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Pu'er Storage Environment Part 2: References, Environment Maintenance

This article covers pu’er storage concerns in greater detail, following an initial article covering basic

issues. It cites reference sources that explore each previously identified subject (subset)

further, adding expert opinion or well-developed research based input on specific issues.

These topics are arranged by source reference from most general to specific subject scope, covering

background context and moving onto more specific solution suggestions.


The sources cited are blogs, with the exception of one informative tea forum post. Peer-reviewed

context research articles do cover some related scope, (eg. analyzing bacteria and fungus growth in

sheng and shu samples), and such references are cited in some of these sources, but the topic of ideal

pu’er storage conditions to support fermentation and practical advice about maintaining such

environments are practical concerns, not academic study scope. Currently blogs serve the function of

collecting and presenting the best available information across those source scopes.


A summary of this scope clarifies this:


TeaDB: research oriented blog input on humidity level concerns and “pumidor” / controlled

environment solutions

Tea in the Ancient World: a second personal review and research oriented tea blog, covering sealing /

wrapping in storage as a factor, relative versus absolute humidity, and ideal natural climate conditions

Death by Tea: personal history experiment-based tea blogger input

Tea Addict’s Journal: subject expert blog input on general storage issues and humidity level concerns

Late Steeps: experiment based series of results on split-storage environment testing related to

temperature range

Tea Forum discussion: dedicated web forum post summary of home-made alternative solution to use of

commercial humidity control packs


Tea DB tea blog on level of humidity for storage and pumidor solutions:


A main Western tea blog discusses these central topics regularly, with the post referenced written by

James Schergen, one of two blog-series contributors. “Pumidor” is a commonly used informal reference

to a controlled storage environment used to store pu’er, a variant of the more general English-language

based term “humidor.” An older post from August 16, 2014 cited a recommended range for humidity level:

The proper idea of humidity and temperature for dry storage will vary, usually falling somewhere

between 60-70 relative humidity and 60-70 F. Drier stored teas will age more slowly and are a much

longer-term game plan for those that engage it.


One qualification: that and any other blog author’s views can change somewhat over time, with greater

experience and exposure, so statements related to ideals might need to factor in all related conditions

and a learning curve. This later September 8, 2018 post on preventing mold risk and tied to his own

storage solutions help clarify the context of that earlier statement, with more of a recommendation tied

to personal experience:

You may hear instances of people storing tea in 70-80s RH in east Asia and directly translate those to

your home storage. I think this is a bad and risky idea.. It’s better to edge lower than parameters we

may associate with Hong Kong or Guangzhou. Airflow is a highly polarizing topic, but by putting our

pu’erh in a fairly confined space it traps the tea with more limited airflow, increasing mold risks.

Unless you are deadset on replicating higher humidity, pumping up humidity to the 70s isn’t a safe

idea for pumidor storage.

How I’m Storing (~4.5 Years): I keep my tea in a mini-fridge, a wine cooler, and plastic bins. I use

Bovedas to add humidity. My parameters:

Temperature ranges from about 68F-78F. It goes higher for maybe 7-10 days a year.

Humidity ranges from 60-68RH, usually centered around 65RH.


It’s a good point, that replicating natural more tropical storage conditions in humid climates within a

small enclosed space may not work.


Tea in the Ancient World on sealed storage, absolute humidity, and ideal natural climates:


This blog is written by the author of this article, John Bickel, an American living in Thailand (which is

coincidentally a good natural storage environment for aging pu’er).


This separate-source Tea in the Ancient World blog post (on November 29, 2017) cites references for

the Hong Kong and Malaysian natural environments, and for Bangkok climate conditions, of course

relating similar results as the prior Tea DB reference.  An earlier section of that same post describes the

meaning of relative versus absolute humidity, explaining how the moisture holding capacity of air changes

over different temperature ranges.

(graphic:  psychometric chart showing relative and absolute temperature relation and ideal pu'er storage window)


A more general reference discussion posted on September 1, 2016 covers environment concerns, cites

background reference sources, and addresses the specific issue of sealing cakes for storage individually

(the air contact issue).  It is commonly accepted that tea cakes should be stored with very limited air

contact but not necessarily sealed (eg. in plastic wrap or in individual ziplock-type storage bags).  Other

references in these citations offer further guidance on that point, with one heated-storage experiment

based input suggesting that it may not be a critical factor, provided that maintaining a specific humidity level

is carefully addressed.


