Pu'er storage and fermentation

A friend recently pointed out a 2013 Tea Chat tea forum discussion thread about how to store pu'er, not focused on typical humidity and temperature issues but instead whether or not to wrap the tea in such a way as to seal it.  It was surprising to me that this is even an issue, since the conventional take on wrapping pu'er or hei cha is so consistent, that it's typically not stored well-sealed, as every other tea type is.


The standard take is that pu'er needs some air contact (very minimal, but some) so storage in the paper wrapper is a given, maybe along with some other form of outer wrapping that also doesn't completely seal the tea.

I'll start with the two different takes expressed in that thread, and move onto what fermentation is as relevant background.  Then I'll close with a bit of feedback from a couple of sources that really know pu'er (always relative that, but seemingly relatively so).  Lots of people have lots of experience with pu'er, and a discussion forum like Tea Chat is a great way to be exposed to some informed takes.  Of course the standard discussion is about ideal storage humidity levels, quite closely related, as in this other more recent Tea Chat discussion.

Tea Chat discussion; to seal or not to seal pu'er for storage  

Not everyone might see this as an open question, but it was raised as such, and brought up some interesting points and discussion in that earlier Tea Chat tea forum discussion.

Initial starting point comment:  This is one of the reasons many people put their cakes into sealed plastic bags to preserve aroma and flavor. Exposure to air will deteriorate this just like with any other tea. It makes sense.

Response:  A lot of people (like Marshal N) believe that doing this stops or severely limits the aging process

Of course that "Marshal N" is a reference to A Tea Addict's Journal blog author, one of the classic tea blogs, still active now.  Others responded that pu'er needs both humidity and oxygen contact to ferment, and later Marshal N himself clarified that further in the discussion:

I don't think constant airflow is the answer. In fact, my storage has very little airflow. I don't believe in sealing the tea, at least not when they're young (when we have 20 years old cakes it's a different problem) but I think airflow is bad.

I store my tea in a cupboard with doors. I only open it when I need something from it. It's not like a fridge door or anything - it will allow a minimal amount of air to circulate, but that's it. In a place like Hong Kong a tight seal runs a very high risk of mold.

And then later, more specific to the point related to sealing:

The point of storing puerh is so that it will change. If your object is to maintain the tea as it is now, then yes, by all means, seal them up, vacuum them, put them in black boxes. But that's not what puerh is - puerh is an aged tea. Those wonderful 50 years old red labels out there? They were, for the most part, not put in plastic bags and sealed up all these years, and they're still wonderful now because the base tea was good and they were stored well. 

...I'm not at all suggesting you should put tea on an open shelf and under the sun - I am saying they don't need to live in plastic bags. ...thousands of drinkers in HK and Taiwan don't seal their tea up when they buy them. They leave them out naturally and let them age. 

That's really the gist of it.  On the one side sealing up pu'er makes sense related to common sense related to storage of anything else, and to the idea that air contact would diminish tea flavors through evaporating off components that cause flavors (that nice smell of tea being stored is molecules you wouldn't get to taste drifting into open air).  The other take is that conventional wisdom demands that pu'er requires minimal air contact to ferment as it ages (not exactly requiring airflow, but not sealed).  A notable justification for the latter is that this is long-standard practice, with a history of effectiveness.  Marshal N says more in his post on pu'er storage, with the same discussion about sealing tea playing out in the comments.  Of course this is all mostly geared towards concerns about aging and fermenting sheng pu'er, not as clear in relation to shou, which does more fermenting during production, speeded up by wet piling processing (for which the question might be if it's most fermentation occurring initially, or all).

Which is right?  The easy way to settle this is to go and buy two each of a number of cakes and store both separately, one of each kind sealed, the other not, and wait a number of years.  Of course results might vary by other factors, temperature and humidity, or by how often the cakes were taken out to sample some, or if they weren't at all.  Cwyn (author of Death by Tea, a funny and informative tea blog) cited an example of someone doing just that in one comment:


One of my tea friends specifically stored three samples of the same tea over a two year period, one in plastic, one in a pumidor [ed. tea version of humidor], and one with no storage (open air). Then he sent unmarked, blind samples of all three to several of us. We all picked the pumidor stored as the best one. The plastic sample in particular was the worst, in my opinion. 


Of course that would seem to depend on his local environment, and it seems possible results could vary by different factors.  Let's check some references about what fermentation is first, and move on to more informed input about these issues after that.

pu'er warehouse storage (photo credit, Seven Cups site)

Storing pu'er, fermentation:  research input

The standard-practice bias will make this harder to evaluate.  It would be possible to find sources advocating sealing pu'er for storage (a comment in that discussion mentioned the Hojo vendor advocates this) but more others will go with what they know, the other more standard approach.  A vote, the count of approach endorsed per reference, might not be a great indicator of likely accuracy.  All the same lets start with Hojo's input:

It is said that a pu-erh tea should be fermented in the original paper wrapping and stored in an open space. However, this method is not practical in most of our living environment. This traditional storage method only works in Yunnan or any place where it is located at an equivalently high altitude, as the weather in Yunnan is very dry with low humidity.

