Numerous methods such as spectrometry [7,8], chemiluminescent
[9] and fluorescent [10] have been proposed for
determining the levels of tannins. Regarding spectrometric
methods, Folin–Ciocalteau [11] and ferrous tartrate methods are
adopted as the official methods in the USA and Japan, respectively.
The Folin–Ciocalteau (or Folin–Denis) method based on
redox chemistry is one of the most widely used methods. Owing
to complex-formation with tannin, the ferrous tartrate method
is not affected by the co-existing ascorbate [12]. Nonetheless,
trivial preparation of reagents, time-consuming incubation and
heat processing for colorful complex-formation of spectrometric
methods make measurement inconvenient and on-line application
difficult