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Natural Food Pigments
Benjamin K. Simpson, Soottawat Benjakul, and Sappasith Klomklao
Introduction
The Major Natural Pigment Types
The Heme Pigments (Mb and Hb)
Structures and Functions
Mb and Meat Color
Discoloration of Meat
Heme Pigments and Health
Measurement of Mb
Carotenoid Pigments
Structures, Sources, and Functions
Properties and Uses of Carotenoids
Health Benefits of Carotenoids
Methods for Measuring Carotenoid
Pigments
Chlorophylls
Structure and Functions
Properties, Sources, and Uses
Chlorophylls and Health
Anthocyanins and Anthocyanidins
Properties, Structures, and Functions
Uses of Anthocyanins
Anthocyanins and Health
Measurement of Anthocyanins
Flavonoids
Properties, Structures, and Functions
Biological Functions and Uses
Flavonoids and Health
Betalains
Properties, Structures, and Functions
Uses of Betalains
Melanins
Properties and Functions
Melanins and Health
Measurement of Melanins
Tannins
Properties and Functions
Tannins and Health
Quinones
Properties and Functions
Quinones and Health
Xanthones
Properties and Functions
Other Pigments
Phycocyanin and Phycoerythrin
Summary/Concluding Remarks
References
Abstract: Nature has endowed living organisms with different pigments for a plethora of functions. These include enhancing the visual
appeal of the source material, a means for camouflage and concealments, as an index of food quality, and even as sex attractants.
The interest in natural pigments as food-processing aids derives
from the increasing consumer aversion to the use of chemicals and
synthetic compounds in foods. The chapter provides information on
the major types of natural food pigments and their major sources,
their functions and uses in food, their health benefits, and their fate
under various processing and storage conditions. The chapter also
provides information on their relative advantages over their counterparts perceived as artificial food colorants.
INTRODUCTION
Nature relies on a variety of compounds to impart colors to
living organisms and to carry out various functions. The colors may be red, purple, orange, brown, yellow, green, blue, or
their various shades. In the case of raw unprocessed foods, these
compounds or natural pigments enhance their visual appeal to
consumers who associate the colors of food with quality and
freshness. These natural compounds are widespread in animals,
plants, and microorganisms (including algae, fungi, and yeasts).
In all these organisms, these compounds display various shades
of black, blue, brown, green, orange, pink, red, or yellow colors.
Examples of fresh foods with dark or black colors are blackberries, grapes, black beans, black olives; those with brown colors
include seaweeds (brown algae), mushroom, cocoa, coffee, and
tea; some others with blue or purple colors include blueberries,
Food Biochemistry and Food Processing, Second Edition. Edited by Benjamin K. Simpson, Leo M.L. Nollet, Fidel Toldrá, Soottawat Benjakul, Gopinadhan Paliyath and Y.H. Hui.
C 2012 John Wiley & Sons, Inc. Published 2012 by John Wiley & Sons, Inc.
704
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37 Natural Food Pigments
aubergines, raisins, currants, plums, pomegranates, and prunes;
the kinds with green colors include asparagus, avocado, broccoli,
Brussels sprouts, cabbage, celery, cucumber, green beans, green
onion, kiwi fruit, lettuce, okra, spinach, and zucchini; examples
of the types with orange or yellow colors are crustacea (crabs,
lobster, scampi, and shrimps), salmonids (Arctic char, salmon,
and trout), carrots, apricots, yellow peppers, mangoes, lemon,
pumpkin, pineapple, squash; and the ones with shades of red
colors include several fruits and vegetables (e.g., beets, cranberries, raspberries, red bell peppers, strawberries, tomatoes, and
watermelon), meats (e.g., beef, hog, and poultry), and seaweeds
(red algae).
Although it is their visual appeal that first attracts the consumers’ attention to foods, these compounds also carry out a
plethora of key functions in living organisms. In certain species,
the compounds function as attractants or mating signals to gain
the attention of their mates; in some cases, the colors provide
camouflage against predators; some of the compounds participate in metabolic processes (e.g., chlorophylls or carotenoids
in biosynthetic reactions and energy generation); some others
serve as antioxidants and protect vulnerable biomolecules from
oxidative damage; some carry out a transport function (myoglobin (Mb) and hemoglobin (Hb) for oxygen (O2 ) transport
and energy generation); while others play a role in vision or
protect skins and membranes against the damaging effects of
sunlight and radiation. Thus, it is not surprising that interest in
these compounds extends beyond their visual appeal and consumer preferences in foods to also encompass other aspects such
as the health benefits they provide. Each of these interests in itself can form the subject matter of an entire book; thus, the focus
in this chapter will be on the major natural food pigments and
their sources, structures and functions, properties and uses, and
some of their important health benefits.
are colorless to yellow or brown colored pigments found in the
barks of certain trees like the oak or sumac; the quinones are a
group of pale yellow to dark brown or black pigments also found
in plants and microorganisms; and xanthones are yellow-colored
compounds found mostly in the plant families of Bonnetiaceae,
Clusiaceae, and Podostemaceae.
THE HEME PIGMENTS (MB AND HB)
Structures and Functions
The two main heme pigments of interest in foods are Mb and
Hb. They are responsible for the red color of meats. The principal function of these two pigments in animal tissues is O2
transport for the purpose of energy generation. Mb comprises a
single polypeptide chain called globin with a molecular weight
of approximately 16.4 kDa. This single protein chain is bound
to an essential nonprotein four-member ring compound known
as porphyrin (or tetrapyrrole) through a central iron (Fe) atom.