Death by Tea blog experiment-based tea blogger input:


This pu’er-oriented US tea blogger has spent years communicating results and advice related to trial-

and-error storage solutions. Most notably Cwyn (a writing pseudonym) relates successes and failures

tied to using crocks (semi-permeable stoneware) as small storage environments, as in this October 7,

2014 post “Pu’er Storage is a Crock.” One limitation: in trying to follow and use methodologies based

on her successes it works best to read through multiple related posts to see which practices worked out



There are later accounts in that blog to consider, also based on crock or semi-permeable storage

environment solutions. The post titled “Lots More on Puerh Storage,” posted on May 22, 2015, passes

on additional ideas, and this Q & A based review on the topic “Questions I get asked about Puerh

Storage” (posted on April 3, 2018) draws on three more years of results. Content from that post

describes her own context, history, and concerns:

My [2009-2014] storage failure consisted of copying the cardboard box method... This method, even

with a bowl of water in the box, left my tea too dry, flat and flavorless…

My climate in the house is far too dry to leave tea in the open, unless the tea arrives with years of

wetter storage under its belt… during the winter, I experience very dry, desert-like conditions and this

is when I need some sort of storage solution to preserve the progress made during summer. I settled

on traditional farm crock storage used in this part of my country…

People who report mold to me are mainly doing one of two things. One, they are trying to replicate

Hong Kong storage parameters, with 70% RH and warmer than room temperatures. This is very risky

to do in a small storage situation, because you do not have much space and air flow to keep mold

from forming. I prefer a more conservative set of parameters, such as 60-65% RH at room

temperature or slightly cooler…

…The other mold situation is storage of puerh tea in plastic containers, such as plastic tubs. Plastic has

no ability to breathe. There is no air flow, no cracks or anything porous. Plastic is a temporary solution

for students or people moving to a new residence…

That works well as a summary of the other points she makes, but reading that post and the others

mentioned tell a more complete story (or an article posted on May 29, 2015 on storage issues specific to

shu / ripe pu’er).


The input on what range and air-contact conditions can tend to cause mold is especially helpful;

summary take there is that maintaining a 70% RH environment is possible, but requires careful

monitoring. It is interesting that her humidity-control recommendations match James Shergen’s, both

specifying 60-65% relative humidity as an optimum range. Both indicated that a slightly more humid

control setting is possible to facilitate faster aging but this raises risk of problems occurring and requires

much closer monitoring.


The temperature, humidity, and airflow concerns are interesting to revisit in light of a recognized

authority on pu’er in the form of another blog reference, as follows.


Classic storage input reference, the Tea Addict’s Journal blog:


One of the most respected tea blog references on pu’er is A Tea Addict’s Journal, written by Marshal N.

His background and grounding his level of authority on tea isn’t the point here, but related to starting blog

writing at the beginning of 2006 he serves as one of the longest running active references of that type.


A March 9, 2012 post “Ideas of Proper Puerh Storage” covers some basics:

Relative humidity of 70% in a 25C environment is very different from the same relative humidity in a

15C environment. The former is conducive to tea aging, the latter is not, because it’s too cold. Aging

tea requires humidity and temperature, neither of which can be too low.

It's a critical point; a number of factors are relevant. He also touches on the idea of replicating natural

climate or warehouse-controlled storage conditions at home:

…as I have made clear many times before, “traditional storage” is not the same thing as “wet

storage”. You cannot replicate traditional storage at home, even if you try and pump up humidity and

temperature. What you’ll get instead is some nasty tasting, mold covered tea, but the richness and

the flavours that at least some find alluring in traditionally stored teas will be missing. For that, you

need large volume, expert control, and the proper environment for it. You won’t get that at home,

even if you try, unless your home also happens to have a more or less air-tight basement with literally

tonnes of tea and 30C+ temperature.


This discussion there never does zero in on an ideal home-controlled conditions range, but instead talks

around the two related risks of keeping a tea too damp (the mold issue) and too dry (losing flavor, and

not aging over time).

…I do, however, find much fault with the idea that’s sometimes propagated on the internet that

natural = dryness. Even my friends in Beijing, who a few years ago were very wary of traditionally

stored teas, are now trying very hard to find ways to add humidity to their storage precisely because

they now recognize that the natural environment in Beijing tends to produce poorly stored teas

(dryness + coldness). To speed things up, they’d add water in bags in closed plastic boxes in order to

produce something better. Even that doesn’t produce mold. The worry, therefore, is really about

dryness, not wetness. It’s easy to spot tea that is starting to grow mold and even easier to rectify such

a problem – just reduce humidity and temperature, and you’re good…


In another January 22, 2016 post “Storing Pu’er tea – You Are the Final Master” he critiques an online

reference to ideal aging conditions, and comes closer to offering practical advice about what

temperature and humidity conditions will or won’t work.


