Humidity, relative and absolute (pu'erh storage)
When someone mentions humidity in tea discussions (mostly related to storage of pu’erh, of course) they always seem to use relative humidity (a percentage of how much moisture the air can contain at a certain temperature and air pressure, as I understand it). I consistently see people refer to humidity as a simple percentage but as far as I can remember I never saw anyone talk absolute humidity (kg/m3).
Let me give you an example:
Here in Stockholm right now the relative humidity is 80% at 7 degrees C and 998 mm Hg pressure. That makes the absolute humidity 0.006 kg/m3.
In Hong Kong today the relative humidity is “only” 73% but at 24 degrees C and 1014 mm Hg pressure which works out to an absolute humidity of 0.016 kg/m3.
I used an online calculator to get these values.
So if I was comparing humidity with someone in HK using only relative values it would seem like my tea is stored “wetter” than his, while in actuality his tea is stored at almost three times the humidity mine is.
My thought here is: should we not be talking about absolute humidity instead? Surely what affects the tea must be the absolute amount of water in the air, not relative humidity?
Please educate me if I am missing something here.
great observation, thanks. Nothing to add but very interested in any answers that come from this thread
Yes, temperature is often ignored in online discussions. Not everyone who is seeking to store pu erh understands relative humidity and its importance.
Of course those figures don’t reflect indoor storage conditions; my home is warmer and drier than outside, and yours probably is too!
I know the humidity in my home is not all that high but I have found that it is high enough for my ripe. I have gone back and drank ripe stored here for two or three years and it has been fine. I have not done a similar test with my sheng stored outside my pumidor. I suspect sheng is more susceptible to low humidity.
IMO sheng and shu are equally susceptible to poor storage conditions. A poorly stored ripe is really unpleasant. A well stored one is all kinds of sweet and date/jujube
Like i said, I have done a taste test on my ripe and it is fine. I do not know how it will do over the long term as I have only been into puerh for four or five years. But ripe puerh I bought three years ago tastes just as good as when I got it.
The largest amount of my tea comes from Yunnan Sourcing by far, but I have also ordered from Puerhshop, Crimson Lotus Tea, White2Tea, Berylleb King tea, Chawangshop and a variety of other places. Most tea that I have bought by the tong has come from Yunnan Sourcing.
If my tea wasn’t changing for the better over time (3-5 years) then I’d think conditions weren’t appropriate. I know my shou is doing well when my whole room smells sweet from it!
The conditions are what I am stuck with. I suppose I could buy a humidifier for my apartment but I really don’t know what that would do for my asthma. It’s not worth it if my puerh improves and my asthma worsens. Although for all I know high humidity might not have an effect. The conditions of my storage are humid summers and less humidity the rest of the year. It can get really humid in the summers around here.
Temperature is the other factor. Summers here get into the 90s. Winter temps are much cooler and below 25 Celsius, the sweet aromas disappear. It’s still warm right now and with global warming, it’s possible we might not have 7-10 degree C temps at all. Oddly the cool winters don’t come every year now like they used to.
As far as temperature goes it is around 70 degrees most of the year but that is because I have central heat/air. I know this is not ideal for tea but I need the heat to be on in the winter.
“Of course those figures don’t reflect indoor storage conditions”
Sorry I missed this and it took me a while to respond. Sure, I was just illustrating the point. That said indoor conditions still vary (both temp and humidity) from the outdoor conditions in some cases and in other cases (storage in an “open” facility in warmer climates) not. Any way you slice it, absolute humidity seem to be the factor we should be worrying about whether it’s a pumidor in your semi-heated garage, an open warehouse in Hong Kong or an evenly heated apartment in Canada.
Also, indoor relative humidity does not automatically reflect outdoor humidity due to ventilation, air conditioning and so on.
Jay, would your entire room smell from shou even if it were stored in jars? And how much tea are we talking about here…
I wouldn’t use jars myself and if I did, I don’t think there would be any smell. My tiny HK bedroom has maybe 15lbs of shou in it. On a good day I can smell the shou outside the room with the door open.
My warehouse is split into two areas. The pu erh area only has maybe 70lbs of shou. Same deal, I can smell it when I walk into the area on a good day. I hope to get a dedicated warehouse exclusively for pu one day!
Also I accidentally left a shou cake sandwiched between sheng for 18 months. No effect whatsoever
People normally mention a target temperature along with the target humidity. (You’re right that they don’t mention air pressure though!)
(And I think OolongOwl linked this in a related discussion, which is kinda handy: http://www.dpcalc.org/)
And if you look at the “Natural Aging” decay type on there, the rating for your conditions is three times higher than for Hong Kong.
“People normally mention a target temperature along with the target humidity”
Then I suppose our experience differ because I don’t see that as normal, I see mentioning of temperature as rare.
Here’s a quick example: in this thread 4 people mentions humidity and only one also provides temperature.
