”Spoiled“ Tea: What Moisture Does to Tea
Lanzhou Tea Shop - Various Sun-dried and Conventional Green Teas
These mounds of Yunnan green tea that you can find at Lanzhou tea markets (both inside and outside), remain one of the most shocking things I have beheld over the years. The conventional wisdom holds that tea, most especially green tea, will be irrevocably “spoiled” or “ruined” after just a few days of exposure to air and the invisible moisture that floats within it. However, many of the teas pictured above were fantastic in terms of flavor and aroma. To be sure, I have also experienced plenty of absolutely foul green tea from hawkers that sat exposed to the elements and aromas of Central China’s street markets. Yet, the fact remains that “spoiled” tea does not always taste bad.
Many years ago I remember seeing this post on Reddit from user Selderij:
“Today, I found some old middle-steamed (chuumushi) sencha from 2014 stored in an aluminum-lined bag with a grip seal.
I decided to brew the rest of it, and as you might guess, it wasn't like how it used to be when fresh. The leaves were more brownish in color, and they didn't have the aromatic and grassy smell anymore, and the taste had none of the usual grassy and vegetal notes either. Instead, the tea had developed into a warm-natured potion with haylike, woody, nutty and faintly tart properties, with most of the bitterness gone.
The tea had the same kind of satisfying and energizing effect in my head region as many other Japanese green teas, and it reaffirmed my experiences of green teas aging in rather nice ways if you don't hang on to expectations of keeping the first-year freshness that green tea is usually associated with.”
Nothing is wrong with this netizen’s tongue. This last Winter, I experienced something completely similar: a tin of green tea that had sat in our wooden tea cabin for two scorching hot monsoon summers. It fit the description above perfectly. This was a sweet and nutty tea could not be further from how it started out fresh from the factory. There were no foul mildewy, moldy, or fishy flavors. When we compared this aged green tea with one kept at room temperature in an office where the relative air moisture did not exceed 60%, the difference was night and day. The tea that had sat in that non-insulated cabin with air moisture regularly exceeding 80% for 20 months was darkly yellow, almost brown, and had lost any of the original astringency or crispness.
What Moisture Does to Finished Tea
Below, you can see two teas side-by-side. Both are Spring 2025 Orchid Buds, picked and processed by the same farm, then kept in an aluminum-lined resealable plastic bag at room temperature for five months. For the last two weeks, I have kept the tea on the right in that original bag, while the tea on the left has sat out on a napkin. Relative air moisture has sat around 60%. In this short course of time, the tea left out has already visibly yellowed both in dry leaf and soup color. It has taken on an astringent, fishy, mildew taste completely different to the softly vegetal and nutty tea that remained in the bag. Whereas the air moisture worked slowly on the semi-sealed tin of green tea at the cabin pictured above, complete and direct air moisture has had a much more devastating effect on the final product.
Even in this short course of time, several things can happen to a finished tea exposed to moisture.
Green Tea, With and Without Expose to Air Moisture
The most concerning possibility is mold growth. One of the earliest documented experiments with moisture exposure and finished tea dates to 1956. In Chengkou County (Chongqing), a finished tea with 7% moisture content was left out in a room with 80% relative air humidity for five days. By the end of the first day, the water content of the leaves had already reached 8.5%, and by the fifth day, when “some” mold spots were observed, the tea leaves’ moisture content had reached some 11%. The same experiment was then tried in Wanyuan County, with a relative air humidity of 90%, and the result was 12% leaf moisture by the fifth day and a “huge quantity” of mold growth[1]. Nowhere have I personally seen mold growth on green tea with the naked eye, but I have seen white tea with a high moisture content begin to develop a tasteless but very visible white mold after just two weeks in a room with around 70% relative humidity. Obviously, Springtime in Lanzhou, where the mountains of tea in the first picture were being stored, offers a very different environment with air moisture content around just 40%. Storage in drier climates means less potential moisture contact and opportunity for airborne molds to thrive.
