approximately 1500 colored compounds

7/28/2019 Approximately 1500 Colored Compounds

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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.

approximately 1500 colored compounds

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