Hundreds of flavone-like pigments are widely distributed
among plants. On the basis of their chemical structure,
these pigments are grouped in several classes, the most
important of which are listed in Table II. The basic structure
of all these compounds comprises two benzene rings,
A and B, connected by a heterocycle. The classification of
flavonoids is based on the nature of the heterocycle (which
is open in one class).
Most of these pigments are yellow (Latin, flavus
important exception is the anthocyanins, which display a
great variety of red and blue hues. Because of the strong
visual impact of anthocyanins on the marketing of fruits
and vegetables, these pigments will be discussed in greater
detail than other flavonoids.
The name of these pigments was originally coined to designate
the blue (kyanos
) pigments of flowers (anthos
is now known that not only the blue color, but also the
purple, violet, magenta, and most of the red hues of flowers,
fruits, leaves, stems, and roots are attributable to pigments
chemically similar to the original “flower blues.”
Two exceptions are notable: tomatoes owe their red color
to lycopene and red beets owe theirs to betanin, pigments
not belonging to the anthocyanin group.
Anthocyanins are glycosides of anthocyanidins, the
latter being polyhydroxyl and methoxyl derivatives of
flavylium. The arrangement of the hydroxyl and methoxyl
groups around the flavylium ion in six anthocyanidins
common in foods is shown in Fig. 4.
|Figure 4 Six anthocyanidins common in foods. The electric charge shown at position 1 is delocalized over the
entire structure by resonance.
There are at least 10 more anthocyanidins in nature,
practically always appearing as glycosides. The number
of anthocyanins far exceeds that of anthocyanidins, since
monosaccharides, disaccharides, and at times trisaccharides
glycosylate the anthocyanidins at various positions
(always at 3, occasionally at 5, and seldom at other positions).
Eventual acylation with p
-coumaric, caffeic, and
ferulic acids increases the number of natural anthocyanins.
An example of acylated anthocyanin is the dark purple
eggplant pigment delphinidin, 3-[4-(p-coumaroyl)-Lrhamnosyl-(
The color of anthocyanins is influenced not only by
structural features (hydroxylation, methoxylation, glycosylation,
acylation), but also by the pH of the solution in
which they are present, copigmentation, metal complexation
The pH affects both the color and the structure of anthocyanins.
In very acidic solution, anthocyanins are red,
but as the pH rises the redness diminishes. In freshly prepared
alkaline or neutral solution, anthocyanins are blue
or violet, but (with the exception of certain multiacylated
anthocyanins) they fade within hours or minutes.
In acidic solution four molecular species of anthocyanins
exist in equilibrium: a bluish quinoidal (or
quinonoidal) base A, a red flavylium cation AH+
, a colorless
carbinol pseudo-base B, and a colorless or yellowish
chalcone C (Fig. 5).
|Figure 5 Four anthocyanin structures present in aqueous
acidic solutions: R is usually H, OH, or OCH3. Gl is glycosyl.
[Adapted from Brouillard, R. (1982). In “Anthocyanins as Food
Colors” (P. Markakis, ed.), Academic Press, New York.]
|Figure 6 Absorption spectra recorded immediately after dissolving
an anthocyanin (malvin chloride) in buffers of pH 2, 6, and
10. The absorption peaks at pH 6 and 10 disappeared within 1
to 3 hr. (Adapted from Brouillard, R. (1982). In “Anthocyanins as
Food Colors” (P. Markakis, ed.), Academic Press, New York.]
At very low pH (below 1), the red cation AH+
but as the pH rises to 4 or 5, the concentration of
the colorless form B increases rapidly at the expense of
, while forms A and C remain scarce. In neutral and
alkaline solutions, the concentration of base A rises and
its phenolic hydroxyls ionize, yielding unstable blue or
violet quinoidal anions A−
TABLE II Major Classes of Flavonoids
||Quercetin (apples, grapes)
red apples, red grapes)
Although it is true that the reaction of most plant tissues
pigmented with anthocyanins (fruits, flowers, leaves)
is slightly acidic, pH alone cannot explain the vivid colors
encountered in these tissues. One mechanism leading
to the enhancement and stability of anthocyanin
coloration is copigmentation, that is, the association of
anthocyanins with other organic substances (copigments).
This association results in complexes that absorb more
visible light (they are brighter) and light of lower frequency
(they look bluer—the bathochromic effect) than
the free anthocyanins at tissue pH. Most of these copigments
are flavonoids, although compounds belonging to
other groups (e.g., alkaloids, amino acids, nucleotides) can
function similarly. A stacked molecular complex between
an acylated anthocyanin and a copigment (flavocommelin)
is shown in Fig. 7.
|Figure 7 Stacked molecular complex of awobanin and flavocommelin;
p-C. denotes p-coumaroyl. [From Osawa, Y. (1982).
In “Anthocyanins as Food Colors” (P. Markakis, ed.), Academic
Press, New York.]
Self-association is the binding of anthocyanin
molecules to one another. It has been observed that the
complexes absorb more light than the sum of the single
molecules. This explains why a 100-fold increase in the
concentration of cyanidin 3,5-glucoside results in a 300-
fold rise in absorbance.
Certain anthocyanins form complexes with metals (e.g.,
iron, aluminum, magnesium), and the result is an augmentation
of the anthocyanin color. At times the complexes
involve an anthocyanin, a copigment, and a metal.