The Fe can exist in two oxidation states, the ferrous (Fe2+ ), and
the ferric (Fe3+ ) forms. Both of these forms have a coordination
number of six, giving heme the capacity to bind or accept up
to six ligands (Fig. 37.1). In both heme pigments (Mb and Hb),
the central Fe atom uses four (4) of its coordination positions to
bind to the four nitrogen (N) atoms in the porphyrin ring, and
its fifth coordination site to bind with a N atom of one of the
H
N
Pryol
THE MAJOR NATURAL PIGMENT TYPES
For the purpose of this discourse, the term major natural food
pigments is used to encompass the heme pigments (Mb and
Hb), carotenoids, chlorophylls, anthocyanins, flavonoids, betalains, melanin, tannins, quinones, and xanthones. The heme
pigments are the major pigments found in meats; they are watersoluble red, purplish or brownish compounds and they perform
O2 transport and energy generations functions in animal tissues;
the carotenoids are bright red, orange, and yellow fat-soluble and
water-insoluble pigments widespread in plants, animals, and microorganisms; the chlorophyll pigments tend to occur with the
carotenoids and are found in the plastids of photosynthetic organisms (i.e., plants, algae, and certain bacteria); the anthocyanins
are water-soluble red, blue, and violet pigments found in several
plants (fruits and vegetables) and microorganisms; flavonoids
(also known as anthoxanthins) are water-soluble pigments ranging from colorless to yellowish and are found in plant and microbial sources; the betalains are red and yellow water-soluble
pigments found extensively in plants; melanins are shades of
brown to dark colored pigments formed by the enzymatic oxidation of polyphenolic compounds and are responsible for the
dark colorations in animals, plants, and microorganisms; tannins
Globin
N
N
Tetrapyrolle ring
(structure of myoglobin)
N
N
Fe
N
N
H2O
Figure 37.1. Tetrapyrolle ring structure of heme pigments.
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histidine residues in globin. The sixth coordination site of the
central Fe atom is available for binding with several groups (e.g.,
CO2 , CO, CN, H2 O, H2 O2 , NO, O2 ). Which of these groups (or
ligands) can bind to the central Fe atom largely depends on the
oxidation state of the Fe. For example, :O2 , :NO, and :CO bind
to Fe in the Fe2+ state, while -CN, -OH -SH and H2 O2 , bind to
Fe in the Fe3+ form.
The synthesis of the heme pigments has initial stages that are
common to those of the chlorophyll pigments, and lead to the
formation of the tetrapyrrole or porphyrin ring structure. The
common initial steps (for the heme and chlorophyll pigments)
involve the formation of a 5-carbon intermediate compound,
5-aminolevulinic acid (dALA), from smaller molecules like
glycine and succinate or glutamate (Beale 1990). Next, two of the
dALA molecules combine and cyclicize to form the pyrrol ring
compound, porphobilinogen (PBG). Four molecules of the PBGs
subsequently form hydroxymethylbilane (HMB) via deamination and complexation reactions, and the HMB molecule formed
next undergoes dehydration to form the tetrapyrrole ring compound, uroporphyrinogen (UPP) (Jordan 1989). The heme pigments are formed from UPP via a series of enzyme catalyzed
reactions including the insertion of Fe into the tetrapyrrole structure (by ferrochelatase), and its eventual binding to the protein,
globin. The chlorophylls, on the other hand, are formed from
UPP via a series of enzyme-catalyzed reactions including chelation with magnesium (Mg) by magnesium chelatase, and attachment of the phytol side chain (from phytyl pyrophosphate by
chlorophyll synthetase).
In terms of concentrations or levels, Mb is the major pigment
of meat muscle. Approximately 80% of muscle pigment is Mb
with the balance (approximately 20%) as Hb. Mb traps O2 in
muscle cells as a complex known as oxymyoglobin (MbO2 )
for the purpose of producing energy for biological activity in a
continuous fashion. Hb, on the other hand, occurs mostly in the
blood vessels for O2 transport. Hb is a tetramer of four Mb units
and has a molecular weight of approximately 64 kDa. For this
chapter, the focus will be more on Mb than Hb.
state of the meat, the muscle protein in its native state imparts
a reddish/purplish color while the denatured form is pinkish to
brownish. Furthermore, exercised muscles tend to be darker in
color than nonexercised muscles. Thus, the color in the muscles
of the same animal can vary depending on the state of activity
of the animal. In terms of age, the Mb content tends to increase
as the animal gets older (Kim et al. 2003), and the color of
female foals were reported in a study to be darker than their
male counterparts (Sarriés and Beriain 2006), although Carrilho
et al. (2009) found male rabbit meat to be more intensely colored
than females rabbit meat. The diets of the animal also affect meat
color. For example, cattle raised on pasture has a darker meat
color than their counterparts raised on concentrates (Priolo et al.
2001), and there appears to be seasonal variations in meat color
with the color being darker in the winter season than in the spring
and autumn seasons (Kim et al. 2003).
The color of meat also depends to a large extent on the oxidation state of the Fe present in the porphyrin ring, as well as on
the nature of the ligand bound to the sixth coordination position
of the central Fe atom. When meats are cooked, the color of Mb
changes depending on the extent and/or intensity of the cooking.
For example, Mb retains its native color when dark meats are
cooked “rare.” However, medium cooking causes denaturation
of the protein group (globin) and oxidation of the central Fe2+
to the Fe3+ form, and changes the meat to a tan-colored product known as hemichrome. When meats are “well done,” the
amount of hemichrome formed increases and the meat acquires
a darker brown hue. When white meat is cooked, it changes from
a translucent pallor to an opaque or whitish color.
The interconversions between Mb, MbO2 , and MetMb are
summarized in Figure 37.2. O2 adds to purple colored Mb (Fe2+ )
by oxygenation to form the bright red colored MbO2 (with the
central Fe atom still in the Fe2+ form). MbO2 thus formed may
be deoxygenated to regenerate Mb, or oxidized via electron
loss to form the brown-colored metmyoglobin (MetMb, Fe3+ ).
MetMb may then undergo reduction by electron gain to re-form
Mb (Fig. 37.2).
Mb and Meat Color
Meats are classified into two types, that is, red (or dark) versus
white meats based on the muscle type and the Mb content. The
Mb content varies in cells; the higher the Mb content, the more
intense the red or dark color of the meat. Thus, red (or dark)
meats have relatively higher content of Mb than white meats.
