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Approximately 1500 colored compounds, also known as natural food pigments, have been
isolated from foodstuffs. On the basis of their chemical structure, these food pigments can be
grouped in the following six classes: heme pigments, chlorophylls, carotenoids, flavonoids,
betalains, and miscellaneous pigments.
1. Heme PigmentsHeme (from the Greek for blood) is responsible for the red color of two important animal
pigments: hemoglobin, the red pigment of blood, and myoglobin, the red pigment of muscles.
Practically all the red color of red meat is due to myoglobin. Other colored muscle compounds
(cytochromes, vitamin Bi2, flavoproteins) do not contribute significantly to the color of red meat.
FIGURE 1 Structure of heme.
The myoglobin in meat is subject to chemical and color changes. Freshly cut meat looks
purplish. On exposure to air, the surface of the meat acquires a more pleasing red hue (blooming
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of the cut). The color change is due to the oxygenation of myoglobin. Severe oxidative
deterioration may result in the formation of green pigments (sulfmyoglobin, chole-myoglobin).
When meat is cooked, the protein moiety (globin) of myoglobin is denatured and the heme is
converted chiefly to nicotinamide hemichrome, the entire pigment acquiring a brown hue.
In cured meats, in which nitrite is used, many reactions occur, some of which lead to color
changes. Among the established reactions are the following: (1) the nitrite salt is converted to
nitric oxide (NO), nitrate, and water; (2) the NO replaces the H2O attached to the iron of heme
and forms nitrosyl myoglobin, which is reddish; (3) on heating, the nitrosyl myoglobin is
transformed to nitrosyl hemochrome, which has the familiar pink color of cured meats; and (4)
any metmyoglobin present in the cured meat is similarly nitrosylated, reduced, and finally
converted to nitrosyl hemochrome.
2. ChlorophyllsSeveral chlorophylls have been described. Two of them, chlorophyll a and chlorophyll b, are of
particular interest in food coloration because they are common in green plant tissues, in which
they are present in the approximate ratio 3:1, respectively. Their structures resemble that of heme
since they are all derivatives of tetrapyrrole.
As food pigments, chlorophylls impart their green color to many leafy (spinach, lettuce, etc.) and
nonleafy (green beans and peas, asparagus, etc.) vegetables and to unripe fruits. They are not
very stable pigments. Ethylene, a gaseous plant hormone, destroys chlorophylls, and it is
occasionally used to degreen fruits.
3. CarotenoidsMany of the yellow, orange, and red colors of plants and animals are due to carotenoids,
pigments similar to those of carrots. The basic structure of carotenoids is a chain of eight
isoprenoid units. Some carotenoids are hydrocarbons and are known as carotenes, while others
contain oxygen and are called xanthophylls. The carotenoids of foods, however, are usually in
the all-trans form.
Carotenoids occur free or as esters of fatty acids or as complexes with proteins and
carbohydrates; for example, in paprika, capsanthin is esterified with lauric acid. In live lobster,
astaxanthin is complexed with protein; the astaxanthin-protein complex is blue-gray, the color of
live lobster, but on heating, the complex is broken and the freed astaxanthin imparts its red color
to the cooked lobster.
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Carotenoids are present in a large variety of foods, from yeast and mushrooms, to fruits and
vegetables, to eggs, to fats and oils, to fish and shellfish. As fat-soluble substances, carotenoids
tend to concentrate in tissues or products rich in lipids, such as egg yolk and skin fat, vegetable
oils, and fish oils.
Types of Carotenoids and Their Natural Sources
Name and sou
f-Carotene (car
egg, orange, chfat)
Xanthophyll
(vegetables, eg
chicken fat)
Zeaxanthin (ye
corn, egg, liver
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Cryptoxanthin
yellow corn, or
Physalien
(asparagus, ber
Bixin (annatto
seeds)
Lycopene (tom
pink grapefuit,
oil)
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Capsanthin
(paprika)
Astaxanthin
(lobster, shrimp
salmon)
Torularhodin (R
otorula yeast)
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Canthaxanthin(mushrooms)
f -Apo-8-carot
(spinach, orang
Additional information (not necessary to right)
The stability of carotenoids in foods varies greatly, from severe loss to actual gain in carotenoid content
during storage. Carotenoid losses amounting to 20 or 30% have been observed in dehydrated vegetables
(e.g., carrots, sweet potatoes) stored in air. These losses are minimized when the dry product is stored in
vacuum or inert gas (e.g., nitrogen), at low temperatures, and protected from light. The main degradative
reaction of carotenoids is oxidation. Oxygen may act either directly on the double bonds or through the
hydroperoxides formed during lipid autoxidation. Hydroperoxides formed during enzymatic lipid oxidation
can also bleach carotenoids by a coupled lipid-carotenoid oxidation mechanism. On the other hand, certain
vegetables, such as squash and sweet potatoes, in which carotenoid biosynthesis continues after harvesting,may manifest an increase in carotenoid content during storage
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4. Flavonoid PigmentsHundreds of flavone-like pigments are widely distributed among plants. 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.
Most of these pigments are yellow (Latin, flavus). One important exception is the anthocyanins,
which display a great variety of red and blue hues.
a) AnthocyaninsThe name of these pigments was originally coined to designate the blue (kyanos) pigments of
flowers (anthos). It 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 color of anthocyanins is influenced not only by structural features (hydroxylation,
methoxylation, glyco-sylation, acylation), but also by the pH of the solution in which they are
present, copigmentation, metal complexa-tion and self-association.
