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.

 

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