This is because red or dark meat comprises muscles that require
more uptake of O2 to be able to generate a constant supply
of energy to carry out activities over protracted periods. White
meat, on the other hand, has muscle types with lesser Mb content
and relies on glycogen as the source of energy for rapid spurts
of activity for only brief periods of time. In addition to muscle
type, other factors like species, age, sex, and physical activity
all influence the Mb levels (and the color) of meat. For example,
fresh beef tends to be bright red in color, pork is pinkish, veal
is brownish pink, lamb and mutton are pale to brick red, poultry
(chicken, ducks, and turkeys) are white to dull red, and most fish
tend to range from white to gray. With regards to the physical
Reduced myoglobin
Oxygenation
Mb, Fe
Purple
Oxymyoglobin
(MbO2, Fe2+)
2+
Deoxygenation
Bright Red
Reduction
Oxidation
(Electron gain)
(Electron loss)
Metmyoglobin
(Met-Mb, Fe3+)
Brown
Figure 37.2. Interconversions between myoglobin (Mb),
oxymyoglobin (MbO2 ), and metmyoglobin (MetMb).
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Discoloration of Meat
CAROTENOID PIGMENTS
The color of meats is important because it is used by consumers
as an index of quality. In addition to the color changes ascribed
previously to the chemical state of the central Fe atom, other factors may also influence the meat color. For example, pale, soft,
and exudative (PSE) meats that are characterized by pale colors, soft textures and low water-holding capacities, tend to have
considerably more moisture on their surfaces. This is caused
by increased glycolysis and a rapid decline in muscle pH that
denature meat proteins to adversely impact the texture. Unlike
PSE meats, some meats may become dark, firm, and dry due to
relatively higher pH levels (>6.0), which make the meat proteins bind water molecules more tenaciously and cause the meat
surface to dry out, and reduce light absorption making the meat
color to darken. The elevated pH also promotes meat spoilage
from microbial proliferation and metabolism. An increase in
microbial activity could enhance production of compounds such
as amines (via deamination of amino acids) that can react with
nitrites (used to “cure” meats) to form nitrosamines in the meats
and/or impart a reddish color; or catalase negative bacteria (e.g.,
lactic acid bacteria) may produce H2 O2 to give meats a greenish tinge. Exposure of meat to light may cause O2 to dissociate
from MbO2 in meats to cause fresh meat color to become paler.
Antioxidant compounds, for example, vitamin E (α-tocopherol)
or vitamin C (ascorbic acid), may be applied to meats to curtail the oxidation of Mb and/or MbO2 to MetMb, and thereby
stabilize meat color. The use of nitrites to “cure” meats is primarily to safeguard against Clostridium botulinum, but the process
also imparts a pinkish color to meats. Vacuum packaging films
also affect the relative proportions of Mb, MbO2 , and MetMb
in packaged meats to impact the color from the differences
in O2 permeability or O2 barrier properties of the packaging
films.
Structures, Sources, and Functions
Heme Pigments and Health
Red meats are a good source of Fe (by virtue of Fe being part
of the heme). When Fe is deficient, it can cause a reduction in
cognitive abilities particularly in children, or result in anemia
(i.e., Fe deficiency anemia or IDA). The body requires Fe to
synthesize red blood cells and to regulate body temperature,
among other things. When a person has IDA, the red blood cell
levels decline and the victim’s health suffers. People with IDA
tend to feel cold on a continuing basis, because regulation of the
body’s temperature is impaired. Because it is part of Hb and Mb,
Fe-deficiency causes people to tire easily since their bodies have
insufficient O2 for both biosynthesis and energy generation.
Measurement of Mb
A number of procedures have been described in the literature
for the measurements of Mb. These include the optical density
method by atomic absorption spectroscopy (Weber et al. 1974),
differential scanning calorimetry (Chen and Chow 2001), spectrophotometry (Tang et al. 2004), and NIR spectrophotometry
(van Beek and Westerhof 1996).
707
Carotenoids are ubiquitous in nature and several different members (>400) have been identified in living organisms. One of the
best known naturally occurring carotenoid pigment is the yelloworange colored compound ß,ß-carotene, which was first crystallized from carrot as far back as in the early 1830s. It is from this
source material (carrot) that the name for the entire class of these
compounds (carotenoids) is derived. Most carotenoids are water
insoluble, but are fat or oil soluble and heat stable. They are found
in fruits and vegetables like oranges, tomatoes, and carrots, in
the yellow colors of many flowers, in animal species including crustacea (e.g., crab, lobster, scampi, shrimp), fishes (e.g.,
goldfish and salmonids like Arctic char, salmon, trout) in birds
(e.g., canaries, flamingos, and finches), and insects (phasmids,
lady bird, and moths); in seaweeds (e.g., Undaria pinnatifida,
Laminaria japonica); in yeasts (e.g., Rhodotorula rubra, Phaffia rhodozyma); in algae (e.g., Xanthophyceae, Chlorophyceae);
in fungi (e.g., Blakeslee, Xanthophyllomyces); and in bacteria (e.g., Corynebacterium autotrophicum, Rhodopseudomonas
spheroides). Carotenoids may occur in the free form (e.g., fucoxanthin in seaweeds), or complexed with proteins to form
the relatively more stable carotenoproteins as found in several
invertebrates (e.g., crustacea). When complexed with proteins,
the colors of carotenoids may be altered from orange, red, or
yellow to blue, green, or purple. In general, carotenoids are
present in low amounts in most organisms, but good sources
of this class of pigments include algae and plants (the predominant sources for humans), from fruits (cantaloupe, pumpkin, watermelon, pineapple, citrus fruits, red or yellow peppers,
tomatoes, mangoes, papaya, and guava) and vegetables (carrot, rhubarb, sweet potato, kale, parsley, cabbage, collards, and
spinach).
