If you look at the dictionary definition for tannin then there is
nothing wrong with the term 'leached tannin'. Tea will stain your
teeth and can be used as a wood dye which is more or less the
historical reference to the tannin use. I think it is a tannin still
used with cotton today. Don't let any dry on a granite countertop.
It is not a trivial tannin to remove. If you're talking about
biochemistry that is something else. I didn't know there was a
Chinese character for the sheen characterization but all things tea in
China has a name. Does anybody know what it is or at least the
pinyin.

Jim

Will Yardley wrote:
> On 2007-05-18, Space Cowboy > wrote:
> > magicleaf wrote:
>
> >> i have noticed on several occassions depending on the type tea , I
> >> notice very slight amounts of oily looking slicks on the surface of
> >> the hot liquid after poured comes apparant under bright light , where
> >> does the oil come from . I have recieved many samples form different
> >> suppliers and enjoy tasting all the good with the bad however the
> >> oily stuff concerns me. Any answers on this ?
>
> > Leached tannins on the water surface look like an oil slick. I use
> > the term sheen. It's what gives the patina color to a pot.
> > Sometimes you see it sometimes you don't.
>
> I thought we established that tea doesn't have tannins.
>
> http://en.wikipedia.org/wiki/Tannin#Tea
> http://www.teatalk.com/science/chemistry.htm
>
> "The term "tannins" has been used by many to describe certain
> tea constituents. In industrial and botanic literatures, tannins
> are characterized as plant materials that give a blue color with
> ferric salts and produce leather from hides. Thus, tannins are a
> group of chemicals usually with large molecular weights and
> diverse structures. Monomeric flavanols, the major components
> in green tea, are precursors of condensed tannins. It would be
> more appropriate to use the term "tea polyphenols" or "tea
> flavanols" because they are quite distinct from commercial
> tannins and tannic acid."
>
> That said, I don't think the sheen is from oil used in the
> roasting process, and IIRC, there is a name for this phenomenon
> (at least in Chinese), and that the sheen is (if anything) a
> desirable thing.
>
> w