Alarge number of the anthocyanins present in fruits and
vegetables have been identified. It is not unusual for a plant
tissue to contain several anthocyanins (17 in certain grape
varieties), all genetically controlled. Table III shows the
anthocyanidin moieties of anthocyanins in common fruits
TABLE III Anthocyanidins Present as Anthocyanins in Fruits
|Fruit or vegetable
|Apple (Malus pumila)
|Blackberry (Rubus fructicosus)
|Black currant (Ribes nigrum)
|Blueberry (lowbush,Vaccinium angustifolium;
|Delphinidin, petunidin,malvidin, peonidin,cyanidin
|Cherry (sour, ‘Montmorency,’ Prunus
‘Bing,’ P. avium)
|Cranberry (Vacinnium macrocarpon)
|Elderberry (Sambucus nigra)
|Fig (Ficus carica)
|Gooseberry (Ribes grossularia)
|Grape (red European. Vitis vinifera)
||Malvidin, peonidin, delphinidin, cyanidin, petunidin, pelargonidin
|Grape (‘Concord,’ Vitis labrusca)
|Mango (Mangifera indica)
|Mulberry (Morus nigra)
|Olive (Olea europea)
|Orange (‘Ruby,’ Citrus sinesis)
|Passion fruit (Passiflora edulis)
|Peach (Prunus persica)
|Pear (Pyrus communis)
|Plum (Prunus domestica)
|Pomegranate (Punica granatum)
|Strawberry (Fragaria chiloensis and F. virginiaca)
||Pelargonidin, little cyanidin
|Beans (red, black; Phaseolus
||Pelargonidin, cyanidin, delphinidin
|Cabbage (red, Brassica oleracea)
|Corn (red, Zea mays)
|Eggplant (Solanum melongena)
|Onion (Alium cepa)
|Potato (Solanum tuberosum)
|Radish (Raphanus sativus)
Generally, the attractive color of anthocyaninpigmented
foods is not very stable. Canning of red cherries
or berries results in products with considerable bleaching.
Strawberry preserves lose one-half of their anthocyanin
content after a few weeks on the shelf, although the
browning reaction may mask the loss. And red grape juice
is subject to
extensive color deterioration during storage.
Exposure to high temperatures and contact with the oxygen
of the air appear to be two factors affecting anthocyanin
stability most adversely. Ascorbic acid accelerates
the destruction of anthocyanins, and so does light. Certain
oxidizing enzymes, such as phenol oxidase, and a
hydrolyzing enzyme known as anthocyanase may contribute
to the degradation of anthocyanin pigments. Oxidizing
enzymes act on the anthocyanidin moiety, while
anthocyanase splits off the sugar residue(s); the freed anthocyanidin
is very unstable and loses its color spontaneously.
Sulfur dioxide, which is used for the preservation
of some fruit products (pulps, musts), bleaches
anthocyanin pigments, but on heating of the fruit prduct
in vacuum the SO2
is removed and the anthocyanin color
reappears. Large concentrations of SO2
, combined with
lime, decolorize anthocyanins irreversibly and are used in
the preparation of maraschino cherries. Anthocyanins act
as anodic and cathodic depolarizers and thereby accelerate
the internal corrosion of tin cans. It is therefore necessary
to pack anthocyanin-colored products in cans lined with
special enamel. In aging red wines anthocyanins condense
with other flavonoids and form polymeric (MW ≤ 3000)
redbrown pigments (Fig. 8). On continued polymerization
these pigments become insoluble and form sediments in
bottled red wines.
|Figure 8 Proposed structure for a polymeric red-brown pigment
in aging red wine. [From Somers, T. C. (1971). Phytochemistry 10,
Anthocyanins possessing more than one acyl group
show extraordinary color stability over a wide pH
range. One of them, peonidin-3-(dicaffeyl sophoroside)
5-glucoside, isolated from ‘Heavenly Blue’ morning glory
flowers (Ipomoea tricolor
), has been shown to “produce a
wide range of stable colors in foods and beverages which
have a pH range of 2.0 to about 8.0.” United States patent
4,172,902 covers its use as a colorant in foods.
2. Other Flavonoids
Among flavonoids other than anthocyanins, the catechins,
flavonols, and leucoanthocyanidins have the widest distribution
in foodstuffs, while flavonone glycosides are of
special interest in citrus fruits.
|Figure 9 Structure of
Catechins, or flavan-3-ols, are present mainly in woody
tissues. Among common foods, tea leaves contain at least
six catechins representing about 25% of the dry weight
of tea leaves. Tea catechins are excellent substrates for
the catechol oxidase that is present in tea leaves and participates
in the conversion of green tea to black tea. The
reddish brown color of tea brew is due to a mixture of
pigments known as theaflavins and thearubigins. The
structure of one of them is shown in Fig. 9.
Flavonols, like anthocyanidins, exist almost exclusively
as glycosides. Three common flavonols are kaempferol,
quercetin, and myricetin, resembling pelargonidin, cyanidin,
and delphinidin, respectively, in the hydroxylation
pattern of theBring. Flavonol glycosides impart weak yellow
hues to apples, apricots, cherries, cranberries, grapes,
onions, plums, potatoes, strawberries, tea, tomatoes, and
are compounds of the general formula
- shown in Fig. 10. They have no color of their own,
but in acidic environments and at elevated temperatures
they are converted to colored anthocyanidins.
- This reaction
is in competition with the condensation to a dimeric
- Low temperature favors the formation
of the dimeric compound, which can polymerize
to yield products with pronounced tanning properties.
The most common leucoanthocyanidins are leucopelargonidin,
leucocyanidin, and leucodelphinidin,
which are converted to the corresponding anthocyanidins.
This conversion results in the undesirable “pinking” of certain
products such as canned pears, canned banana puree,
processed brussels sprouts, and beer. On the other hand,
polymerization to tannins leads to astringency and the formation
of haze in beer (insolubilization of beer proteins).
|Figure 10 Basic structures of leucoanthocyanidins (1), anthocyanidins
(2), and dimeric leucoanthocyanidins (3).
|Figure 11 Two major pigments of red beets.