Carotenoid compounds are synthesized by plants, bacteria and
microalgae from the low molecular weight precursor molecules,
pyruvate and acetyl CoA, to initially form a 5-carbon intermediate compound known as isopentenyl pyrophosphate (IPP).
IPP has a molecular weight of 246.1 Da and is transformed
by a series of enzyme-assisted reactions to form the 40-carbon
polyunsaturated hydrocarbon compound known as phytoene
(C40 H64 ) with a molecular weight of 544.94 Da. Phytoene (Fig.
37.3) subsequently undergoes four desaturation steps to form
lycopene (C40 H56 ), a hydrocarbon carotenoid with thirteen double bonds (Fig. 37.3). Lycopene thus formed, may isomerize
and/or cyclicize to form various other hydrocarbon carotenoids
such as α-carotene, β-carotene, and γ -carotene. The hydrocarbon carotenoids may then be hydroxylated by hydrolases to
form their oxygenated counterparts such as lutein, astaxanthin,
canthaxanthin, cryptoxanthin, and zeaxanthin. Figure 37.3 has
the structures of some of the common carotenoid compounds.
Various raw materials contain different levels of carotenoids.
For example, lutein is abundant in many green plants; carrots
and most green fruits and vegetables are rich in α-carotene
and ß-carotenes; astaxanthin, canthaxanthin, and astacene are
the major carotenoids in crustacea and salmonids; astaxanthin
is also abundant in mushroom, algae, yeasts, and bird feathers
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Lycopene
Phytoene
β-Carotene
α-Carotene
γ-Carotene
Lutein
Astaxanthin
Figure 37.3. Structures of some common carotenoids (continued ).
(e.g., flamingos and finches); cryptoxanthin, antheraxanthin, and
zeaxanthin are the principal ones in maize; cryptoxanthin and
zeaxanthin are also predominant in papaya and cantaloupe; red
bell peppers have a high content of capsanthin; tomatoes have
high levels of phytoene, lycopene, ββ–carotene, neurosporene,
and phytofluene; and seaweed and many algae have fucoxanthin,
fucoxanthinol, and lactucaxanthin as the major ones.
Unlike plants and microorganisms, animals do not synthesize
carotenoid compounds de novo, and must derive these compounds from their diets for various processes, although animals
may transform dietary carotenoids to other forms for various
functions (Tanaka et al. 1976).
Carotenoids like α-carotene, β-carotene, zeaxanthin, and βcryptoxanthin are important sources of dietary vitamin A. These
molecules are cleaved by dioxygenase in the gastrointestinal
tract to release, at least a molecule of retinal, which is subsequently reduced to retinol (or vitamin A); thus, such carotenoid
compounds are classed as having provitamin A activity. Vitamin
A or retinol is needed in the retina of the eye for vision. Other
biological functions of carotenoids include their role in lightharvesting and energy transfer reactions in photosynthesis, and
in photo-protection of tissues against the ravaging effects of oxidation by exposure to light. The antioxidant role of carotenoids
is well known. As antioxidants, carotenoids protect membranes
and tissues from oxidative damage and prevent or minimize
serious illnesses such as cancer, heart disease, and nutritional
blindness. The antioxidant capacity of carotenoids enables them
to ward off degenerative diseases by “quenching” harmful free
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37 Natural Food Pigments
Astacene
Canthaxanthin
Tunaxanthin
Violaxanthin
Fucoxanthin
Figure 37.3. (Continued )
radicals and reactive O2 species such as singlet O2 from causing damage to cells or tissues. Evidence for this behavior by
carotenoids includes studies in men that showed that carotenoids
could reduce the risks of heart attacks significantly. The same
free radical “quenching” behavior of carotenoids enables them to
act as anticarcinogens, whereby they act to lower the incidence
of certain cancers (e.g., lycopene in tomatoes and prostate cancer). There is also evidence that carotenoids like lycopene suppress the oxidation of low-density lipoprotein (LDL) and helps
to lower blood cholesterol levels. Thus, carotenoids play crucial
roles in vision and are effective in slowing down age-related
eye disorders, and inhibiting oxidation of LDL that promotes
platelet formation and aggregation and cardiovascular diseases,
as well as free radical scavenging to prevent certain cancers and
membrane or tissues damage from light (Hadley et al. 2003).
Carotenoids also boost the immune system by increasing the
activities of lymphocytes, or protecting macrophages and the
immune system from damage by ultraviolet (UV) light and Xrays. However, the efficacy of antioxidants like β-carotene in
protecting against cancers has been questioned (Albanes et al.
1996), and it has even been suggested that it may actually increase the risk of lung cancer in heavy smokers, and cause a
higher incidence of cardiovascular diseases as well as high mortalities (Albanes et al. 1996).
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An oxygenated carotenoid pigment touted for its health benefits is fucoxanthin (Fig. 37.3) found in the brown algae (kelp).
Fucoxanthin has a molecular formula of C40 H60 06 , a molecular weight of 658.9 Da, and studies have shown it to induce
apoptosis and/or antiproliferative effects on certain cancer cells
(Hosokawa et al. 2004, Kotake-Nara et al. 2005). For these and
other reasons, there is intensive research afoot on the antioxidant
effects of carotenoids, and their role as regulators of the immune
system, and there is considerable interest in the production of
carotenoids by biotechnological approaches to increase supplies
for the demand for these compounds.
Properties and Uses of Carotenoids
The market for carotenoids is estimated at approximately a billion US$ per annum (Fraser and Bramley 2004). They are used
as colorants (e.g., β-carotene, annatto) to impart colors to many
processed foods (e.g., margarine, butter, dairy products, confectionery, and baked goods) beverages (e.g., soft drinks), drugs
and cosmetics (Britton 1996). They are also used as nutritional
supplements and in animal feeds, for example, as colorants for
farmed animals (e.g., poultry eggs, salmon, trout, shrimp, and
lobster) (Lorenz and Cysewksi 2000). Carotenoids (e.g., astaxanthin, lycopene, and zeaxanthin) can reduce the adverse side
effects such as inflammation and/or pain associated with administration of cyclooxygenase inhibitor drugs (Kearney et al.
