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Encyclopedia of Physical Science and Technology EN006C-254 June 28, 2001 19:52

Food Colors 109

disappearance of chlorophyll. The intensity of the yellow purple, violet, magenta, and most of the red hues of flow-
color of certain animal products, such as egg yolk and ers, fruits, leaves, stems, and roots are attributable to pig-
milk fat or butter, depends on the carotenoid content of ments chemically similar to the original “flower blues.”
the feed the animals ingest. In view of this dependency, Two exceptions are notable: tomatoes owe their red color
the seasonal variation in the color of these products is to lycopene and red beets owe theirs to betanin, pigments
understandable. A nutritionally important interconversion not belonging to the anthocyanin group.
of carotenoids is the formation of retinol (vitamin A) from 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.
There are at least 10 more anthocyanidins in nature,
β-carotene and other carotenoids possessing a β-ionone
practically always appearing as glycosides. The number
ring and known as provitamins A.
of anthocyanins far exceeds that of anthocyanidins, since
The stability of carotenoids in foods varies greatly,
monosaccharides, disaccharides, and at times trisaccha-
from severe loss to actual gain in carotenoid content dur-
rides glycosylate the anthocyanidins at various positions
ing storage. Carotenoid losses amounting to 20 or 30%
(always at 3, occasionally at 5, and seldom at other posi-
have been observed in dehydrated vegetables (e.g., car-
tions). Eventual acylation with p-coumaric, caffeic, and
rots, sweet potatoes) stored in air. These losses are mini-
ferulic acids increases the number of natural anthocyanins.
mized when the dry product is stored in vacuum or inert
An example of acylated anthocyanin is the dark pur-
gas (e.g., nitrogen), at low temperatures, and protected
ple eggplant pigment delphinidin, 3-[4-(p-coumaroyl)-L-
from light. The main degradative reaction of carotenoids
rhamnosyl-(1 → 6)-D-glycosido] 5-D-glucoside.
is oxidation. Oxygen may act either directly on the double
The color of anthocyanins is influenced not only by
bonds or through the hydroperoxides formed during lipid
structural features (hydroxylation, methoxylation, glyco-
autoxidation. Hydroperoxides formed during enzymatic
sylation, acylation), but also by the pH of the solution in
lipid oxidation can also bleach carotenoids by a coupled
which they are present, copigmentation, metal complexa-
lipid–carotenoid oxidation mechanism. On the other hand,
tion and self-association.
certain vegetables, such as squash and sweet potatoes, in
The pH affects both the color and the structure of an-
which carotenoid biosynthesis continues after harvesting,
thocyanins. In very acidic solution, anthocyanins are red,
may manifest an increase in carotenoid content during
but as the pH rises the redness diminishes. In freshly pre-
storage.
pared alkaline or neutral solution, anthocyanins are blue
or violet, but (with the exception of certain multiacylated
D. Flavonoid Pigments anthocyanins) they fade within hours or minutes.
In acidic solution four molecular species of antho-
Hundreds of flavone-like pigments are widely distributed
cyanins exist in equilibrium: a bluish quinoidal (or
among plants. On the basis of their chemical structure,
quinonoidal) base A, a red flavylium cation AH , a color-
+
these pigments are grouped in several classes, the most
less carbinol pseudo-base B, and a colorless or yellowish
important of which are listed in Table II. The basic struc-
chalcone C (Fig. 5).
ture of all these compounds comprises two benzene rings,
+
At very low pH (below 1), the red cation AH domi-
A and B, connected by a heterocycle. The classification of
nates, but as the pH rises to 4 or 5, the concentration of
flavonoids is based on the nature of the heterocycle (which
the colorless form B increases rapidly at the expense of
is open in one class).
AH , while forms A and C remain scarce. In neutral and
+
Most of these pigments are yellow (Latin, flavus). One
important exception is the anthocyanins, which display a alkaline solutions, the concentration of base A rises and
great variety of red and blue hues. Because of the strong its phenolic hydroxyls ionize, yielding unstable blue or
−
violet quinoidal anions A (Fig. 6).
visual impact of anthocyanins on the marketing of fruits
Although it is true that the reaction of most plant tis-
and vegetables, these pigments will be discussed in greater
sues pigmented with anthocyanins (fruits, flowers, leaves)
detail than other flavonoids.
is slightly acidic, pH alone cannot explain the vivid col-
ors encountered in these tissues. One mechanism lead-
1. Anthocyanins
ing to the enhancement and stability of anthocyanin
The name of these pigments was originally coined to des- coloration is copigmentation, that is, the association of
ignate the blue (kyanos) pigments of flowers (anthos). It anthocyanins with other organic substances (copigments).
is now known that not only the blue color, but also the This association results in complexes that absorb more