Although the natural environment in which teas grow obviously affect how they taste, processing, for oolongs at least, is king. The sheer number of variables is astounding, and the range of tastes that are possible, from the really light and floral baozhongs to the really dark and heavy wuyis are what make oolongs so much fun (and why so many people drink them).
I’ll try to proceed in the order in which these things happen in the production process: oxidation, rolling, roasting, aging.
Oxidation is the first step in oolong production that makes it distinct from green teas, and it really happens immediately after the tea leaves are plucked. When leaves get harvested, they usually go through a withering stage, and then they are bruised so that the cell structure breaks down, so that the enzymes can get to work and oxidation can begin. How much bruising, how much time for oxidation, under what conditions, etc, are the kinds of things that create particular flavours in a tea and are also the domain of a master tea maker. I suspect, for example, that darjeeling oolongs have generally turned out to be similar to their first flush is because they haven’t quite gotten the hang of the oxidation process yet, so everything still taste sort of vaguely similar. I have talked to folks who tell me that they have to control for everything from weather, to time of day, to moisture level in the air, etc, and they know when to stop the oxidation process and start the kill-green by the way the leaves look and smell. That’s stuff that I think I will only be able to learn if I become a tea farmer and work on it for thirty years.
The kill-green stops the oxidation process, and then you have to roll the tea — literally rolling them in the old days, in cloth bags with the farmer’s feet doing the rolling. These days, that’s more often than not done with a rolling machine. The purpose of rolling here is more or less like the purpose of rolling for puerh — squeezing out liquids, and basically allowing a lot of the dissolvable materials to stick on the surface of the leaves rather than remaining inside. The rolling process can take a while, and depending on the area in which this is happening, rolling will also determine, to a large extent, the final shape in which the tea takes. Just look at any dancong and compare it to a Taiwanese gaoshan oolong and you’ll know what I mean.
The drying and roasting process is then the step in which tea becomes tea — drinkable, brewable leaves. This can be done in different ways, but generally speaking, this is mostly done through machines again. At what temperature and for how long is really a matter of the craft of the teamaker again, because the retained moisture at the end of this process affects how the tea will taste by the time it gets to you. Even though leaves look dry, there’s always some moisture in them, and the amount of drying/roasting that it goes through affects this value, which then changes the way it keeps and the way it ages through time. That’s why, for example, vacuum sealed packs of somewhat wet leaves don’t keep too long and need to be left alone in the fridge — they go bad, fast.
Generally speaking, the drier the leaves, the longer/better they keep. Roasting is a process through which moisture gets taken out of the leaves, and re-roasting, which was done often, was something that tea merchants would routinely do themselves in order to refresh a stock of leaves — reigniting (and changing) the aromas of a tea, and to take out excess moisture that usually ends up imparting a sour flavour on the leaves. Oolongs can go from virtually no roasting to really heavy, pitch black roasting, and the skill of the roaster in handling this again has a direct and immediate effect on the way the teas come out. There is also a regional preference here, with Wuyi teas generally being of higher roast, for example, and modern day tieguanyin from the mainland are increasingly little to no roast — nuclear green, in other words, which I personally find terrible to drink. There’s literally something for everyone here, depending on one’s likes and dislikes.
Then there’s the question of aging, which I have written plenty about before. I don’t think all oolongs will age well — only a select few do. Badly aged oolongs are usually sour and pretty disgusting, and sometimes re-roasting them will fix the problem. However, there are lots of fake aged oolongs out there that are simply heavily roasted teas pretending to be aged teas. They can be nice, but they’re not necessarily very old. I personally find aged teas to be most fascinating, and since I don’t drink nearly as fast as I buy tea, I end up having some teas that I age myself without really having intended to do so — such as the cup of 2006 Beidou that I’m drinking right now. Over time, a properly aged oolong should have a reddish appearance in both the leaves and especially the liquor, and the taste should be sweet and aromatic. Then they eventually acquire the type of taste that all aged teas get — hard to explain, but you know one when you see one.
The interesting thing here is that the permutations of various factors – location, processing, aging – combine to form all types of flavours and aromas that you can get from oolongs. I can safely say that almost no two oolongs are the same, and every time I go to a store in China that specialize in some type of oolong or another, each batch that I try are going to be different in some way or another. Because so much of it also depends on post-processing and storage, even after the same batch of tea left the factory, the ultimate result in your cup may still differ. I suppose that’s what makes it fun.