Sources and Health Effects of Dietary Ellagitannins - Chemistry and Biology of Ellagitannins - page 300

The method based on acid hydrolysis is the most practical choice for
routine quantification of complex ellagitannins in food samples. It can be
used for various food materials and allows simultaneous analysis of all
forms of ellagitannins. It uses commercially available, inexpensive
ellagic acid as standard, while such standards for ellagitannins are
lacking. Comparable levels of ellagitannins in berries were obtained by
the analysis of soluble ellagitannins as gallic acid equivalents and by the
analysis of ellagic acid equivalents released by acid hydrolysis (Määttä-
Riihinen et al. , 2004).
The contents of ellagitannins reported in literature are variable
(Table 8.1), since different conditions of extraction and acid hydrolysis
have been used and these significantly affect the yield of ellagic acid.
Attention should be paid to optimization of the hydrolysis conditions and
selection of the solvent for solubilization of the released ellagic acid.
Poorly soluble ellagic acid may form unrecognized precipitates resulting
in underestimation of the content of ellagitannins.
Daniel et al. (1989) used methanol extraction and hydrolysis with
trifluoroacetic acid for 2 h at 100 °C, but this method was later on found
to result in poor recovery of ellagic acid (Häkkinen et al. , 2000). For
simultaneous screening of flavonols and ellagic acid in berries, the
samples were hydrolyzed for 16 h at 35 °C in 50% methanol and 1.2 M
HCl (Häkkinen et al. , 1999). Later, the method was optimized for ellagic
acid analysis (Häkkinen et al. , 2000), and this procedure (20 h at 85 °C
in 50% methanol and 1.2 M HCl) has also been applied with slight
modifications in other studies (Häkkinen and Törrönen, 2000, Mattila
and Kumpulainen, 2002, Määttä-Riihinen et al. , 2004, Koponen et al. ,
2007). However, Vrhovsek et al. (2006) noted that this treatment resulted
in incomplete hydrolysis even after 20 h. Instead, they suggested a 6-
hour hydrolysis with 4 M HCl and extraction with aqueous acetone for a
method of choice for ellagitannin analyses.
In addition to ellagic acid, its conversion products are detected in
acid-hydrolyzed samples. In the earlier studies (Daniel et al ., 1989,
Häkkinen et al. , 2000, Mattila and Kumpulainen, 2002), only ellagic acid
was quantified. Määttä-Riihinen et al. (2004) observed that berry
ellagitannins were degraded by acid hydrolysis into ellagic acid and a
less polar derivative ( ca . 20-26% of the content), and they quantified