2006).
Health Benefits of Carotenoids
The health benefits of carotenoids include protection against
cardiovascular diseases (by inhibiting the oxidation of LDL
cholesterol), certain cancers (e.g., cervical, gastrointestinal tract,
lung, skin, uterine), and age-related macular degeneration and
cataracts. Carotenoids inhibit the growth of unhealthy cells while
promoting the growth of healthy cells to invigorate the body.
Most are antioxidants and are able to absorb light/filter the UV
rays of the sun to reduce photo-oxidation, and thereby act to
protect cells and tissues against oxidative damage potentiated
by free radicals and reactive O2 species (Krinsky and Johnson
2005, Dembinska-Kiec 2005).
2008), resonance Raman spectroscopy (Bernstein et al. 2004),
and by UV/Vis and mass spectra chromatography (MeléndezMartı́nez et al. 2007).
CHLOROPHYLLS
Structure and Functions
Chlorophylls are fat and oil soluble green pigments that occur
in the plastids of most plants, algae, and certain bacteria. In
these organisms, chlorophylls participate in the biosynthesis of
complex biomolecules (C6 H12 O6 ) from simpler ones (CO2 and
H2 O) by the process of photosynthesis. They are the most widely
distributed natural plant pigments and are present in all green
leafy vegetables, and also occur abundantly in the green algae
known as chlorophytes. The chemical structure of chlorophylls
comprises a tetrapyrrole (or porphyrin) ring system similar to
those of Mb and Hb, except that the central atom of chlorophyll
is Mg, and the side chain is a 20 carbon hydrocarbon (phytol)
group that is esterified to one of the pyrrole rings (while Mb or
Hb has the protein, globin, bound to the fifth coordination position of the central Fe atom). Thus, four pyrrole groups are linked
together by a central Mg+2 ion to form a porphyrin ring, which
together with phytol (a 20-carbon hydrocarbon chain) makes
the chlorophyll molecule (Fig. 37.4). When foods containing
chlorophyll (e.g., vegetables) are cooked, the chlorophyll may
undergo changes in color and/or solubility. The colors that form
may be dull green, bright green, or brownish. These different
colors are due to changes in the chlorophyll molecule such as
loss of the phytol side chain, or the removal of the central Mg2+
Methods for Measuring Carotenoid
Pigments
Carotenoid pigments (as astaxanthin) may be measured quantitatively by measuring the absorbance at 485 m in a spectrophotometer (Saito and Regier 1971). Other methods that have
been used to study the molecular properties of carotenoids include field desorption mass spectrometry (Takaichi et al. 2003),
thin layer chromatography and analysis by gas chromatography
(Renstrom and Liaaen-Jensen 1981); high-pressure liquid chromatography (HPLC) (Yuan et al. 1996), or by negative ion liquid
chromatography-atmospheric pressure chemical ionization mass
spectrometry (negative ion LC-[APC]-MS) (Breithaupt 2004).
Other methods described for the measurement of carotenoids include reversed phase HPLC (Burri et al. 2003, Graça Dias et al.
Phytol
Figure 37.4. Structure of chlorophyll.
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37 Natural Food Pigments
Chlorophyll
+ weak acid
+ chlorophyllase / ROH
+ strong
acid
– Mg 2+
– phytol
– Mg 2+
– phytol
+ chlorophyllase
Pheophyn
+ alkali
& O2
+ acid
Pheophorbide
Chlorophyllide
– Mg 2+
– phytol
+ acid or
alkali & O2
Chlorin purpurins
Figure 37.5. Interconversions between chlorophyll pigments.
atom (Fig. 37.5). The phytol side chain may be removed by
the action of enzymes (chlorophyllase) and/or acidic conditions,
while Mg2+ may be removed from exposure to acid and heat
treatment. It is the 20 carbon hydrocarbon side chain (phytol)
that makes chlorophylls nonpolar and water insoluble. So, its
removal from the molecule renders the residual compound water soluble. Alkaline conditions and exposure to light can also
induce changes in the chlorophyll molecule (Fig. 37.5).
Because of their light trapping and electron transfer roles in
photosynthesis, chlorophyll pigments are considered as the basis
of all plant life. Chlorophylls are synthesized via the formation
of a porphyrin ring, and the initial steps in the biosynthesis
pathway are similar to those of the heme pigments, Mb and Hb,
described in Section “Structures and Functions.” Chlorophylls
exist in nature in different forms (e.g., chlorophylls a, b, c1 , c2 ,
and d). The differences in these forms arise from subtle changes
in either the tetrapyrrole ring itself or in the side chain groups.
For example, chlorophylls a and b differ only in the substituent
group on C7 of the molecule (i.e., CH3 for chlorophyll a, and
CH O for chlorophyll b), while chlorophylls a and b both
have CH CH2 group at the C3 position unlike chlorophyll d
that has a CH O group.
Chlorophyll is commonly extracted from plant species like
spinach, alfalfa, nettles, or grass using organic solvents (e.g.,
acetone and hexane), preferably in dim light to minimize degradation of the pigment. Exposure to light, air, heat, and extreme
pH all adversely affect its stability. Nevertheless, the stability
of chlorophylls against degradation may be enhanced by deesterification of the chlorophyll and complexation with copper
ions.
Properties, Sources, and Uses
Chlorophylls are safe for human consumption and since ancient
times humans have ingested chlorophylls in their fruits and vegetables. Commercial sources of chlorophylls include the green
algae chlorella, the blue-green algae Spirulina, the string lettuce
(Enteromorpha) and the sea lettuce (Ulva); all the sources mentioned previously are used as human food (Ayehunie et al. 1996)
in salads and soups. Chlorophylls are used as food colorant and
in this regard, can be safely added to several foods either in the
pure form or complexed with copper. They are also used in several other applications including cosmetics (soaps, creams, and
body lotions), oral hygiene products (mouthwash, toothpaste) as
well as confectionary (gums and candies) because of their intense green color. Chlorophyll-colored products are best stored
under dry conditions and protection from air, light, and heat
to better retain the color. Certain countries have restrictions on
the use of copper complexes of chlorophylls in foods and drugs
because of the toxicity of copper.