These statements are the points he is discussing and generally refuting:

There are two kinds of storage – dry and wet. Wet storage is when humidity is 50% or higher. Dry

storage is “much drier condition.”…

Temperature – between 55 and 80F, that’s 13 to 27 [C]…


His response, a partial rejection along with further clarification:

Wet (I think they mean traditional in my usage) is a lot wetter than “50%” humidity – in fact, 50% is

positively dry. Anything drier is going to kill your tea, and even a constant 50% will pretty much

ensure your tea never really age at all…

Temperature – well, this is a sort of reasonable, if somewhat low, range. Temperature is not going to

kill you here, but if it’s too cold for too long your tea won’t change much either. The reason Malaysian

teas age a bit faster is because they’re generally hotter there. If your temperature is a constant 15

degrees your tea once again won’t age much at all…


It's again not an ideal controlled-environment recommendation but it does specify the range that won’t

work well, at or below 50% relative humidity, and at or below 55 F / 13 C, at least to support aging

transition (fermentation). It’s my understanding based on other reading and discussion that keeping tea

cool will just stop the aging process, as stated. It’s an interesting concern about to what extent a stalled

aging process and less favorable storage conditions can be recovered from (eg. how cold a tea can get,

or how dry, that still allows for resumption of uninterrupted resumption of micro-biome activity).


A bit on airflow qualifies where that stands:

Airflow is pretty much a bad idea… Fresh air can do a lot of things, but most of it will destroy your tea.

If you want your tea to retain its aroma and age well, stick it in a place with low airflow that isn’t too



As with other factors the idea here is to achieve the right balance. Too much air contact can ruin tea;

enclosure in too confined and too damp a space can cause it to become musty or to mold. An

experiment in the next reference source on long-term storage conditions that don’t provide significant

air contact, even oxygen supply, seems to indicate that sheng can age well even with minimal air



The next set of references on storage condition experiments addresses some of that, but on the side of

optimization versus problems resulting from poor storage conditions, attempting to isolate ideals

through experimentation.


(photo:  air contact limited by bamboo outer wrapper, tong of Menghai Tea Factory 7742 cakes, credit Wikimedia Commons, CC BY-SA 2.5, image created and uploaded by Jason Fasi)


Late Steeps blog post series on pu’er storage environment experiments:


This is a unique reference source, actually testing different environment controls by setting up long-term

divided-case storage experiments (located at All of the posts related to

different experiments are worth a read, but one of the early basic versions, “Storage experiment: 2016

W2T Bosch, one year in” published on August 4, 2018, highlights temperature as an important storage

transition factor:

The best dead-simple system I could come up with involves three pieces of technology: first, 1-gal

Mylar ziploc bags, second, a coleman cooler, and third, a temperature controller attached to a 17W

seedling mat. For about 50$ and no assembly whatsoever, you can individually ziploc your cakes and

keep them at a balmy 32 degrees C for months. After the cakes have been properly conditioned

(which for me could mean anywhere from 63-69%RH), this system will maintain that humidity with no

maintenance required…

One of the first teas I put into the hotbox was one of White2tea’s most popular cakes of 2016, the

Bosch. Each half of my Bosch cake was kept in a mylar bag; one went into the hotbox at 32C, and the

other stayed out at around 23C (room temperature). After one year (i.e., today), I did a simultaneous



So this experiment relates to sealing teas, already conditioned (pre-set) to a certain humidity level by

isolating the cakes together with a humidity control pack. It’s not identical to “pumidor” type controlled

environments but functionally it might not be so far off that. It raises the question of how much oxygen

contact the bacteria and fungus biome requires to be sustained, but for now that can be set aside.

One might wonder how it would be possible to measure the amount of moisture actually present inside

compressed tea, which isn’t a point that comes up in any of these references. The short version: that’s

not necessary, because a tea cake stored in a controlled 60% RH environment will absorb a

corresponding amount of moisture over a reasonable period of time. It is possible to measure that

amount by weighing a sample of stored tea, heating that to well over 100 C to evaporate off all moisture

content, then re-weighing the sample, but in general this isn’t required or useful.


The heated storage experiment results (comparing 32 C and 23 C environment results):

The most surprising result is that there was a very clear difference between the two teas, after just

one year of heated storage. The effect was not at all subtle, and the heated tea was much preferred

by both of us…

…Both teas have quite pungent dry leaf, but the heated tea is slightly more pungent and with a

sweeter smell. The unheated tea has a strong high-pitched floral aroma. The wet leaf is more

pungent, sweeter, and less green for the heated tea…

…Even visually, the [heated] tea has clearly aged more quickly. The harsh grassiness has been much

reduced and the sweet pungency has been enhanced…


A more complete review section, more description of control conditions, and other experiments varying

individual factors are all worth reviewing. The simple point here is that tea stored at higher

temperatures (32 C versus 23 C, relating to 90 F versus 73) ages faster, based on a relatively equivalent

humidity level condition.