Admittedly the pressure doesn’t make a big difference so that can probably be omitted. So disregarding air pressure, relative humidity + temp is the same as absolute humidity but the calculating is pushed onto the reader. My point was, maybe we should skip that step and just talk about absolute values?
Ok – I’ve not been around that long and have seen it a lot, but it has been in posts specifically about long-term aging of puerh (not necessarily on this forum, though I’m pretty sure some of them have been!)
I think it’s mostly because hygrometers show relative humidity and temperature, so it’s easy to just take readings off them if you use those values. The absolute value is harder to relate back to local conditions. (And, as said below, the same absolute humidity probably isn’t going to have the same effect on your tea if the temperature is very low).
I take your point about it being easier to post relative values. However I believe it is a source of confusion since I so often see relative percentages posted without temp.
I ran through a bit on absolute versus relative humidity in a post awhile back (in a post on compressed white tea; just when I happened to get around to it). A psychrometric chart clarifies how the air can hold vastly different amounts of water at different temperatures, so relative humidity is actually a proportion, as others have said.
In a sense it would be too simple only talk about the absolute amount of water in the air—and the notation format itself is strange—because the main concern is really an optimum environment for the bacteria and fungus responsible for fermentation, but not so optimum that mold also thrives, and temperature is a main factor along with humidity related to that. I suppose it’s relevant background that I’m an engineer (industrial, with my part of my current work relating to cooling issues in data center environments).
Mold is a layman’s term for fungus. You want some fungal activity, but not too much and not the really scary species
Right, you want the kind that doesn’t look like much that makes your tea tasty, not the kind growing as white fuzz that makes you sick. I’ve mentioned this before on Steepster but this research article describes testing identifying the good kind: http://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0157847
Excess humidity in storage can cause some serious mycotoxins to build up too. No bueno
This paper doesn’t go into storage condition issues (as the other doesn’t) but it does review measured results related to mycotoxins in pu’er. They’re there, in some versions, even in significant amounts: http://www.lifescienceglobal.com/pms/index.php/jbas/article/viewFile/2217/1318
That’s the paper I was thinking of too! Thanks for saving me the trouble of looking for it :) That paper gave me quite the scare a little while ago
“clarifies how the air can hold vastly different amounts of water at different temperatures, so relative humidity is actually a proportion”
Yeah but so what? The relative value is still not interesting except as a datapoint to calculate the absolute value which is still what you are talking about. That air can hold “vastly different amounts of water at different temperatures” is exactly my point. That’s why relative humidity means nothing in itself.
Unless someone can come up with a compelling argument why I am wrong about that assumption.
It’s 23 Celsius in HK now, and 86% humidity. I broke bits off several cakes today as samples. At the peak of summer, I can just snap bits clean off cakes! Right now, I’m getting a lot more dust than I would at a higher temperature. It’s making breaking bits off for samples a real chore!
Interesting. So what would be a proper range for puerh in absolute humidity?
This discussion went off in different directions (and that’s fine) but to bring it back to the point I was originally making:
If we all agree that relative humidity is irrelevant and that absolute humidity is the only thing that matters for tea (do we?)… should we not stop talking about “relative plus temp” in order to avoid confusion and try to go for absolute values in discussions?
And again: I can not see that relative humidity have any value except as a datapoint for calculating absolute humidity. Yes, it is easier to post rel + temp, but in order for the data to have ANY value to anyone someone have to calculate the absolute humidity at some point. If I am wrong about this, please convince me but so far I have not heard one argument that counters this. I still think that this is confusing for many people and that the relative percentage is seen as informative in itself which (if we agree to the above) it’s not.
Yes, that is the interesting question.
I wouldn’t be able to resolve this, but these considerations did lead me to read up a bit on some classic discussion of pu’er storage concerns, so I’ll pass on those references and some thoughts.
Related to temperature as an issue, in general it is referenced that pu’er should only be stored within a moderate range of temperatures. It’s a problem that there really isn’t one definitive, well-agreed guideline for any of this, a range of absolute optimum and also what lies beyond that as acceptable. In a Tea Addict’s Journal article (written by an author regarded as one authority on this topic, who is familiar with other inputs) he cites a range of 20 to 30 C as being typical / acceptable (70 to 85 F, roughly). Although he doesn’t state a clear humidity range in the blog article (which I’ll link to later) for dry and wet storage optimums but we could rough that out in a typical range of 60 to 80 % RH; much below that and tea won’t age, just above that it may well mold, and towards the top and bottom level wet and dry storage are represented. Mind you I’m not intending this as a last word; I mention specific values for the sake of further discussion, not final definition.
