Agriculture Reference
In-Depth Information
Effect of processing
Stability of anthocyanins during processing and subse-
quent storage depends on the composition of food as well
as the temperature, light, and organic chemicals, such as
ascorbic acid and contamination with metal ions of prod-
ucts (Markakis, 1982; Macheix et al., 1990). Anthocyanins
are degraded into a brown-colored products in the pres-
ence of oxygen. Degradation of anthocyanins during ther-
mal treatment depends on the temperature and duration of
heat treatment. Application of high temperature-short time
(HTST) processes improves the retention of anthocyanins.
Conversely, thermal treatments that employ high temper-
atures for prolonged time, such as sterilization and con-
centration, cause degradation of anthocyanins (Mazza and
Miniati, 1994).
Flavones and flavonols
Flavones and flavonols are generally present in foods as
aglycones and glycosides. These classes of flavonoids pos-
sess a double bond between C-2 and C-3. Flavonols are dif-
ferent from flavones in that they possess a hydroxyl group at
C-3 position. The content of flavones and flavonols in fruits
depends on a number of factors including growing condi-
tions, degree of ripeness, size of fruits, and their variety.
The common flavonols found in tropical and subtrop-
ical fruits include kaempferol, quercetin, myricetin, and
their derivatives. Typically, a combination of kaempferol
and quercetin is found in fruits (Macheix et al., 1990).
Oranges contain flavones, such as naringenin and hesper-
itin, while lime and grapefruit contain hesperitin and narin-
genin, respectively.
Flavones are less common in fruits than flavonols. Citrus
fruits contain a number of polymethoxylated derivatives
of flavones, which are mainly found in the flavedo and in
the essential oils. The total content of polymethoxylated
flavones in peel oil of citrus fruits ranged from 1.88 g/liter
for orange (
Citrus sinensis
) to 6.49 g/liter for common
mandarin (Gaydou et al., 1987). The content of flavonol
glycosides in fruits varies from 0.2 to 30 mg/l00 g fresh-wt
(Macheix et al., 1990).
Flavanols
Flavanol molecules possess a natural tendency towards con-
densation during which they form stable compositions of
two, three, four, or more flavan-3-ol molecules bonded to
each other. Flavan-3-ols are present in the free form in fruits
with exception of grapes, where (
)epicatechin-O-gallate
was identified (Su and Singleton 1969). The common
flavanols in fruits include (
−
+
)catechin, (
+
)gallocatechin,
(
−
)epicatechin, and (
−
)epigallocatechin (Macheix et al.,
Tannins
Plant polyphenols capable of precipitating proteins from
aqueous solutions are known as tannins. Tannins also form
complexes with certain types of polysaccharides, nucleic
acids and alkaloids (Ozawa et al., 1987). Depending on their
chemical structure, they are conventionally subdivided into
hydrolyzable and condensed tannins.
1990).
Flavanones and flavanonols
Flavanones and flavanonols are distinguished from other
flavonoids by the presence of saturated 3-carbon chains and
oxygen atoms in the C-4 position. The difference between
the flavanones and flavanonols is due to the presence of a
hydroxyl group in the C-3 position of the middle ring in the
flavanonols, which is lacking in the flavanones.
Flavanones and flavanonols in tropical and subtropical
fruits are mainly found in citrus fruits. Presence of four
flavanones, namely, naringenin, eriodictyol, hesperetin,
and isosakuranetin, in citrus fruits was reported by
Rouseff et al. (1987). Flavanone glucosides including
hesperidin, naringin, and neoericitrin contribute 50-80%
of the total flavanoid content in citrus fruits (Kanes et al.,
1992). Naringin, naringenin 7-neohesperidoside, narirutin,
and naringenin 7-rutinoside are the major glycosides
in grapefruits. Sour oranges contain 7-neohesperido-
sides, namely, naringin, neohesperidin, and hesperetin
7-neohesperidoside, while sweet oranges contain
mainly 7-rutinosides, namely, narirutin, hesperedin,
and hesperetin 7-rutinoside (Rouseff et al., 1987).
Condensed tannins
Condensed tannins are also known as proanthocyanidins.
They are polymers of 2 to 50 or more flavonoid units that are
joined by carbon-carbon bonds, which are not susceptible
to being cleaved by hydrolysis. While hydrolyzable tannins
and most condensed tannins are water-soluble, some very
large condensed tannins are insoluble in water.
Condensed tannins are present in higher amounts in
the skin and peel of the fruits although they are dis-
tributed throughout the flesh of fruit. These tannins are
more widely distributed in the fruits than hydrolyzable tan-
nins. Proanthocyanidins can be divided into procyanidins,
propelargonidins, and prodelphinidins. Among these, pro-
cyanidins are the predominant proanthocyanidins, but sig-
nificant quantities of prodelphinidins are found in banana
and kiwifruit (Matsuo and Itoo, 1981).
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