The cause seems obvious enough: the bacteria and fungus responsible for fermentation experience

more of an optimum living-condition biome at that warmer temperature. Different bacteria and fungus

could potentially thrive under different conditions, so aging results may vary, but related to this simple-

variable experiment final results (taste and character of stored tea) improved at higher temperature

setting, which is the most important consideration.


Tea Forum discussion on home-made alternative to humidity control packs

A well-developed article by a Tea Forum member, identified as Pedant (a user name), describes how to

make an alternative to Boveda and other manufacturer’s humidity control packs. This post was labeled

Humidity Control: DIY Salt Packs, located here:

This section covers their version of humidity control background, with a couple key points about their

designed solution:

The most obvious question is which humidity point to design for. I arbitrarily chose 70% RH. The

general consensus online seems to be that 60-80% RH is acceptable, but mold growth in sealed

pumidors is a serious concern. One article suggests (6):

≤60% RH: molds don't grow

70% RH: molds don't grow much aside from xerophiles (e.g. P. chrysogenum germinates at 73% RH on

leather and paper)

≥80% RH: molds grow readily

I found experimentally that a mixture of 6:25 sucrose:NaCl by mass gives about 70% RH at room


Just perfect, discussion of ideal humidity control from the relevant condition of preventing mold growth.

It’s interesting to consider that the fermentation itself is occurring through the activity of bacteria and

fungus (microbes related to mold), so the point is to get the right types of microbes thriving to the right

extent, not restricting that biome.


Some earlier statements describe the project from the main starting point, what a humidity control salt

pack does and how:

Salt packs, sold under the brand name Boveda among others, are humidity control devices that

employ a salt solution to maintain a water vapor equilibrium in a sealed environment.

…This fixed humidity point is ultimately not affected by reasonable moisture sinks (like dry puerh

cakes). The salt concentration remains unchanged as moisture leaves the salt pack because some salt

precipitates out of the solution.

It provides two-way humidity control: if the RH goes above the salt pack's humidity point, the solution

behaves hygroscopically and draws moisture out of the air. Some of the excess salt dissolves, and

again the solution concentration remains unchanged…


Basically the packs are designed so that they give up moisture to the local environment if that humidity

range is below the designed equilibrium for the pack, and absorb it if the humidity is above that range.

In a large-space very damp or dry environment the packs would simply dry out or become completely

saturated, but in a confined space at the right proportion the humidity level would stabilize to the pre-

designed level instead.


Per the first statement mixing ordinary salt and sugar enables adjusting a maintained threshold using a

replacement solution, versus those commercial packs. A complete description and assembly process is

described in the initial post (with discussion following; it is a forum thread), but the basics are simple

enough: salt, sugar, and water are combined in a mason jar (canning jar), with a semi-permeable

material used as a cover, a Tyvek envelop. That device adds or removes humidity from the environment

that it’s in, supporting maintenance of a controlled level.


It’s brilliant; simple, effective, and low risk. The rest of the post details covers technical background and

materials details (eg. if type of salt is a concern, if the jar would leak if tipped over, device size concerns),

so if using this type of approach is of interest that thread would make for critical reading.


(photo:  current global relative humidity mapping, April 22, 2019, image credit




The range of ideas covers too much scope to summarize. The citations and opinions seemed to overlap

enough to map out a general approach and functional ranges people might follow in their own attempts

at setting up controlled pu’er storage environments. Each reference cited includes additional supporting

references and further discussion.


In terms of summary, it seems as long as indoor humidity stays above 50% RH there is minimal risk of

completely stopping the fermentation process. Teas wouldn’t age / transition much at that low

humidity level, and maintaining moderate room or storage temperatures is also a factor.

The most ideal solution would depend on individual goals. For some using higher levels of humidity and

slightly elevated temperatures to enable teas to ferment would be optimum, replicating tropical climate

storage conditions. Temperature and airflow / air contact are closely inter-related factors.


Many sources here recommend that humidity control range over 70% relative humidity is possible but

potentially problematic, requiring more careful control and monitoring. For others a goal might be to

simply avoid teas drying out and loosing flavor over time, and maintaining an environment at any level

at or above 50% RH would seem to achieve that. In the middle more moderate conditions might strike a

balance between those two extremes in goals and control requirements, and result in less active

monitoring demands, with lower risk of problems occurring.


Article by John Bickel

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