Page 34 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

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

106 Food Colors

color. In certain instances, the original color of the food of blood, is composed of four heme groups attached to
must be preserved, as is the case with most fruits and veg- four polypeptide chains.
etables. In other instances, culinary art is required to create The myoglobin in meat is subject to chemical and color
new, pleasing colors, as when turkey is roasted, bread is changes. Freshly cut meat looks purplish. On exposure
baked, or potato chips are fried. In still other instances, col- to air, the surface of the meat acquires a more pleasing
ors (colorants) are added to foods, as is done with many red hue (blooming of the cut). The color change is due
beverages and candies. to the oxygenation of myoglobin (an oxygen molecule is
The coloring matter of foods is discussed under three attached to the heme group in a fashion parallel to the oxy-
headings: natural food colors, food browning, and food genation of hemoglobin). The oxygenated myoglobin is
color additives. called oxymyoglobin. When meat is packed in plastic film,
the oxygen permeability of the film should be sufficient to
keep the myoglobin oxygenated. In both myoglobin and
II. NATURAL FOOD PIGMENTS oxymyoglobin the heme iron is in the Fe 2+ form. In the
presence of oxygen, myoglobin is eventually oxidized to
Approximately 1500 colored compounds, also known as brown metmyoglobin, in which the heme iron is in the
natural food pigments, have been isolated from foodstuffs. Fe 3+ form. Both the oxygenation and oxidation processes
On the basis of their chemical structure, these food pig- are reversible. Severe oxidative deterioration may result in
ments can be grouped in the following six classes: heme the formation of green pigments (sulfmyoglobin, chole-
pigments,chlorophylls,carotenoids,flavonoids,betalains, myoglobin).
and miscellaneous pigments. 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. Heme Pigments
a brown hue. These changes are irreversible. Heated meat
Heme (from the Greek for blood) is the basic chemical is also subject to the browning reactions discussed in Sec-
structure (Fig. 1) responsible for the red color of two im- tion III. A simplified scheme of the red-pigment changes
portant animal pigments: hemoglobin, the red pigment of in fresh and heated meat is shown in Fig. 2.
blood, and myoglobin, the red pigment of muscles. Prac- In cured meats, in which nitrite is used, many reactions
tically all the red color of red meat is due to myoglobin, occur, some of which lead to color changes. Among the
since the hemoglobin is removed with the bleeding of the established reactions are the following: (1) the nitrite salt
slaughteredanimal.Other colored musclecompounds (cy- is converted to nitric oxide (NO), nitrate, and water; (2)
tochromes, vitamin B 12 , flavoproteins) do not contribute the NO replaces the H 2 O attached to the iron of heme
significantly to the color of red meat. and forms nitrosyl myoglobin, which is reddish; (3) on
Myoglobin is a protein that facilitates the transfer of heating, the nitrosyl myoglobin is transformed to nitrosyl
oxygen in muscles. It was the first protein to be fully elu- hemochrome, which has the familiar pink color of cured
cidated with regard to the three-dimensional arrangement meats;and(4)anymetmyoglobinpresentinthecuredmeat
of its atoms. Hemoglobin, the oxygen-carrying pigment is similarly nitrosylated, reduced, and finally converted to
nitrosyl hemochrome.

B. Chlorophylls
Several 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. An important
difference is that the central metal atom is iron in heme
and magnesium in the chlorophylls. Another difference
is that the pyrrole unit IV in the chlorophylls is hydro-
genated. In addition, the chlorophylls contain a 20-carbon
hydrophobic “tail,” the phytyl group (Fig. 3).
The chlorophylls are located in special cellular bodies,
FIGURE 1 Structure of heme. the chloroplasts, where they function as photosynthetic