Storage experiment: 2016 W2T Bosch, one year in

// Published August 4, 2018 by mgualt

No topic in the puerh tea world is fraught with more controversy and misinformation than that of storage. Each collector must decide which storage system they will use for their tea, hoping that it won’t be ruined by the time they actually get around to drinking it. Some of us are more ambitious (or delusional!), with plans that our 2009 Dayi 7542 will, before we pop our clogs, transform into a sublime masterpiece rivaling the legendary teas of yesteryear.   Collectors with a substantial investment in their tea collection are often very touchy on the topic of storage, and in the unlikely event that they are willing to discuss it openly, will defend their chosen method… usually using self-contradictory arguments… to the death.

My experimental system:

For me, the main goals of puerh storage are

  1. to maintain as much as possible of the aspects I enjoy, such as pungency, resinous content, sweetness/juiciness, complexity, and energy effects,
  2. to reduce as much as possible the harsh bitterness, greenness, and astringency which young tea can have, and, potentially,
  3. to develop in new unexpected ways, with emergent flavours and aromas as well as deepening energetic effects.

One year ago, in an attempt to achieve these goals, I started testing a storage system which would also fulfill the following criteria:

  • Most importantly, no maintenance required, so that during long absences nothing need be done;
  • Minimize risk of damage, for example by mold, pests, drying, etc.
  • Maintaining individuality of teas, no mixing of aromas/tastes
  • Cheap

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.  Note that the cooler serves only as insulation – this helps stabilize the temperature and saves a lot of energy.  I also put a spacer between the mat and the mylar bags, so that the bags aren’t in direct contact with the mat.

What are the results?

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 tasting: 4g each in two identical 60ml gaiwans, with one drinking partner.

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.  In the photos below, the heated tea is on the left.

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.

  1. Sweetness on the left, slightly thicker, much less grassy. On R, green, grassy, medium thickness, oolong-like as in my initial tastings for Bosch 2016. Menthol on both teas.
  2. L is noticeably thicker and sweeter than R.
  3. With a 30s steep, L is remarkably sweeter and thicker than R, and R is quite bitter and green. The difference is quite remarkable. The sweetness on L is not only in the arrival but also in the finish.
  4. Testing a 45s steep, both are quite strong. L is softer, sweeter, less green.  R is starting to show quite some harshness and greenness.  L has some harshness too but the sweetness comes as well.
  5. R is very much not drinkable at high steep times whereas L has a thicker complexity with finishing sweetness which is pleasant and encouraging.

Heated Bosch 2016 on the left, unheated on the right

These results were quite a surprise to me; I was expecting the results to be much more subtle. Even visually, the tea has clearly aged more quickly.  The harsh grassiness has been much reduced and the sweet pungency has been enhanced. At this point, I am strongly encouraged to put more teas into the hotbox.  I also have another hotbox set at a higher temperature, and hope to report on that one in not too long a time.


  1. MattCha
    August 4, 2018 @ 3:15 am


    I would like to order 4 of these. I will remit my payment via PayPal tonight. Thanks again.


    • mgualt
      August 7, 2018 @ 5:37 pm

      We should do a Kickstarter, Matt!!

  2. nonen
    August 7, 2018 @ 3:03 pm

    How did you select 32C (89.6F)?
    Ninety degrees is quite warm and well outside the “standard range” of temps we often see recommended in Western storage discussion. Did you have any fears of mold growth at that temp?

    • mgualt
      August 7, 2018 @ 5:32 pm

      “How did you select 32C (89.6F)?”

      I selected 31-33C and 39-41C, although I haven’t said anything about the second box, yet. 40C is approximately where the tea enzymes have maximum activity. 32C is a lot more practical to maintain. Also, 32C is simulation of hot temperatures in Malaysia. You do know about Malaysian storage, yes?

      “Ninety degrees is quite warm and well outside the “standard range” of temps we often see recommended in Western storage discussion.”

