Polyphenols - Fruit Ripening: Physiology, Signalling and Genomics - page 166

Among polyphenols, fl avanones, fl avonols
and fl avan-3-ols have been reported to taste
bitter.
Flavanones, and especially naringin, are
responsible for the bitter fl avour of
grapefruit and sour orange (Horowitz and
Gentili, 1969; Rouseff, 1990). Replacement
of the neohesperidose (2- O - D - L -rhamnosyl-
E - D -glucose) by its isomer rutinose (6- O - D - L -
rhamnosyl- E - D -glucose) results in tasteless
compounds, while naringenin 7-glucoside
is only mildly bitter. Members of the
fl avanone family have other important taste
properties. Flavanone aglycones such as
eriodictyol and homoeriodictyol have been
reported to mask the bitterness of caffeine
without exhibiting intrinsic fl avour (Ley et
al. , 2005). Moreover, opening of the C-ring
of fl avanone neohesperidosides to form the
dihydrochalcones

during development (Sugiura and Tomana,
1983). However, the products of catechin
condensation with acetaldehyde were
perceived to be as astringent as PAs of
equivalent chain length and were more
bitter (Vidal et al. , 2004c). The other
mechanism involves increased solubiliz-
ation of pectins that can interact with PAs
(Taira et al. , 1998) and reduce astringency
when added to procyanidin solutions
(Vidal et al. , 2003b). Indeed, postharvest
deastringency treatments induce genes
encoding xyloglucan endotransglycosylase/
hydrolase that catalyse degradation of
xyloglucans, the major hemicellulosic
polysaccharides in plant cell walls
(Nakatsuka et al. , 2011).

generates extremely 9.3.3 Health benefi ts

sweet compounds.
Flavonols have also been described as
bitter and astringent in water, 5% ethanol,
and beer (Dadic and Belleau, 1973; Delcour
et al. , 1984). Flavonols extracted from
blackcurrant (Schwarz and Hofmann,
2007) and bilberry (Laaksonen et al. , 2010)
were also perceived as bitter and
astringent. However, supplementing the
bilberry juice with fl avonol extracts did not
modify its sensory profi le (Laaksonen et
al. , 2010), and there is no report of their
contribution to fruit taste.
Flavan-3-ols taste less bitter and more
astringent as their chain length increases
(Lea and Arnold, 1978). Interactions of
tannins with proteins increase with the
number of phenolic groups, and thus with
DP and galloylation, and so does
astringency (Vidal et al. , 2003a). Larger PAs
(beyond DP8 or so) have classically been
considered insoluble and thus unable to
exhibit astringency (Lea, 1990), but other
studies have established that larger
polymers are highly astringent (Vidal et al. ,
2003a). Most persimmon cultivars show
intense astringency and require postharvest
treatments (with CO 2 , ethanol or ethylene)
to reduce it (Kato, 1990). Two mechanisms
have been proposed. The fi rst involves
condensation of PAs with acetaldehyde
(Tanaka et al. , 1994) released from seeds

Dietary polyphenols are attracting great
interest for their potential benefi cial health
effects. This assumption arises from
epidemiological studies relating higher
consumption of plant-based foods and
beverages to a lower incidence of certain
degenerative diseases. Health effects of
polyphenols are often attributed to their
antioxidant activity, involving a variety of
mechanisms (i.e. reduction or scavenging
of reactive oxygen species, chelation of
transition metal ions and inhibition of
enzymes involved in oxidative stress)
(Dangles, 2012). Numerous in vitro studies
have shown antioxidant activity of
phenolic compounds, but scientifi c
evidence is not yet conclusive and more
human studies are required (Törrönen,
2009). Indeed, the relevance of such in vitro
studies is questionable because of the poor
bioavailability and extensive metabolism of
phenolic compounds in the digestive
process (Williamson and Stalmach, 2012).
This applies in particular to the potential
health benefi ts of stilbenes, and especially
of resveratrol, suggested by over 4000 in
vitro studies, much highlighted by media
coverage. Although resveratrol shows
potential to reduce the incidence of chronic
diseases, clinical trials are still needed to
establish its effects in humans (Smoliga