2.2.3. Antioxidant properties of tannins
Antioxidant capacity of tannin fractions was measured using
the CAT (conjugated autoxidizable triene) procedure developed
by Laguerre et al. (2008). Briefly, this assay is based upon the high
sensitivity to oxidation of the a-eleostearic acid present in the triacylglycerols
of stripped Tung (Vernicia fordii) oil. Eleostearic acid
(ELA) is a linolenic acid containing a conjugated triene part: it thus
has a strong UV absorbance at kmax = 273 nm. Upon oxidation, the
degradation of the conjugated triene system into a conjugated
diene is accompanied by a decrease in the signal at 273 nm (and
an increase at 234 nm). Tung oil oxidation was initiated by 2,20-
azobis(2-amidinopropane) dihydrochloride (AAPH) in a water in
oil emulsion at 37 C, in the presence or absence of the tested antioxidant.
Spectrophotometric monitoring of the UV signal decay at
273 nm determines the ability of a molecule to protect tung oil
from oxidation. Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-
2-carboxylic acid) was used as reference antioxidant. The CAT
value is expressed as micromoles of trolox equivalents (TE) per
gram of dried tannins.