Controlling microbes in your Pu-erh cave.

[mr mopu]

Between 7% and 12% of leaf water content,

Thanks much for this. Can you say more about leaf water content and how it is measured? How is this related to relative humidity? I have heard people mention leaf water content thresholds in the context of the end product of manufacturing (or re-roasting) but I would have a hard time knowing how to create a storage environment based on those criteria. (Me, I just aim for 75% relative humidity....) Ox, what're your humidity levels?

I shoot for 70%

[BioHorn]

I usually run 68-72 and am shooting for 75. There is a big difference in the boquet and strength of tea smell between 69-72. I think 75 is a great number for my conditions.

[kyarazen]

100% humidity is around 15-17 grams of water per metre cube (1000L)

tea is hydroscopic, there's an optimal amount of water that it can bind up and extract from the air. i.e. oolong tea dried to 3.5-4% water content, if subjected to simply 60-80% humidity for a couple of days will pick up water to around 12% content

[Drax]

That change in weight should actually be measurable, yes? 100g of tea with 5% moisture that increases to 12% moisture would show a 6-7g increase in weight (to ~106-107g).

I also aim to keep my pumidor ~70 percent.

However.

I noticed this winter that my pumidor was having more trouble than normal maintaining humidity. I have a digital system that measures the relative humidity and then turns fans (over reservoirs of water) on or off.

I've had this sys tem operating over 7 years, and I realized I hadn't ever re-calibrated it. And I wondered if it had drifted a lot, and that's why it was having such a hard time (e.g., trying to get to a much higher humidity than it had to). So a couple months ago, I bought a hygrometer, calibrated it with a saline pack, and then stuck it in the pumidor. Sure enough, the calibrated hygrometer was reading about 5 percent RH higher than my digital one. So I re-calibrated the digital one. That means, though, that my pumidor had really been riding a bit higher than 70 percent RH over the last few years... (still no problems with mold, though).

[xiaobai]

Yeah, I also do not buy into the theory of 12% moisture after a few days of exposure at > 60% RH: I recently bought a 15 year-old (rolled) Oolong from Lishan. It is not heavily roasted at all (compared to traditional TGY, for instance). Thus, I guess the moisture level in the leaves must have been high (say, %4 to %5 or more) from the start.

Initially the tea was kept in the farmer's house, which must be, I presume, at high altitude and low RH. However, the last couple of years it was kept in a store at Sea level, in a rather warm and humid place of Taiwan, stored in a ceramic (purion-like) jar with a quite loosely fitting lid.

Had the leaves reached 12% moisture or more after a few days of being brought down from the Mountain, the tea would have been spoiled by mold. Yet, in its current state, it shows no indication of being spoiled at all. Upon steeping, it shows also no off tastes. Quite to the contrary, it is very clean, sweet, and fruity. A smooth version of what it must have tasted originally

I have seen many examples of Oolongs preserved like this in Taiwan. I never found them to taste musty or to exhibit any of the off-notes characteristic of wet stored puerh teas.

[kyarazen]

yes it is measurable! if you dont want to waste tea and have a weighing scale that goes to 0.01g you can conduct the experiment with a few grams of tea. weigh out your preferred quantity, transfer it onto a plate and gently roast/toast it until they sound reasonably "crispy". weigh the crispy leaves to see how much water you have lost. then you can leave the leaves by a window or in a humid place, for a few days, and weigh it again to see how much it picks up.

at least where i stay, many exposed teas of any sort, pu-erh, oolong, end up to be around 11-12% water content. well packed yancha (4% water) is easily ruined by a short period of open air exposure

try it!

That change in weight should actually be measurable, yes? 100g of tea with 5% moisture that increases to 12% moisture would show a 6-7g increase in weight (to ~106-107g).

I also aim to keep my pumidor ~70 percent.

However.

I noticed this winter that my pumidor was having more trouble than normal maintaining humidity. I have a digital system that measures the relative humidity and then turns fans (over reservoirs of water) on or off.

I've had this sys tem operating over 7 years, and I realized I hadn't ever re-calibrated it. And I wondered if it had drifted a lot, and that's why it was having such a hard time (e.g., trying to get to a much higher humidity than it had to). So a couple months ago, I bought a hygrometer, calibrated it with a saline pack, and then stuck it in the pumidor. Sure enough, the calibrated hygrometer was reading about 5 percent RH higher than my digital one. So I re-calibrated the digital one. That means, though, that my pumidor had really been riding a bit higher than 70 percent RH over the last few years... (still no problems with mold, though).