While these molds may seldom be toxic to the human body, some can absorb and metabolize the proteins and starches within the tea leaves, changing the original flavor, while others can emit “foul odors” in the process, which I suspect is the source of that pungent fishy smell that sometimes arises in damply stored teas. It is important to note that while the outcome of fungal colonization on green tea is usually not desirable, specific fungal colonization in Anhua dark tea has long been known to improve flavor. More recently, work has been done to see if these and similar fungal species can quicken or enhance the aging of white tea.
Yellowed and Dark, but not Ruined
Finished green tea obviously becomes yellower and darker overtime. Below you can see a picture of green tea in a damp Darjeeling factory that over the course of just a few months in a semi-sealed sack has taken on a noticeably taken on a dark brownish color. The basic principle at play, as mentioned in earlier blog, seems to be the oxidization of chlorophyll in the leaves, magnesium exits chlorophyll as the oxygen comes in, and the appearance changes from green to brown, light to dark.
Browned Green Tea at Darjeeling Tea Factory - January 2025
This is considered undesirable for green tea, but is not necessarily a sign that the tea will be unpleasant when brewed up. Across the board, finished teas with a greater moisture content tend to be darker. While this appearance alone is unappetizing for some, this color can also be associated with a variety of pleasant flavors. In white tea, a darker browned appearance can mean a slower and less complete drying, which is in turn associated with more mineral or even sour notes, as well as fruity and even chocolately flavors. To be sure, as one study has recently confirmed, a lot of the floral compounds associated with white tea tend to peak between 7-8.5% moisture content[2]. However, at higher levels of moisture, other flavor profiles can form. One recent WeChat blog blasts the intentionally “improper” processing of certain white tea farmers and merchants. They note that moisture content sometimes as high as 15% in the leaves of such “wrongly” made teas can produce cocoa flavors. It is precisely trapped moisture over time that gives browned shoumei its distinct flavor, color, and aroma profile. So long as it does not mold or sour, such teas can be extremely satisfying. Likewise, in sun-dried green teas like Simao’s Chunjian or Mojiang’s Yunzhen, the standard moisture content of finished product is a whopping 10%, well above that of most conventionally dried green teas. This unusual dampness, extra exposure to UV radiation, and the special profile of Yunnan large-leaf cultivars come together to make some of the most strangely fruity, sometimes even creamy green teas I have ever experienced.
Mellowed Flavor
The porous surface of dry green tea, with as little moisture content as 5 or even 3%, is a natural sponge for whatever moisture can be found in the environment around it. Over time, the moisture in air can and will break down the theaflavin and amino acids in tea leaves. These compounds are usually the subject of bragging for green tea producers. As they break down, so will the sharp and fresh sensations associated with green tea[3]. Interestingly, these two compounds and chlorophyll have all been found to decline in the course of yellow tea processing. A longer time smothering (yellowing) means that water-soluble compounds like these are going to be oxidized and transformed in similar ways to aged green tea, albeit with the help of heat and in a much more compressed time interval. This is precisely why some less scrupulous sellers can pass off damply aged green as yellow tea, and is also why the original Reddit poster I saw all those years ago was pleasantly surprised by the “warm-natured potion” that sounds closer to a yellow tea than a green tea.
Wrap-up
Feel free to age your green tea, but be cautious with how not-so-airtight storage in not-so dry environments can affect the teas over time. You may end up with fishy tea, or a moldy monstrosity. If it is relatively sealed and sitting in a room with a pleasant level of humidity, you may instead end up with a warm potion of your own making, transformed completely from what you bought months or years prior.
Further Chinese-Language Reading
Xia Tao. 2014. Zhichaxue. Edition #3. China Agricultural Press.
[1] Yang Yusen. 1989. Lun Chaye Meibian Jiqi Fangzhi. Newsletter of Sericulture and Tea. (02) p.33-34.
[2] Fang Mingfeng. 2007. Chaye Bianzhi De Yuanyin. Newsletter of Sericulture and Tea.128(3): p. 35-36.
[3] Li Yifan.2020. Effects of Water Moisture on Aroma Formation and Release During Tea Roasting. Anhui Agricultural University. Master’s Thesis.