Testing DPcalc.org’s Mold Predictions with Young Raw Puer

The purpose of this experiment is to compare my observations of mold growth on young raw puer to the predictions of dpcalc.org.

For this experiment, I have chosen 2018 Yunnan Sourcing “Impression”.


  • The cake core was removed prior in order to reduce sample variation.
  • Boveda packs were used to control the humidity.
  • Each sample is ~50g tea.

Storage Treatments

  • A – 22°C, 65% RH, sealed in mylar.
  • B – 22°C, 69% RH, sealed in mylar.
  • C – 22°C, 72% RH, sealed in mylar.
  • D – 22°C, 75% RH, sealed in mylar.

DPcalc.org’s Predictions:

  • 65% RH – No mold risk
  • 69% RH – 232 days to mold (September 5)
  • 72% RH – 86 days to mold (April 12)
  • 75% RH – 38 days to mold (February 24)

I will be evaluating the results based on observed signs of mold and how close its occurrences are to the predicted dates. The results aren’t expected to be universally applicable.


Update: September 4, 2020 (241 day mark)

The sample stored at 75% RH has finally grown visible mold. The aroma of the leaf from the bag has been clearly off with minty notes for quite a while, but I wanted to wait for visible mold. Here are some of the affected leaves:

Excuse the subpar photography. As you can probably tell, most of the mold is growing on the stems, and I’m not sure what that indicates. Could it mean that stems have a higher water content relative to the leaves? Or that stems are simply an easier access point for the mold to reach moisture or other nutrients? Whatever the case may be, it does not seem feasible to store young raw puer at a relative humidity as high as 75% at 21-24°C in a sealed environment without encountering mold.

The remaining three samples at 65%, 69%, and 72% currently exhibit no signs of mold, neither in appearance nor aroma. I will continue to check on these samples periodically. If there is no mold on any of the remaining samples by the 1 year mark, I will feel confident in saying that 72% RH is a safe upper limit in the case of sealed storage at 21-24°C.

Update: January 6, 2021 (365 day mark)

The remaining three samples still exhibit no signs of mold.


At a stable ambient temperature of ~22°C, it does not seem feasible to store young raw puer at a relative humidity as high as 75% in a sealed environment without encountering mold. For these particular conditions, 72% relative humidity appears to be a safe upper limit, though personally I would stick with 69% RH as a more conservative maximum.


    1. This experiment is over, so there won’t be further updates. But I do have another experiment on testing the limits of humidity in heated storage. The initial results are promising!


  1. Thanks a lot for such a good info.
    My question is: it is necessary to keep the tea inside mylar bags? Or is it enough as I do? Only inside of a drawer in a wardrobe. Of course with 70 RH. Thanks!


    1. It isn’t necessary to keep tea in mylar bags, but I like them for a few reasons. For one, they keep humidity locked in with no need to replace Boveda packs (as they would otherwise dry out). I also like that they keep outside odors out, and block ambient light from getting in. Mylar bags are much less air-permeable than plastic zip-lock bags, which means that you can even heat them as I do in order to increase the speed of aging without losing moisture. If you’d like to buy good quality mylar bags, I recommend the ones from https://www.topmylar.com/gusseted-pouches.
      If you’d like to verify that your current setup is working as intended, I recommend picking up a hygrometer along with a Boveda calibration kit. Once you’ve calibrated the hygrometer, you can use it to check the RH in your tea storage. This will give you an indication of how well-sealed the storage is. The better the seal, the less often you’ll need to replace dried-out Boveda packs.


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