Chlorophyll content in tissues and food materials may be
measured spectrophotometrically by the AOCS (2004) official
method AK 2–92.
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Chlorophylls and Health
Chlorophylls also have several health benefits for humans. They
have been shown to be capable of rebuilding the bloodstream
(Patek 1936), and to be nontoxic and without harmful side effects even when administered in large doses in various routes
(intravenous, intramuscular, or oral). The high Mg content in
chlorophyll promotes fertility by increasing the levels and activities of the enzymes that regulate sex hormones. Chlorophylls
have antibacterial properties and can be used both inside and
outside the body for this purpose (Bowers 1947). They are able
to clean out drug deposits and deactivate toxins in the body,
cleanse the liver, and reduce problems associated with blood
sugar (Colio and Babb 1948). They are also used in deodorizers, to inhibit oral bacterial infections, promote healing of rectal
sores, and reduce typhoid fevers (Offenkrantz 1950).
ANTHOCYANINS AND ANTHOCYANIDINS
Properties, Structures, and Functions
Anthocyanins are water-soluble, pH-sensitive, red, purple, and
blue pigments found in higher plants. They impart the intricate colors to many flowers, fruits, and vegetables. They also
occur in leaves, stems, and roots of plants particularly in the
outer cells of these parts of the plant. They are found in almost all the plant families, and in fruits and vegetables the main
sources include avocados, blackberries, black currants, blueberries, cherries, chokeberries, cranberries, grapes, prunes, oranges,
mangos, aubergines, olives, onions, and sweet potatoes. Anthocyanins (and anthocyanidins) may be considered as members
of the flavonoid family of compounds (Section “Flavonoids”),
but because of their importance and the extensive attention they
have received, a separate section is devoted in this chapter to the
discussion of these compounds.
The color of anthocyanins is influenced by acidity; they are
red under acidic conditions and turn blue at neutral to alkaline pHs. On the basis of the chemical structures, two types are
distinguished; the anthocyanidin aglycones (or anthocyanidins)
that are devoid of sugar moieties, and the true anthocyanins are
glycosides or sugar esters of the anthocyanidins. They all have
a single basic core structure, the flavylium ion (Fig. 37.6) that
has seven different side groups (denoted as R in the figure) that
may be a hydrogen (H) atom, a hydroxide ( OH) group, or a
methoxy (-OCH3 ) group. The sugars of anthocyanins are esterified to the different side groups, and because there are seven of
these positions and several sugars (monosaccharides, disaccharides, and trisaccharides) in higher plants, it is to be expected
that several different anthocyanins would occur in nature. Furthermore, the sugars may be acylated with the organic acids that
are naturally present in plants (e.g., acetate, citrate, ascorbate,
succinate, propionate, malate, and many others) to form acylated
anthocyanins. The sugar residues may be monosaccharides, disaccharides, or trisaccharides. When there is only one monosaccharide residue esterified to the aglycone, the anthocyanin may
be referred to as a monoside; where esterification is to either
two monosaccharide units or one disaccharide unit, the product
is known as a bioside; where there are three monosaccharides,
or two disaccharides and one monosaccharide, or a single trisaccharide unit esterified, the compound is referred to as a trioside.
Those without sugar molecules are known as aglycones. Some
of the common anthocyanidins include: cyanidin, delphinidin,
pelargonidin, malvidin, peonidin, and petunidin; and some of the
known anthocyanin glycosides are: cyanindin-3-glucoside and
pelargonidin-3-glucoside, as well as the glycosides and diglucosides of cyanidin, pelargonidin, delphinidin, and malvidin (Fig.
37.6). Their biological functions include their action as antioxidants whereby they protect plants against the damaging effects
of reactive O2 species and UV radiation, and also serve as attractant for birds and insects to flowers for pollination.
Uses of Anthocyanins
Anthocyanins are used as food colorant in drinks, food spreads
(jams and jellies), jellos, pastries, and confectioneries because
of their intense blue or red colorations. There are, however, some
drawbacks in the use of anthocyanins in foods. This is due to
their high solubility in water and vulnerability to pH changes.
Thus, red cabbage may exhibit a deep reddish color or a purplish or bluish color depending on the pH of the milieu. Other
factors such as exposure to high temperature, oxidizing agents
(tocopherols, peroxides, and quinones), air, or light are detrimental to the stability of and colors imparted by anthocyanins.
Although the sugars in anthocyanins may provide some protection for the molecule against degradation under ambient conditions, the same sugar moieties could elicit browning reactions in
food products when the latter are exposed to high temperatures
and/or light. The detrimental effects of temperature and air may
be slowed down by low temperature storage, with protection
from light and air/O2 .
Anthocyanins and Health
The toxicity of the anthocyanins is not well documented. But
they are believed to be nontoxic even at high concentrations.
The compounds are potent antioxidants and have been associated with health benefits such as reduced risk of coronary
heart disease, improved visual acuity, and antiviral activity.
Studies with pelargonidin and delphinidin (both anthocyanins)
have shown both of them to be capable of inhibiting aldoreductase, the enzyme that converts glucose into fructose and sorbitol (Varma and Kinoshita 1976). Inhibition of this enzyme
is desirable in preventing complications from diabetes because
without the inhibition, the sorbitol and fructose formed could
diffuse into the myelin sheath along with fluid (by osmotic action) to cause edema of the myelin sheath in nerves (Levin et al.