      I assume by “Western” you mean in English? I actually haven’t encountered that many temperature recommendations, and what I have seen is mostly about mimicking (usually only part of) the storage they wish they had. Finally, I am not making recommendations, I am telling you the result of a 1 year experiment. You can conclude for yourself :).

      “Did you have any fears of mold growth at that temp?”

      Of course, I was trying to avoid mold growth at any temp. You may be under the misconception that mold grows better at higher temp… it’s not the case. If you are at 66% RH, then 32C is extremely safe, but 23C (room temperature) is definitely not safe from mold growth. Interestingly, if you are in the low 70s of RH, room temp is *extremely unsafe* whereas at 40C it is very unlikely (almost impossible) that mold will grow.

      In this experiment, I keep humidity in the mid 60s with *no added humidity source* and at 32C I don’t get any mold growing (with one year running). This is consistent with the data that mold prefers 70-80RH conditions; indeed, at 70%RH or at 75%RH I have been able to grow mold, both at 23C and at 32C. Note also that it is not always simple to tell if mold is growing or not — it may take many months of growth before you can actually see evidence of mold.

      A last comment on English-language storage discussion online: there tends to be a lot of discussion of methods but not much discussion of goals/criteria of storage. In the post you will see my personal criteria, not everyone will share those.

      • nonen
        August 7, 2018 @ 6:33 pm

        Thanks for your informative reply. By Western I mean people living in Europe and the Americas, who by and large are not in warm & humid climates. The parameters I’ve seen discussed are usually around 65-75% RH and 70-80F. If I understand you, this could actually be more dangerous than 65% RH, 90F storage. No I am not familiar with typical Malaysian storage parameters.

        Another thing I wonder about — given the high temperature differential between your warmed cakes and room temp: does this mean the warm-stored cakes are not really available for intermittent drinking? Specifically, is there any reason for concern about condensation that may form on the tea due to those temperature swings, should you pull one out and wish to drink it?


        • mgualt
          August 7, 2018 @ 9:29 pm

          About geography and storage: I don’t think it’s a reliable indicator. People in Southeast Asia love AC just as much as the humid Floridians or the Sweltering Swedes. It’s all about the exact conditions of storage. Nowadays I don’t think you can assume anything.

          As for temp and RH range, you understand me perfectly. At room temperature, 75% is comfortably in the mold growth zone. And, as many have pointed out, room temp is cold for puerh storage.

          As for actually drinking the tea, I have my own system for breaking chunks off cakes and storing or “airing out” small quantities and I don’t disturb the tea in my long term storage every day. Condensation would happen if you took a cake out of the hotbox and left it outside without unzipping the bag… So don’t do that.

  3. Tthx
    August 8, 2018 @ 2:25 am

    Which Coleman cooler did you use? (48qt?) If it has a drain, were you able to route wiring through that? Cool experiment!

    • mgualt
      August 8, 2018 @ 10:56 am

      Any insulated box will do; that size should work as mine is a bit larger. I didn’t bother with wiring through the spout, I just went through the lid.

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  5. Toby
    August 23, 2018 @ 6:32 pm

    I’d been thinking of doing something similar for a while, definitely more inspired now.

    One question though: did you heat seal the Mylar bags? Or just use the Ziploc bit? I expect the former would maintain the humidity better, but wouldn’t allow any air exchange (do pu’er microbes need oxygen?) While the latter would allow some minor amount of air exchange but also possible loss of humidity.

    • mgualt
      August 23, 2018 @ 6:54 pm

      Hi Toby, I only used the zip seal on my mylar bags. The seal is excellent, and doesn’t let any humidity escape over a year timescale.

      The rest of your comment is interesting because I often hear puerh-heads say such things. They imagine that something in the bag needs to “breathe” through some tiny holes, and on the other hand they hope that they won’t lose humidity through said holes. If you dig any deeper into this thought process, you may find the subject changes pretty quickly.

