Because of recent talk in the r/tea subreddit, I thought I would share my “evidence” for why teas can age and how. This is what I have managed to gather from reading Chinese forums, English blogs, and other tea-related research articles.
Once a tea is picked, it begins to wilt and oxidize. We can use the analogy of certain fruits that ripen once they are picked – like mangoes. After picking, “ripening” begins. A mango will go from a tart astringent green fruit, to yellow, and eventually red (orange). A tea leaf left to its own devices follows a similar enough trend. Once a tea has reached the appropriate level of oxidation and withering, the majority of its enzymes are deactivated through a process known as shaqing (kill green). If you were to take that green mango and boil it, the enzymes would deactivate in the fruit, and it would not continue to ripen. Kill-green does this for teas.
There are many different methods of kill-green. In the case of many Chinese teas, pan frying is used. There are acceptable levels of shaqing, but it must not be underdone or overdone. Cooking is an apt analogy used by many. Undercooking a food will fail to get it to reach its desired “form,” while overcooking will burn or tarnish the dish. Imagine frying up some spinach in a pan. When it is finally finished, it is somewhat soggy. If left for a few days, it will rot. Our freshly shaqing’ed teas must also go through further drying to ensure they don’t go bad.
In the case of puerh tea, it is dried either traditionally by sun drying, or by warm air in an industrial process. It is important that the tea does not lose as much moisture content as other teas, otherwise completely dried it essentially becomes a completed tea (like oolongs) and would be best sealed to prevent further water from entering the tea. (Think of how cereal goes stale in more humid climates very quickly). The other important part of sealing teas like oolongs is protecting them from oxidation. Exposure to too much moisture or air will rapidly deteriorate the remaining fermentable content of any given tea. This is why the aging conditions of teas like oolong will differ vastly from puerh.
Due to the processing and base material of many types of teas, the level of antioxidants left in the tea is generally insufficient in keeping them safe from oxidation for extremely long periods of time, depending on levels many years or potentially decades. Think of red wines with very high tannin levels. The reason we can leave puerh out in the open, exposed to oxygen and humidity, is because of the level of antioxidants such as tannins that protect the tea from completely oxidizing too quickly. Think of these tannins as a tea’s armor. A tea with low levels of tannins will have less time to safely age before oxidation degrades the tea. This is why green oolongs are vacuum-sealed, and will generally have oxygen-absorbing packs included inside. Highly-tannic material will help prevent degradation from oxidation for longer. This is why red wines are ideally quite tannic if they are intended to be aged. Floral teas can be considered more akin to white wines, which are not intended to be aged and generally age poorly due to the lack of tannins. If a tea is highly tannic, but the tannins are altered before final processing (like in black teas) they are unlikely to provide the same level of protection. Because most of the enzymes are oxidized/deactivated prior to processing completion, they are less prone to the pitfalls of other lightly oxidized teas. Think here of using ripened bananas for banana bread. Once baked, they will not rot as quickly as they would if left out. Oolongs may also use roasts to remove excess amounts of water, and affect flavor. It is important to note that generally oolong roasts (while temperatures vary for different oolongs) generally do not surpass 100C, and do not completely remove all water content from the leaves. Excess water would result in mold or deterioration of the leaves. Keep in mind that temperatures throughout processing, in both shaqing and roasting or drying, must not be exceedingly high. Not all water is removed from the leaf. Doing so would harm the fragrances and character of the tea. Though levels generally drop very low for teas like oolongs, the process does not completely kill 100% of the teas enzymes. This is important: It allows the teas to have material that can be “transformed” during post process aging.
Because of the more thorough drying upon completion of processing for oolongs, as well as remaining plant-based antioxidants and enzymes, when these teas are stored for aging, they must also ideally have little contact with air and moisture, as they will indeed deteriorate, or if overly humid, mold. Re-roasting may help reduce increased levels of moisture, but may also alter flavors. Ideally, it seems recommended to try to set up good storage for the tea and let time do its thing. Storage of oolong in Taiwan is not 100% sealed, but their container is generally packed full to help fight oxidation. Teas should not be overly humid, but from readings, it appears that some level of moisture will promote the post-fermentation of oolongs.
Moving to white tea. White tea seems to age much more quickly than a tea like puerh. There are many factors to consider here, but I would like to nail down the primary points to consider. White tea is picked, withered, and dried. Though the process may vary, it is important to note that drying temperatures for white tea are relatively low. White tea like any other tea, especially because of its minimal processing, is also subject to natural enzymatic decay via oxidation as any other tea would be. Because no shaqing, roasting, or baking is used in most cases, white teas enzymes are largely left intact to “ripen” or further ferment the tea after processing. Here is a point to consider:
Drying basically dehydrates or removes the moisture from the food and this simple action inhibits the growth of bacteria, mold and yeast. Moreover, it slows down the enzyme action without deactivating them. These factors ensure that food does not spoil easily and hence, makes drying an effective food preservation technique.
Though I cannot claim knowledge on the best ways to store white tea to promote aging, white tea certainly has enzymes remaining that have not been deactivated through processing. These substances will allow for post fermentation. Here is a quick timeline of how aging teas will approximately work:
Processing completion —> Aging Process —> Drinkable Period —> Decay —>Trash or Museum time.
For teas to be immediately consumed, of course, one needs not worry about the aging process. Depending on the materials used, processing, and levels of oxidation of the tea, these “stages” will either be pulled earlier or pushed later. If a tea does not contain enough antioxidant compounds post-production, it is wise to keep the tea away from too much open air, or in some cases, any at all. Oxidations of tannins in teas like puerh will produce different textures in the tea as these compounds oxidize, much like a red wine. How much time the tea can resist oxidation will also determine how much of its flavor will be determined by the process of aging. It will also determine when the tea begins to decay (from oxidation. Other factors such as mold may also decay a tea). For teas with post-fermentation in mind, the right levels of temperature and available ambient moisture will promote fermentation of remaining enzymes more or less quickly. The level of air, moisture, and temperature teas can be exposed to will depend very heavily on original material’s composition and processing style. Teas with less “protection” through tannins and other natural plant-based antioxidants will need to be catered to more specially in their aging conditions to ensure the tea does not decay before material can ferment. This is core to the debate over aging oolongs. Tea can ferment. Due to previously mentioned factors, teas like oolong are not as equipped to weather the elements as puerh tea may be. Most likely because aged oolong is a possibility, the market has been flooded with attempts. These attempts likely form many of the views we’ve seen of late.
Please leave any additional comments you wish to add below. This description is in no way comprehensive, however, it gives beginning insights into the possibilities of aging teas after production based on material, processing, and storage conditions. If you’ve read this far, thank you for sticking along for the ride. Cheers!
References and pieces to consider (I apologize, not all of them are in English):