From the food scientific point of view, any kind of tea must be kept under low humidity. In fact, we highly recommend keeping pu-erh tea in a tightly sealed aluminum bag where the oxygen is trapped inside the bag providing sufficient condition to mediate further maturation compared to those that are kept in open space. Moreover, there is another way to create a unique maturation method for pu-erh by storing the raw pu-erh in a vacuum-sealed bag even though most people may think this storage method is ridiculous. 

Interesting that they squarely address going against conventional wisdom with those recommendations, clarifying that most people wouldn't agree.

A wiki-theme tea site or blogger research isn't likely to help out much, since those would either repeat one or more opinions or cover a bit of range in general research.  For example, the TeaDB post on pu'er storage does that, citing conventional wisdom and mentioning this Hojo post.  I'm not really intending a citation as a final starting point reference, but I'll also mention their input on humidity.  This is likely taken directly from Marshal N's "Tea Addict's Journal" posts, as mentioned by them in crediting him as a primary reference:


This storage method [traditional or wet storage] is usually associated with Hong Kong. Traditional storage involves packing a ton of tea, usually by the jian (42 cakes) into a warehouse. The tea will initially be put into a ground storage unit. These storage units will be intentionally (but not artificially) high in humidity and temperature. These conditions result in the quick aging of the tea. After some time has passed, the tea is moved to a drier storage location... 

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.


It makes you wonder what the relative humidity is in Hong Kong, doesn't it, since 70% sounds like a good bit (from Wikipedia):

Climate data for Hong Kong (Hong Kong Observatory), normals 1981–2010, extremes 1884–1939 and 1947–present
Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year

Average relative humidity(%)	74	80	82	83	83	82	81	81	78	73	71	69	78.0

70% and up; that is humid.  I won't be able to track down the reference but cycling between more and less humidity over time has been cited as a good thing, for the tea to sort of breathe.  That's also probably from Marshal N; I read a lot of his posts while writing this.

It makes me wonder about humidity where I live, Bangkok, since it often feels a bit humid here:

Yep, humid.  For better or worse it really doesn't ever ease up much.

Related to what fermentation really is, and how much air contact is required, most references don't get far.  They tend to focus more on wet versus dry storage definition and basics.  That Tea DB post does mention that Hojo is in the minority for advocating sealed storage, but then Hojo had said that too, and repeat that some claim sealing a tea will stop a tea from naturally aging.

Beyond blogs there are a few decently researched tea information sites out there, with one of those, the World of Tea, weighing in on pu'er storage factors:

Non-Hermetic Seal Aging:  Puer (and other post-fermented teas) are not typically shielded from moisture, rather a controlled level of moisture is employed to influence the aging during storage. In this case, over time, the tea leaves undergo a combination of fermentation AND oxidation. In fact, when maocha is fixed, it is often done so at a lower temperature so that the oxidative enzymes within the leaves are not fully denatured, clearly allowing for further oxidation.

This certainly seems to represent a suggestion not to seal pu'er teas for storage, related to allowing for contact with limited moisture, but it's not so clear if ziplock bag storage of teas that are opened from time to time violates this.  It's not clear from that what other factors come into play, but moisture is represented as the primary factor.  A distinction between oxidation and fermentation is interesting.

Maybe it could help to dig into what fermentation really is.  An interesting reference turned up in a search, a study of components found in types of pu'er, with some insight to what activity related to that:

One of our major findings is that fungal α-diversity is higher in fresh tea leaves than in Pu-erh and that bacterial α-diversity shows the opposite trend, that is, lower in fresh leaves than in Pu-erh...  An interesting finding is that the most sought-after tea, aged raw Pu-erh, has a fungal community more like ripened than young raw Pu-erh, and a similar trend was seen for the bacterial community (Fig 3). This result indicates that the accelerated microbial fermentation of ripened Pu-erh, encouraged by the addition of water and the warmth generated by microbial fermentation, results in a microbial community composition similar to that found in much older, raw Pu-erh. It also provides an ecological explanation for the rapid acceptance and widespread use of the ripened Pu-erh process.

Clear, right?  The rest of the article on methodology and detailed findings really does get a bit technical.  As I'm reading that they're saying both aged sheng pu'er and shou pu'er exhibit similar types of bacteria and fungus growth (which is a good thing, more or less).  There's nothing on storage, obviously, so no extension to what conditions support that.  One read of what that just said, "encouraged by the addition of water" might indicate wet storage supports fermentation but it doesn't seem possible to extend that statement to concerns about specific relative humidity conditions, to what is optimum or too much.

Their conclusion is interesting, especially where it leaves off related to a recommendation:

Next generation sequencing revealed high fungal and bacterial diversity in Pu-erh tea. Fungal diversity drops and bacterial diversity rises as a result of raw or ripened fermentation. The composition of microbial communities changes significantly among fresh leaves, raw and ripened Pu-erh with the aged raw tea having similar community to ripened tea. Age of tea is identified as a significant variable affecting microbial community of raw tea, but not of ripened tea. Multiple mycotoxins were detected from either or both categories of Pu-erh, but all but patulin and asperglaucide were under the safety limit. For safe drinking, we recommend discarding the first brew.