Of course the premise here was that RH means nothing, that between 20 and 30 C (65 and 80 F) the absolute humidity (grams / kilogram of dry air) shifts based on both temperature and RH. There is a psychrometric chart in the blog link I mentioned (mine), and to read from that at 20 C and 60% RH there is around 8 grams water / kg dry air, and at 30 the same RH contains around 16, twice as much (all for sea level pressure, the other variable). Standard forms of discussions would lead us to believe that bacteria and fungus (the biological components causing fermentation; but only the “good” or desired versions of these) would thrive comparatively well under both circumstances, with twice the amount of water—in absolute terms—in the latter case.
I’m not sure that’s actually being implied, though. My take is that an envelope of optimum range is being described between those parameters, which actually varies from that 8 gm water content / kg dry air (at 20C and 60%) up to 25 gm / kg at 30 C at 80%. Just beyond that for moisture level is too wet (tea could mold), much below that is too dry (fermentation would stop). Per my understanding these functional ranges are derived from observing natural temperature and humidity ranges in natural and controlled environments where pu’er aging has been successful. Identifying one optimum where the related bacteria and fungus thrive is a different type of project, but then per some input (not research, from discussion, or hearsay, to spin it negatively) periodic natural environment variation is sometimes considered positive, or maybe even optimum.
All of this is still just a bit vague, isn’t it? The right kind of biologist could move off tea-related research and extrapolate more general micro-biological optimum environment parameter research to say more. Any theoretical optimums would still need to be filtered related to experienced tea-storage practice, since the idea that enabling the most growth of the right bacteria and fungus isn’t necessarily going to work well. We are also concerned with limiting growth of fungus that doesn’t taste good, and other optimum characteristics, which is all going to need to be judged subjectively in the end (tasted, not lab-tested, or both, at a minimum).
If anyone wants to read further I’ll pass on some articles. One I just mentioned, and a forum discussion (with varying input), and a series based on authority input from a reference blog series (Cha Dao; with the Tea Addict’s Journal author contributing to that), and a research paper I cited in a post about storage conditions measuring what components are responsible for pu’er aging.
Related to the central point here note that if we really did want to move to discussing absolute moisture content in air instead (stated in grams of water in kg of dry air, or in dew point temperature levels, how that is phrased in meteorology) we couldn’t work from the same terms used in all those discussion, which is relative humidity. Intuitively related micro-organisms would really have their own optimum range for thriving, and it’s conceivable that implied window I mentioned might be adjusted in the future based on better understanding of that. Actual environment control for that might be problematic; it could depend on control measures used in storage conditions, or if environment variation really is optimum, or could inter-relate with issues like air-flow.
This discussion was so interesting I converted it into a blog post, to distribute the ideas further. I added a review of natural storage conditions (local climate summary) in Hong Kong, Malaysia, and Kunming, and went through the psychrometric graphs to see what difference it really does make in expressing humidity as absolute (amount of moisture as a proportion to air weight, gm / kg) versus relative (% of moisture related to what the air will hold at that temperature). It’s not as if either really solves anything but those details fill in more of the story.
That was a very interesting blog post. I do think there is one thing where we may all be wrong about this. We are assuming that the bacteria cannot survive below a certain humidity level. Bacteria have been shown to survive in many extremes of climate so who says that bacteria cannot survive in the natural humidity of NY. The bacteria that make people sick certainly do or hospitals in NY would have a lot less business.
In the end we would only need to be concerned with effects, with how fermentation results taste and feel, but that is a main gap. I would assume that different bacteria and fungus thrive across different ranges but I really don’t know anything about the subject. It was only included as a brief mention but hearsay about specific effects of letting tea get hot, cold, dry, or wet are interesting for a similar reason.
Good post. it got me thinking more about the relative vs absolute humidity. Chemical reactions tend to go faster at higher temperatures. i would think this also applies to the microbiological processes involved in Puerh aging. If so, then perhaps you don’t need as much moisture, since the temperature is more conducive to the action. This would argue that relative humidity might be an appropriate measure.
Of course you could look at it from the other direction, and say that if the reactions are faster, then they should have more humidity to feed the microbes.
I know next to nothing about biology on that scale but I’d expect there to be a relatively optimum narrow temperature range for where those organisms thrive and a broader range for where they can just exist, I suppose with the same relating to humidity. I don’t see any reason for why that would have to be exactly the same for all bacteria and fungus types. Someone recently passed on an interesting article on mold growth conditions (the fungus in fermenting pu’er is a different variety, but to some extent the same conditions would apply) and a few conclusions stand out from that. Mold can thrive at different relative humidity in different growth mediums, but according to their conclusions the real factor is water availability in that material, which varies. Of course nutrients and other suppressants (fungicides) are a factor, but per their analysis percentage water in the growth material is the main one.
Many enzymes peak at around 36-37 Celsius…above that and they start to denature. Below that and they slow down. The peak varies from enzyme to enzyme and bacteria and fungi also have ideal ranges. Water is the source of life, of course, so humidity is crucial for everything to get cooking. Many species of bacteria and fungi hibernate when conditions are poor, and get back to business when things are within range again.