1980). Thus, substances like pelargonidin and delphinidin that
can inhibit the aldoreductase enzymes would be useful in reducing this type of complications for diabetics. The anthocyanins
petunidin, delphinidin, and malvidin have also been shown
to enhance the activity of glutamate decarboxylase (GAD),
the enzyme that catalyzes the decarboxylation of glutamic
acid (HOOC CH2 CH2 CH(NH2 ) COOH) to γ -amino benzoic acid (HOOC CH2 CH2 CH2 NH2 ) or GABA (for short).
GABA is an important neurotransmitter in the brain, and an
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37 Natural Food Pigments
Basic flavylium ion
Malvidin
Delphinidin
Petunidin
Cyanidin
Peonidin
Pelargonidin
Figure 37.6. Structures of common anthocyanins.
increase in the level of GAD is useful not only because of the
formation of GABA, but also because it can maintain insulin production in people with type 1 diabetes (Ludvigsson et al. 2008).
Petunidin, delphinidin, and malvidin can also inhibit glycerol dehydrogenase, a key enzyme in glycerolipid metabolism (Carpenter et al. 1967), and malate dehydrogenase that participate in both
the tricarboxylic acid (TCA) cycle (i.e., the dehydrogenation of
malate to oxaloacetate) and in gluconeogenesis (i.e., the synthesis of glucose from noncarbohydrate carbon sources) (Carpenter
et al. 1967). Thus, by inhibiting malate dehydrogenase, antioxidants also act indirectly to regulate gluconeogenesis.
Measurement of Anthocyanins
Several methods have been reported for the measurement of anthocyanins in samples. These include spectrophotometric pro-
cedures for measuring anthocyanins in wines (Cliff et al. 2007),
and the use of HPLC and ultra performance liquid chromatography to measure the pigment levels in food samples (Hosseinian
and Li 2008). Other procedures for measuring anthocyanins include pH-differential measurements (Giusti and Wrolstad 2001)
and FTIR (Soriano et al. 2007).
FLAVONOIDS
Properties, Structures, and Functions
The term flavanoids is used here to refer to a family of compounds produced by plants that are sometimes referred to as
anthoxanthins. They are water soluble but have lower coloring
power compared to the anthocyanins or anthocyanidins; they
range in color from colorless to shades of yellow, and they
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HO
O
Genistein
2-Phenyl-1,4-benzopyrone
(The basic backbone
structure of flavonoids)
O
O
HO
O
OH
OH
OH
HO
O
OH
B
Myricetin
OH
A
HO
O
Naringenin
OH
OH O
O
OH
OH
OH
HO
O
Quercetin
B
OH
A
OH O
H3C
HO
OH
HO
O
OH
O
HO
HO
OH
O
O
Naringin
O
O
OH O
OH
OH
B
Kaempferol
A
OH
OH
OH O
OCH3
HO
Hesperetin
O
OH
HO
O
OH
Catechin
O
OH
OH
OH
OH
HO
HO
O
H 3C
OH
OH
O
HO
O
Apigenin
OH
O
HO
HO
OCH3
O
O
Hesperidin
O
OH
OH O
Figure 37.7. Structures of common flavonoids.
have a common core structure, the benzopyrone ring (Fig. 37.7),
which bears a structural resemblance to the flavylium ion of
anthocyanins or anthocyanidins (Fig. 37.6). Flavonoids are subdivided into five sub-classes: the flavanols, the flavanones, the
flavonols, the flavones, and the isoflavones. These five groups of
compounds exhibit subtle differences in structure, such as degree of saturation or unsaturation, and/or position of substituent
groups (Fig. 37.7). Examples of the flavanols are catechin, epi-
catechin, epigallocatechin, epicatechin gallate, epigallocatechin
gallate, and theaflavins; examples of the flavanones are butin,
hesperidin, hesperetin, naringenin and naringin; the flavonols
include azaleatin, quercitin, kaempferol, myricetin, morin, and
rhamnetin; members of the flavones family include apigenin,
baicalein, chrysin, luteolin, tangeritin, and wogonin; and the
isoflavones are typified by daidzein, genistein, and glycitein.
The flavonoids have lower coloring power compared with the
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anthocyanins or anthocyanidins and range from colorless to
shades of yellow. Some common food sources at flavonoids are
apples, broccoli, citrus fruits, chocolate, cocoa, grapes, green
tea, oolong tea, parsley, red peppers, white tea, yellow onions,
celery, soybeans, and thyme.
Like anthocyanins, some flavonoids are esterified with one
or more sugar residues. Apart from flavanols (catechins and
proanthocyanidins), the other known flavonoids appear to occur in plants and foodstuffs in the esterified forms. A lot of
these flavonoid glycosides appear to survive the cooking process, as well as the preliminary digestion steps in the mouth and
stomach. However, in the small intestines, it appears only the
aglycone forms and the flavonoid glucosides are absorbed and
metabolized to form various secondary metabolites; and colon
bacteria metabolize flavonoids to facilitate absorption (Manach
et al. 2004). Nevertheless, the flavonoids have low bioavailability due to inadequate absorption and their fast removal from
the body. Of the flavonoids, the isoflavones are said to be the
most bioavailable while the flavanols are the least bioavailable
(Manach et al. 2005).
715
also known to inhibit the enzyme 3-hydroxy-3-methylglutarylcoenzyme A reductase (HMG-CoA reductase), the regulatory
enzyme in the mevalonic acid pathway that leads to the formation of isoprenoid compounds, for example, carotenes, cholesterol, and steroids. The effects of these compounds on drug
metabolism have been shown to affect the bioavailability and
toxicity of drugs such as amiodarone, atorvastatin, cyclosporine,
saquinavir, and sildenafil (Bailey and Dresser 2004, Dahan and
Altman 2004). It is partly for this reason that patients are cautioned to avoid taking grapefruit juice when they are on some
of these drugs, as grapefruit juice has high levels of quercetin
and naringenin, as well as furanocoumarins. Copious consumption of purple grape juice has also been found to inhibit platelet
aggregation, and the inhibition has been attributed to the high
levels of flavonoids in the juice (Polagruto et al. 2003). On the
basis of this, it has been speculated that high intake of some of
these flavonoids in conjunction with anticoagulants could make
some patients more prone to bleeding (Polagruto et al. 2003).
