I don't actually necessarily agree with the suggestion Robert made about the fermentation beginning when the tea is compressed.entropyembrace wrote:From wyardley's link above.
I wonder about the sheng maocha sold to consumers? Old sheng maocha has clearly aged not lost it's flavour and aroma as green teas generally do and the young sheng maocha tastes much more like a sheng pu-erh cake than green tea.
[next paragraph quoted from a new reply I posted to that thread]
I'm not sure that the pressing is what changes the tea -- the tea is dried completely after it is compressed, and I think it's pretty well established that while maocha and compressed pu'er ferment slightly differently, both will ferment, and both can have similar end results (i.e., both will become dark and fermented with time). Your other observation (about the sealed bag) is a little more to the point - without air and moisture, the tea won't ferment. And of course, maybe green tea would end up fermenting as well, given sufficient air and moisture.
Keep in mind that Pu'er is a different major sub-varietal of the tea plant from the one used for most other tea plants, and that the production methods may also be slightly different from [other] green teas. Also, the way it's typically prepared differs slightly from the way you make most green tea. Plus, green teas have tons of taste differences from each other, which come from both differences in the varietal, time of harvest, exact method of production, etc. I don't think too many people have tried storing green tea in a warm, humid and oxygenated environment for 10-30 years, but I wouldn't be surprised if at least some sort of fermentation didn't take place if you did. Generally, green tea is prized for its freshness, and once it's sat around for 2-3 years, it may taste kind of stale / boring (pu'er tea can also go through some "awkward phases").
I prefer to just say "post-fermented" tea, and leave the question of whether young sheng is hei cha or not alone. From the limited research I've done, I believe the subject of at what point (if any) sheng becomes "dark" or "black" is a subject on which there isn't widespread consensus.