[Evan Draper]

yes it is measurable! if you dont want to waste tea and have a weighing scale that goes to 0.01g you can conduct the experiment with a few grams of tea. weigh out your preferred quantity, transfer it onto a plate and gently roast/toast it until they sound reasonably "crispy". weigh the crispy leaves to see how much water you have lost. then you can leave the leaves by a window or in a humid place, for a few days, and weigh it again to see how much it picks up.

at least where i stay, many exposed teas of any sort, pu-erh, oolong, end up to be around 11-12% water content. well packed yancha (4% water) is easily ruined by a short period of open air exposure

So it seems you are assuming
1) that roasting the leaves as you describe takes them to 0% water content (or some other reliable benchmark?)
2) there is not another factor in this process that would cause them to lose or gain any significant mass
3) at a constant rate of humidity, the tea's water content and mass will stabilize "in a few days"
4) a tea roasted in this fashion retains the same water absorption properties as the portion you didn't roast in this fashion

I'm not saying you're wrong; I just want to understand the mechanisms at work here. I'm also imagining that you would have to repeat this experiment with every tea at issue, because compression, leaf style, roast, and other variables could affect water absorption.

[kyarazen]

1) of course there could be some loss attributed to the volatiles, i.e. tea oils and all that, but since tea oils are in such low content in the leaf it could be negligible. "Loss on drying" moisture analysis is routinely done in many test labs for foods and QC etc. i think akira hojo does such measurements too.

my own measurements on a sartorius instrument (using 5g) seem to suggest that 11-12% water content measured by this method, is comfortable for sheng pu-erh tea without going moldy. my moldy point is somewhere around 14%. i steamed some tea leaves to mimic the pre-compression steaming process, and the soft leaves were around 16+% water content.

3) it should! unless it is compressed, that will need way more time. some highly compressed cakes can have very dry and very anoxic interiors.

i think humidity is just a guide, the more important factor is how much water the tea leaves have in them during the aging process. they have to absorb water from the moist/damp air

So you are assuming
1) that roasting the leaves as you describe takes them to 0% water content
2) there is not another factor in this process that would cause them to lose or gain any significant mass
3) at a constant rate of humidity, the tea's water content and mass will stabilize "in a few days"
I'm not saying you're wrong; I just want to understand the mechanisms at work here. I'm also imagining that you would have to repeat this experiment with every tea at issue, because compression, leaf style, roast, and other variables could affect water absorption.

[BioHorn]

1) of course there could be some loss attributed to the volatiles, i.e. tea oils and all that, but since tea oils are in such low content in the leaf it could be negligible. "Loss on drying" moisture analysis is routinely done in many test labs for foods and QC etc. i think akira hojo does such measurements too.

my own measurements on a sartorius instrument (using 5g) seem to suggest that 11-12% water content measured by this method, is comfortable for sheng pu-erh tea without going moldy. my moldy point is somewhere around 14%. i steamed some tea leaves to mimic the pre-compression steaming process, and the soft leaves were around 16+% water content.

3) it should! unless it is compressed, that will need way more time. some highly compressed cakes can have very dry and very anoxic interiors.

i think humidity is just a guide, the more important factor is how much water the tea leaves have in them during the aging process. they have to absorb water from the moist/damp air

So you are assuming
1) that roasting the leaves as you describe takes them to 0% water content
2) there is not another factor in this process that would cause them to lose or gain any significant mass
3) at a constant rate of humidity, the tea's water content and mass will stabilize "in a few days"
I'm not saying you're wrong; I just want to understand the mechanisms at work here. I'm also imagining that you would have to repeat this experiment with every tea at issue, because compression, leaf style, roast, and other variables could affect water absorption.

This is all fascinating and quite another way to think about tea health.
So a 70 pound pumidor would have about 7+ pounds of water! Imagine those Hong Kong warehouses!

[Drax]

That's why there's usually an equilibration period when adding new tea to the pumidor (or at least, from what I've noticed). That is, new tea is usually lower in water content (from its shipping trip), and the pumidor initially has to operate at a higher level as the new tea absorbs water and lowers the RH in the pumidor. Eventually the new tea reaches its equilibrium at the pumidor RH, and the pumidor doesn't have to turn on as frequently...

[BioHorn]

That's why there's usually an equilibration period when adding new tea to the pumidor (or at least, from what I've noticed). That is, new tea is usually lower in water content (from its shipping trip), and the pumidor initially has to operate at a higher level as the new tea absorbs water and lowers the RH in the pumidor. Eventually the new tea reaches its equilibrium at the pumidor RH, and the pumidor doesn't have to turn on as frequently...

Exactly! The other week I added a leather 2 cigar carrier to the pumidor (sometimes I add cigars to my pumidor.) It made a sudden and very large shift down by about 20%. I learned a lesson!

Bew bings really do take some time to equilibrate. In this pumidor maybe 2-3 weeks. I always try to give them 2-3 weeks rest after shipping before going into the pumidor.

[shah82]

Reading this, I am glad I do not need or want my puerh to be well-browned. I already have enough trouble keeping my tropical plants alive during the winter (and keeping the lemon outside alive as well, which is harder). Taking care of the pumidor on top of that? Geez.

Of course, it feels rewarding when I have grimal variety Jaboticabas (like a more delicate muscadine grape) to enjoy in the early spring!