveda pack can emit, because without the cake, the reading goes up to 68-70 in only a couple of hours.

So, this should give you an idea of how long it takes to condition a cake.  There are two experiments I wish to do next:

  1. Once the cake reaches around 63-65%, I will remove the Boveda and see whether the cake remains at this humidity level over the longer term
  2. I will repeat this experiment with another dry cake (I need to find one) but carefully weighing the cake before and after the conditioning.  I didn’t do this in this case because I was not intending for this to be an experiment, it happened by accident.

Another interesting finding: at this point, the cake is much less musty-smelling than at the beginning.  It isn’t completely clear of its former aroma but it has been dramatically reduced, without any “airing” or exposure to circulating air. Only through humidity exposure at slightly higher temperature.



  1. Jonny山内
    March 10, 2018 @ 3:34 am

    Some yeasts and moulds naturally inoculated into the cake may have been triggered into spore formation from the dry conditions hence the mustiness. These spores are easily damaged by warmer and wetter conditions hence the improvement seen

  2. TONY
    October 11, 2018 @ 11:34 am


    How do you measure the RH? Did you measure inside the Ziplock bag or ousite?

    • mgualt
      October 11, 2018 @ 11:41 am

      Hi Tony,
      RH is measured with a sensor in the bag. Measuring outside would not make sense, as the bag is essentially air tight.

      • Matt
        June 11, 2019 @ 9:29 pm

        Would you mind sharing what specific product/method you used for rh/temp monitoring in the bag?

        I think your experiments are excellent. Good methods descriptions are needed for us to better understand how to store our Puerh. Thanks!

  3. TONY
    October 11, 2018 @ 11:40 am

    Regarding the condition part, did you put the Boveda pack together with the cake into the ziplock bag?

    • mgualt
      October 11, 2018 @ 11:48 am

      Yes, that is how I do it.

  4. TONY
    October 11, 2018 @ 11:56 am

    While you are conditioning the cake, does it have to be in high temperature 32C? Will it still work in a lower temperature environment(24C or lower)?

    • mgualt
      October 11, 2018 @ 11:59 am

      It will work, it will just be slower.

  5. TONY
    October 11, 2018 @ 12:14 pm

    Is the Boveda bag safe for food? Can we use distill water and a sponge instead?

    • mgualt
      October 11, 2018 @ 12:53 pm

      Yes, it is safe for food. Sponge with water… I wouldn’t do that. Sorry, Tony, I am not giving advice, just explaining the results of my tests. Cheers!

  6. Five Things I Like & Dislike 12. Measuring Pu’erh Conditions, Steaming Iron Cakes, Anonymous Vendors | TeaDB
  7. Emil
    April 26, 2020 @ 4:06 pm

    Do you use a mylar bag to store the cake + boveda pack for conditioning? Or an ~airtight zip bag and then move the conditioned cake to a mylar for heated storage?

    Did you conduct the next experiment 1?

    • mgualt
      April 26, 2020 @ 4:58 pm

      I just use Mylar. The details are maybe not so important if you understand the main point of all this. If you are trying to humidify a cake it makes no sense to use a partially sealed container. Using Mylar the cakes do keep their humidity over a 3 year timescale.

      • Emil
        April 27, 2020 @ 4:04 am

        Thanks a lot for clarifying! Trying to wrap my head around the principles laid out here.

  8. Why Shelf-Stored Pu’erh Storage in the West Is Usually Bad | TeaDB
  9. morgan
    June 7, 2021 @ 7:28 pm

    Hi Marco. Thank you for sharing your knowledge and experiments. A clarification and a couple questions, if I may.

    I have some shelf stored cakes that are probably a bit dry. Based on what I’ve read, a reasonable approach might be to 1) place a dry cake in Mylar and assess its humidity 2) add a boveda pack to raise the humidity as needed (target cake humidity in the bag when conditioning is complete and after removal of the boveda at room temp should be about 63-65 RH), 3) with only the cake now in Mylar, place in a hotbox (if I did this with a humidistat in the bag, I should expect the RH to rise with the addition of heat, but, as per your explanation above, that should be ok as the temperature is also higher), 4) monitor and maintain / enjoy. Have I followed the logic here correctly?

    For the questions: When conditioning a cake inside a bag with a boveda, would you allow the cake and boveda to touch or should they just be roommates?

    You made a comment under a different post that: “If you are at 66% RH, then 32C is extremely safe, but 23C (room temperature) is definitely not safe from mold growth.” Is there a reference somewhere or have your experiments shown what a mold-safe RH range might be for a cake in a bag at room temperature?

    Thank you patience with newbie questions.

    • mgualt
      June 7, 2021 @ 10:57 pm

      1. I don’t disagree with this strategy
      2. It doesn’t seem to matter if they touch. I usually leave the wrapper for the cake on.
      3. There is a range of mold and yeast commonly found on raw puerh and it has well known growth conditions. I consulted the tables in the literature and added an extra margin for error. It seems to work well for me.

      • morgan
        June 7, 2021 @ 11:13 pm

        Thanks again – this is much appreciated.

  10. Heated Storage System - Empirical Tea

Leave a Reply

Your email address will not be published.