Polyphenols - Fruit Ripening: Physiology, Signalling and Genomics - page 162

but strawberry and raspberry fruit also
contain (epi)afzelechin units, and black-
currant, persimmon, grape and banana
contain (epi)gallocatechin units. Flavan-
3-ol glycosides are found in cherry, kiwi
and various nuts and galloylated
derivatives in grape and persimmon fruit.
Most fruits contain only B-type PAs, but
A-type ones are encountered in cranberry,
plum, avocado and groundnut (Gu et al. ,
2003).
PA-rich fruits include persimmons and
most berries, whereas citrus fruits are
devoid of them. Quantities ranging from
4 mg (in banana and kiwi) to 540 mg (in
chockeberries) per 100 g of fresh weight
(FW) have been determined by normal-
phase high-performance liquid chroma-
tography (HPLC) analysis (Gu et al. , 2004).
However, PA content is often under-
estimated. Methods relying upon acid-
catalysed depolymerization in the presence
of a nucleophile followed by HPLC
analysis showed that they are the most
abundant polyphenols in common fruits
such as apple (Guyot et al. , 2002; Wojdylo
et al. , 2008), grape (Mané et al. , 2007) and
strawberry (Buendia et al. , 2010), account-
ing for 71-90% of total phenolic com-
pounds in apple (Vrhovsek et al. , 2004)
and 60-93% in grape (Mané et al. , 2007).
Large varietal differences are also reported
and contents ranging from 1.3 to 19.8 g
kg −1 of dry weight (DW) have been
determined in apple varieties (Wojdylo et
al. , 2008). Moreover, PAs differ in terms of
the average degree of polymerization (aDP).
For instance, aDP values in apples varied
from 3.2 to 28.7 (Wojdylo et al. , 2008), and
higher values (up to 100) have been
reported in cider apples (Guyot et al. ,
2001).
In addition, PAs are only partly
extracted by the usual solvents. The
unextractable fraction can be determined
by performing acid-catalysed depolymeriz-
ation directly on the plant material (Guyot
et al. , 2001) or on the residue after
extraction (Downey et al. , 2003; Verries et
al. , 2008; Hellstrom et al. , 2009). The
proportion of extractable PAs reported by
Hellstrom et al. (2009) varied from 0% in

banana to 90-95% in apple, although
much lower values (10%) were obtained
for apples in another study (Arranz et al. ,
2009). The poor extraction rate of banana
PAs may be related to their high aDP, as
higher-molecular-weight PAs adsorb to
plant cell-wall material (Le Bourvellec and
Renard, 2005). Differences in sample
preparation (freeze drying versus freezing),
in extraction and analysis protocols, and in
the development stage (green versus ripe)
may explain the large discrepancy between
the PA levels reported in banana (cv.
'Cavendish') by different authors: 420-
500 mg per 100 g FW, extractable (Uclés
Santos et al. , 2010); 64 mg per 100 g fresh
weight, unextractable (Hellstrom et al. ,
2009); and 4 mg per 100 g FW, extractable
(Gu et al. , 2004).

Other fl avonoids
Flavonoids also include minor groups such
as fl avones, fl avanones (syn. dihydrofl a-
vones), fl avanonols (syn. dihydrofl av-
onols), chalcones and dihydrochalcones.
Flavones (Fig. 9.1d) and fl avanonols
(Fig. 9.1e) are seldom present in fruits, but
small amounts of fl avones (7-glycosides
and 6- and/or 8-C-glucosides of apigenin
and luteolin) are found in citrus fruits (Gil-
Izquierdo et al. , 2001) and dihydrofl avonol
3-rhamnosides in grapes (Trousdale and
Singleton, 1983; Vitrac et al. , 2002).
Fruit fl avanones (Fig. 9.1f) are usually
encountered as 7- O -diglycosides (e.g. nar-
ingin (naringenin 7- O -neohesperidoside),
narirutin (naringenin 7- O -rutinoside) and
hesperidin (hesperetin 7- O -rutinoside)),
along with smaller amounts of C-glycosides.
Naringenin derivatives have been detected
in tomato (Slimestad and Verheul, 2005;
Vallverdu-Queralt et al. , 2010). However,
fl avanones are present in large amounts
only in citrus fruits. Flavanone com-
positions are specifi c for the different Citrus
species (Rouseff et al. , 1987). Major
fl avanone glycosides are eriocitrin
(eriodictyol 7- O -rutinoside) and hesperidin
in lemon and lime, and naringin and
narirutin in grapefruit (Mouly et al. , 1994).
Sweet oranges are a rich source of