The pH affects both the color and the structure of an-thocyanins. 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.
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Major Classes of Flavonoids
Class Structure Example (source)
FlavonesApigenin
(chamomile)
Flavan-3-ols
(catechins)
()-Epicatechin
(cocoa)
Flavan-3, 4-diolsLeucocyanidin
(peanut)
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Flavanones
Naringenin,
hesperidin (citrus
fruits)
Flavonols
Quercetin (apples,
grapes)
Flavanonols Taxifolin (Prunus)
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Isofiavones Genistein(soybeans)
Anthocyanidins
Pelargonidin,cyanidin,
delphinidin (berries,
red apples, red
grapes)
Chalcones Butein (Butea)
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Aurones Aureusidin (Oxalis)
FIGURE 4 Six anthocyanidins common in foods. The electric charge shown at position 1 is
delocalized over the entire structure by resonance.
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Generally, the attractive color of anthocyanin-pigmented 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.
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 redbrown
pigments. On continued polymerization these pigments become insoluble and form sediments in
bottled red wines.
b) Other FlavonoidsAmong 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.
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.
Anthocyanidins Present as Anthocyanins in Fruits and Vegetables
Fruit or vegetable Anthocyanidin
Apple (Malus pumila) Cyanidin
Blackberry (Rubus fructicosus) Cyanidin
Black currant (Ribes nigrum) Cyanidin. delphinidin
Blueberry (lowbush,Vaccinium Delphinidin, petunidin,
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angustifolium; highbush, malvidin, peonidin,
V. corymbosum) cyanidin
Cherry (sour, Montmorency, Prunus Cyanidin, peonidin
cerasus;sweet, Bing, P avium)
Cranberry (Vacinnium macrocarpon) Cyanidin, peonidin
Elderberry (Sambucus nigra) Cyanidin
Fig (Ficus carica) Cyanidin
Gooseberry (Ribes grossularia) Cyanidin
Grape (red European. Vitis vinifera) Malvidin, peonidin,
delphinidin, cyanidin,
petunidin, pelargonidin
Grape (Concord, Vitis labrusca) Cyanidin, delphinidin,
peonidin, malvidin,
petunidin
Mango (Mangifera indica) Peonidin
Mulberry (Morus nigra) Cyanidin
Olive (Olea europea) Cyanidin
Orange (Ruby, Citrus sinesis) Cyanidin. delphinidin
Passion fruit (Passiflora edulis) Delphinidin
Peach (Prunus persica) Cyanidin
Pear(Pyrus communis) Cyanidin
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Plum (Prunus domestica) Cyanidin, peonidin
Pomegranate (Punica granatum) Delphinidin, cyanidin
Raspberry (Rubus ideaus) Cyanidin
Strawberry (Fragaria chiloensis Pelargonidin, little cyanidin
and F. virginiaca)
Beans (red, black; Phaseolus Pelargonidin, cyanidin,
vulgaris) delphinidin
Cabbage (red, Brassica oleracea) Cyanidin
Corn (red, Zea mays) Cyanidin, pelargonidin
Eggplant (Solanum melongena) Delphinidin
Onion (Alium cepa) Cyanidin, peonidin
Potato (Solanum tuberosum) Pelargonidin, cyanidin,
delphinidin, petunidin
Radish (Raphanus sativus) Pelargonidin, cyaniding
Flavonols, like anthocyanidins, exist almost exclusively as glycosides. Flavonol glycosides
impart weak yellow hues to apples, apricots, cherries, cranberries, grapes, onions, plums,
potatoes, strawberries, tea, tomatoes, and other commodities.
The most common leucoanthocyanidins are leu-copelargonidin, 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).
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5. BetalainsBetalain is a relatively new term used to describe a class ofwater-soluble plant pigments
exemplified by the red-violet betacyanins and yellow betaxanthins. Betalains owe their name to
the red beet (Beta vulgaris), from which they were originally extracted. Other foods containing
betalains include chard, pokeberries, and Indian cactus fruits. The major red pigment of red beetsis betanin, and their major yellow pigment is vulgaxanthin.
Betalains are stable in the pH range 3.5-7.0, which is the pH range of most foods, but they are
sensitive to heat, oxidation, and light.
6. Miscellaneous Natural Food ColorsThere are several hundred additional natural pigments that are not as widely represented in
foods. Among them are the quinones and xanthones, which are yellow pigments. An example ofa quinone is juglone, which is present in walnuts and pecans. Mangiferin, a representative of
xan-thones, is found in mangoes. Tannins include two types of pale yellow to light brown
compounds, characterized by their property to convert animal hides to leather. One type consists
of condensed tannins, to which reference was made in relation to the leucoanthocyanidins, and
the other type comprises hydrolyzable tannins, which are esters of a sugar, usually glucose, with
gallic acid, ellagic acid, or both. Corilagin is an example of a gallotannin, in which glucose is
esterified with three gallic acid molecules. A yellow pigment that has attracted much attention
because of its toxicity to humans and nonruminant animals is gossypol. It is present in
cottonseeds, which are used as animal feed and have been considered a potential source of
protein for human use. Several biologically very important food constituents are colored, such as
phytochrome (yellow), vitamin B2 (riboflavin, orange-yellow), and vitamin B12 (red), although
their contribution to food coloration is negligible.