'lo Sabine,
SB> I just cook as much as I can drink within 12 hours. These days 
In that case unless it's kept  in a very warm place, I think we can
indeed rule out a fermentation. 
Are you using fruits that could also  make jam? It just might be
that you are extracting pectin from the fruit as well as flavour,
and this is what is giving the tea an odd consitency.(Pectin is the
stuff that helps give preserves a jelly-like consistency, apart from
the sugar.
SB> Home brew shops are not usual in Germany.
Oh dear, because home wine makers often use an enzyme to destroy the
pectin whem making a country wine from fruits like plum or damson
because the pectin would spoil the wine.
Does this mean that there are few country wine makers in Germany? 
Good luck with the tea anyway,
Duncan
  
... Hmmmm, I wonder what this button doeÈ– Aç±NO CARRIER
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