Page 35 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

P1: FPP Final
Encyclopedia of Physical Science and Technology EN006C-254 June 28, 2001 19:52

Food Colors 107

FIGURE 2 Pigment changes in fresh and heated red meat.

agents. As food pigments, chlorophylls impart their green C. Carotenoids
color to many leafy (spinach, lettuce, etc.) and nonleafy
Many of the yellow, orange, and red colors of plants
(green beans and peas, asparagus, etc.) vegetables and to
and animals are due to carotenoids, pigments similar to
unripe fruits. They are not very stable pigments, however.
those of carrots. The basic structure of carotenoids is a
Ethylene, a gaseous plant hormone, destroys chlorophylls,
chain of eight isoprenoid units. Certain isoprenoid deriva-
and it is occasionally used to degreen fruits. The acids
tives with shorter chains (e.g., vitamin A) are also con-
naturally present, formed, or added to plant tissues dur-
sidered carotenoids. Most of the structural differences
ing food processing convert the bright green chlorophylls
among carotenoids exist at the ends of the chain. Some
to dull olive brown pheophytins by replacing the magne-
carotenoids are hydrocarbons and are known as carotenes,
sium of the molecule with hydrogen. Unfortunately, no
while others contain oxygen and are called xanthophylls.
fail-safe procedure has been proposed for preventing this
The structures of several carotenoids, along with the foods
discoloration in heated and stored green vegetables. Freez-
or tissues in which they are present, are shown in Table I.
ing storage is an effective method of preserving the green
Becauseofthenumerousdoublebondsinthecarotenoid
color of vegetables.
molecule, a large number of cistrans isomers are theoret-
ically possible. The carotenoids of foods, however, are
usually in the all-trans form (Table I). Trans to cis trans-
formation is possible and is accelerated by heat, light, and
acidity.
Carotenoids occur free or as esters of fatty acids or
as complexes with proteins and carbohydrates; for exam-
ple, 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.
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 sub-
stances, carotenoids tend to concentrate in tissues or prod-
ucts rich in lipids, such as egg yolk and skin fat, vegetable
oils, and fish oils.
Plants and microorganisms synthesize their own
carotenoids, while animals appear to obtain theirs
from primary producers. In the development of many
FIGURE 3 Structure of chlorophylls a and b. [From Aronoff, S.
(1966). In “The Chlorophylls” (L. P. Vernon and G. R. Seeley, eds.), fruits (e.g., citrus fruits, apricots, tomatoes) ripening is
Academic Press, New York.] associated with the accumulation of carotenoids and the