      My opinion is that action of fungi and bacteria enzymes are slightly exaggerated in the press. They play a part in aging, of course, but for natural dry aging of sheng, a very important role is played by the enzymes inherent in puerh tea. It depends what kind of storage you want to give the tea.

      In any case, if you want to grow mold in a sealed mylar bag, it’s easy… If you increase humidity and add a couple drops of water, seal the bag and you will rapidly have a mold colony.

      • Fred
        August 24, 2018 @ 3:01 am

        Toby’s question about heat sealing vs ziplock-only is one I was wondering about as well. Oxygen is required for enzymatic browning of the tea to occur. How much? I don’t know. But a heat sealed Mylar bag will create a zero-O2-flux environment, whereas a merely zip locked one will allow some O2 transfer.

        Again, the question of whether “heat sealed vs zip only” makes a difference with respect to the oxygen required for aging comes down to the specific quantities and rates involved. It may make no difference over the course of a single year. But on longer timescales it could, if all the O2 had been consumed by oxidative processes over time, and wasn’t replenished by slow influx through the ziplock. Again it depends on how much was sealed inside, and the rate of consumption. I should look into how to estimate that latter figure.

        • mgualt
          August 24, 2018 @ 2:23 pm

          Thanks for your comment. There is O2 transmission even with heat seal, and I don’t find the heat seal vs zip seal question very compelling, but I think we would agree that there is an important problem here, which is to estimate the oxygen requirements of puerh tea for aging. I think we lack even an order of magnitude estimate at this point.

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  7. TONY
    October 11, 2018 @ 3:20 pm

    How much extra money do you have to spend for the electricity monthly?

    • mgualt
      October 11, 2018 @ 5:15 pm

      Because of the insulation and the heating mat, it is the same as having a 14W light bulb on for about 4hr a day. In other words, it is microscopic.
      Best wishes!

  8. TONY
    October 12, 2018 @ 2:38 pm

    Thank you for your reply Mgualt!
    Will it cause any problems if putting the a couple of cakes with the original paper warp in the mylar bag?
    Will the mold grow inside the bag?

    • mgualt
      October 12, 2018 @ 7:02 pm

      Paper wrap is not a problem. As long as humidity is not too high for the chosen temperature, and it is insulated with no additional moisture, mold should not grow.

  9. TONY
    October 12, 2018 @ 2:41 pm

    One more thing, how often do you recondition the cake during the hotbox process in a year?

    • mgualt
      October 12, 2018 @ 6:59 pm

      Hi Tony, it is not necessary to do it more than once, since it does not lose humidity if the bag is a good one.

  10. Jeff Cornelius
    December 3, 2018 @ 2:11 pm

    Hi Marco. What kind of spacer do you use and how much space is necessary above the seedling mat?

  11. jan
    December 18, 2018 @ 7:50 am

    trying out this experiment ive found that the seedling mat has a very strong (new) plastic smell. doesnt that negatively interfere with the puerh storage given its with the cakes in so small & enclosed space?
    and do you think that taking the cakes out to break pieces for regular usage really alters/interferes negatively the aging?
    i also wondered whether one could keep several cakes stored tightly together in a well-closed plastic storage foodbox and put the whole box in the warm cooler instead of wrapping them individually?
    id appreciate your opinion on this!
    greetings an many thanks for the great idea!

    • mgualt
      December 18, 2018 @ 9:51 am

      Hi Jan,
      The tea is in a sealed mylar, and won’t be affected by smells in the vicinity. The only thing you have to make sure of is that the interior of the mylar doesn’t smell and that the seal is intact.

      I don’t think that small interruptions will have a major impact on aging.

      Finally, the mylar bags I use can be large enough to store entire tongs, pairs of cakes, or even multiple tongs. Any odorless sealed container would work. The thing is that normal Tupperware or Ziploc bags do not have a good seal. The only things I’m aware of that have a truly good seal are heat sealed mylar, latex seal stainless steel canisters or mason jars.

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