So much for drinking the rinse.  The rest of that part on mycotoxins was interesting, if a bit hard to follow.  

Clicking around for input on risks I found this WebMD reference to pu'er effects (positive in this case):

There is interest in using pu-erh tea for lowering cholesterol because, unlike other teas, it contains small amounts of a chemical called lovastatin. Lovastatin is a prescription medicine used for lowering cholesterol. Investigators think that bacteria that sometimes contaminate pu-erh tea may somehow make the lovastatin in the course of their normal life cycle. Animal research suggests that pu-erh tea might lower certain blood fats called triglycerides as well as total and “bad” low-density lipoprotein (LDL) cholesterol. It might also raise “good” high density lipoprotein (HDL) cholesterol.

And I was just explaining to someone that essentially all tea bloggers are very skeptical of all health claims related to teas.  I'll stay a little skeptical but it sounds better than the other earlier content on mycotoxins, which can't be good.

Expert input on pu'er storage (well informed input, anyway)

I asked a few people that would have informed opinions on this, with some of that cited here.  I'll start with a local tea friend, a tea drinker that lives in Bangkok and stores pu'er here.  He makes no claims related to being an expert, but he does have significant first hand experience.


I keep all my cakes in ziplok bags. Some use vacuum sealing (like Hojo). For me, the exposure to air can increase the oxidation level, which diminishes flavor and aroma. Fermentation is another matter. According to Hojo, when the cake is pressed, the fermentation process is already under way. By keeping the cake enclosed or vacuum sealed, there is still enough moisture to keep the fermentation process going.

The main point is about humidity and aging. With high humidity, there can develop a wetness that will make the cakes taste and smell musty. The HK Chinese like probiotic :-) foods that stink. So do the Taiwanese. One could say it's a developed taste. Personally, I don't like it. Maybe the cakes age faster exposed to high humidity without ziplocking. I'm happy with the cakes I have and how they are progressing. I've tasted teas from Wistaria Tea House in Taiwan and didn't like them. I would never buy these musty cakes. But, now we get into taste and what you are taught to like. Who is to say what is right? As long as your cakes are not developing mold, they should be safe. 


Interesting, right?  Issues of personal preference and local climate fold into all that, tying together a lot of the previous ideas.  I'm still curious about how much oxygen content is necessary for a pu'er to age, and how humidity really does relate, but some reading up and discussion will only go so far.

or you could just leave it lay around (credit Tea and Mountain Journals)

Another online friend strikes me as more of an expert reference, not in terms of public record background credentials, but related to regularly passing on input that others have referred to as "encyclopedic."  Of course I'd need to extrapolate a lot from what little I do know to evaluate the relative value of the input, but as far as that indicates anything he's someone to listen to.  It might seem odd accepting ideas from a source without even an online nickname to go on (which is all that is accessible anyway); take that part as you will.


1) Completely sealing the bag limits microbrial processes to anaerobic stuff. That can mean some fruitiness, and it can also mean sourness, depending on the metabolic pathways of our micro friends.

2) Loose package sealing is nice to reduce the loss of volatiles, not really for better aging process.

3) Stagnant humidity promotes way too much activity, leading to toxic accumulation of molds.

4) Puerh does oxidize, but the processes involved in making puerh is meant to reduce such things. Small leaf puerh tends to be more prone to oxidizing to black tea. Browning is fermentation from microbes, like what see in cheese.


And this seems more or less off the top of the head input; he really should write a book about tea.  Mind you he isn't even someone I've spoke to much, just one more person I bother with questions, who is kind enough to humor them.

That last point has lots of room for following up--I'm not so clear on how oxidation and browning vary by input--but this is running long as it is.  I won't really get into all that's covered in those points, most of which are clear on their own.  The first point seems most interesting, delving a little into the variable introduced in that academic journal article, extending it to potential flavors changes.

I discussed the subject a little with Cwyn by message, that other blogger I'd mentioned, and she said this:


For storing sheng in sealed plastic, Hojo is the primary proponent of this method of storage. It works only in humid climates, the idea is to seal out the outside air and prevent the tea from too wet storage. Sealed plastic still leaches air. In dry climates, the tea will actually just dry out slowly and fade. I have one cake preserved this way, mainly because the cake has a large intact leaf across the top. Makes a good demonstration cake that people can hold in their hands without affecting the tea. Otherwise, sealed plastic is not an effective method for my dry household climate. If your climate is humid, it is worth considering.



So sealing the tea really may be a great alternative for pu'er storage in Bangkok, which is humid, depending on preference for fermentation and other results.  Or maybe sealing those teas, even in looser ziplock storage bag format, does all but stop fermentation, to an extent that per most individual preference sealing the pu'er would be an unacceptable solution.  Personal preference related to all the variables is a difficult aspect to fully factor in.

Altogether all those ideas did all fall together better than I'd expected.  The inputs settled into one clear running theme of people saying similar things and arriving at different conclusions that weren't quite as clearly contradictory as they seemed at first.