Pumidor Adventures – Part 1

Special storage for pu’er

One of the things that makes pu’er teas special are the live microbes that remain on the leaves after an intentionally incomplete kill-green processing step. Given certain conditions, these microbes ferment the tea over time, changing its character significantly.

I’ve now accrued a modest collection of young raw pu’er cakes. They can be categorised into cakes that are either:

  1. ready to drink right now, or
  2. currently too rough and need time to ‘calm down’ (age a bit) before being enjoyable.

I had assumed that my ‘ready’ cakes would be fine being kept ‘on the shelf’ and only my ‘rough’ cakes would need special humidity-controlled storage to help them age into something nice.

After thinking about it more, I realised that the ready cakes are typically the more expensive ones. Not trying to age more complexity out of them would be a wasted opportunity. Since I was planning on some kind of storage solution for the rough cakes anyway, I might as well throw them all in together.

Learning about pumidors

I decided to educate myself using TeaDB’s resources on pu’er storage and pumidors 101 (the word ‘pumidor’ is a portmanteau of “pu’er humidor”). I learnt that hobbyist artificial pu’er storage is a complex topic with a lot of theorising by people and scarce historical evidence. Sydney has average climate conditions ranging from 13ºC (55ºF) lows with 50% relative humidity (RH) to 27ºC (80ºF) highs and 60% RH. The generally accepted wisdom is that 65-75% RH hits the right balance between microbial activity (‘ageing’) and mould growth (‘sadness’), so a pumidor should be worthwhile for me.

Making a pumidor

At the time of writing I have constructed a low-effort pumidor and am currently experimenting to see how it goes. It consists of:

  • a small esky (ice box), cleaned and aired out (no residual smell),
  • a cup of water, and
  • a hygrometer.

The hygrometer I have has a wireless sensor that I keep inside the esky that reports data to an external display. This lets me monitor the conditions inside without having to open the esky and losing precious humidity.

On my first attempt, I boiled some water and let it cool before pouring it into the cup inside the esky. I then loaded up the esky with the pu’er cakes and sensor and closed the lid. Unfortunately, I did not wait long enough for the water to cool so water started to condense inside (very bad!).

I took out everything and dried it all out before trying again with boiled water that had cooled down much more. After this, the conditions inside the esky were 25ºC (77ºF) with a little over 80% RH (I checked several times to confirm no condensation was happening), but over the next few hours this steadily dropped to ~75% RH. I assume this was from the cakes absorbing some of the moisture. I tried putting a bamboo skewer inside the cup of water to increase the surface area available to diffuse water, although I’m not sure if it would make any difference. After a few days it became a bit mouldy so I removed it.

Monitoring conditions

After several days of operation, the conditions are currently stable at 25ºC and 75% RH, assuming the sensors are accurate.

Since I’m running at the dangerous end of the ideal humidity range, I will need to be on the look-out for mould. One important metric for this is the dew point: the point at which air starts condensing, which varies by temperature and RH. I found online tools to calculate the dew point, and dpcalc.org even has an algorithm for estimating how many days until mould growth at the specified conditions (36 days in my case). I’m not sure how accurate or meaningful this number is though.

Being an ice box, it has some level of insulation. Combined with being inside the house, it doesn’t look like sudden temperature changes outside would cause the dew point to be reached. I guess I will have to wait and see how things go.

I’ll post updates in the future to share anything I learn. Feel free to comment with any personal experiences!

Update: 2017-01-07

I have acquired some 75% RH Boveda packs and replaced the cup of water with a couple of satchels. I’m currently experimenting to see how many I need to reach target RH.