BETALAINS
Biological Functions and Uses
Properties, Structures, and Functions
Flavonoids also have antioxidant properties and are effective
free radical scavengers (Chun et al. 2003). However, most circulating flavonoids are believed to be metabolized intermediates or
by-products that have relatively lower antioxidant capacity than
the parent flavonoid compounds. Thus, the relative contribution
of dietary flavonoids to antioxidant behavior in the body is suggested to be miniscule (Lotito and Frei 2006). By virtue of their
polyphenolic nature, flavonoids are also able to act as metal ion
chelators to stop ions such as Cu+ /Cu2+ and Fe2+ /Fe3+ from
promoting the formation and release of free radicals (Mira et al.
2002). The chelating behavior of flavonoids could also adversely
affect intestinal absorption of Fe, thus it is recommended for Fe
supplements not to be taken together with beverages high in
flavonoids or flavonoids supplements (Zijp et al. 2000).
The name betalain is derived from beet vegetable (Beta vulgaris)
from which it was first extracted. They are red or violet and yellow or orange water-soluble aromatic indole pigments found
in plants and fungi (Basidiomycetes). Betalains are subdivided
into two categories based on their colors as betacyanins (reddish
to violet) and betaxanthins (yellowish to orange). Examples of
the betacyanins are amaranthine, isoamaranthine, betanin, isobetanin, phyllocactin, and iso-phyllocactin; and representatives
of betaxanthins include dopaxanthin, miraxanthin, indicaxanthin, portulacaxanthin, portulaxanthin, and vulgaxanthin. The
betalains are synthesized from tyrosine via several routes. The
amino acid tyrosine may be converted to betalains via either
dihydroxy phenylalanine (l-DOPA) or dopaquinone; or through
spontaneous combination with betalamic acid to an intermediate
portulacaxanthin II (Tanaka et al. 2008). The structures of betanin and vulgaxanthin are shown in Figure 37.8. The betalains
are most stable within the pH range of 3.5 and 7.0, but they are
susceptible to light, heat and air.
Flavonoids and Health
Although various suggestions have been made to the effect that
high intakes of flavonoids from plant sources do not elicit adverse effects in humans due to their low bioavailability and rapid
metabolism and removal from the body, there are some reports
indicating that high doses of some flavonoids can cause adverse
health effects in some people. For example, some flavonoids like
quercitin have been reported to induce nausea, headaches and/or
tingling sensations in some people, or renal toxicity or vomiting,
sweating, flushing, and dyspnea in certain cancer patients (Ferry
et al. 1996, Shoskes et al. 1999). Other reports attribute liver toxicity, abdominal pain, diarrhea insomnia, tremors, dizziness, and
confusion sometimes observed from intake of caffeinated green
tea extracts to the flavonoids present in tea (Jatoi et al. 2003,
Bonkovsky 2006). The flavonoids quercitin and naringenin have
also been reported to inhibit cytochrome P450 that is involved in
the oxidation of various biomolecules such as lipids, hormones,
and xenobiotics (drugs) (Bailey and Dresser 2004). They are
Uses of Betalains
Extracts from betalains have been used to color foods since time
immemorial, and beetroot extract is currently approved for use
as food color in many countries to impart color to juices and
wines. In addition, different varieties of beet have been used as a
source of sugar (sugar beet), as vegetable (beetroot), and fodder
(fodder beet). Beetroot tuber is cooked and eaten like potatoes,
and it is pickled and used in salads.
Because of their high solubility in water and susceptibility to
light and heat, the use of betalains to color food may be limited,
although the dried form of betanin is quite stable. High sugar
content has also been shown to impart protection to betalains,
thus they are used in soft drink powders, frozen dairy products, and in confectioneries. Gentile et al. (2004) reported that
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OH
OH
H
OH
H
H
HO
O
H
O
OH
N+
HO
Betanin
O–
OH
N
H
O
O
OH
NH2
O
O
N
Vulgaxanthin
OH
HO
O
N
H
O
OH
O
+
N
O–
Indicaxanthin
HO
O
N
H
by polymerization to form the large molecular weight melanins.
The enzymes catalyzing the oxidation of the phenolic compounds are polyphenoloxidases, also known as phenolases or
tyrosinases. In animals and humans, melanins occur in the eyes,
hair, skins, and peritoneal lining where they protect tissues from
the detrimental effects of factors like light or UV radiation from
the sun’s rays. Damage from UV radiation is potentiated in various ways such as dimerization of thymine in DNA molecules, or
by the effects of reactive O2 species (e.g., singlet O2 and superoxide radicals (Jagger 1985, Tyrell 1991). Examples of melanins
are eumelanin and pheomelanin. Pheomelanin is dark red-brown
in color and is thought to be responsible for red hair and freckles
in humans, while eumelanin imparts black and brown coloring
to skin and hair (Meredith and Riesz 2004).
Microorganisms synthesize melanins as part of their normal
metabolism, from where they are passed along the food chain
to higher forms of life. In crustacea, the formation of melanins
is sometimes referred to as melanosis or “blackspot” formation,
and connotes spoilage to consumers—mainly due to their lack
of visual appeal. The “blackspot” phenomenon has been extensively studied to develop strategies to control this undesirable
effect and products such as glucose oxidase, glucose, catalase
mixes, or 4-hexyl resorcinol were found to be effective in controlling blackspot formation in raw shrimp (Ogawa et al. 1984,
Ferrer et al. 1989, Yan et al. 1989, Chen et al. 1993, Benjakul
et al. 2005). Nevertheless, these same compounds are desirable
in other situations, such as protecting against UV radiation from
sunlight to prevent damage to the skin and also to minimize glare
in the eyes. This feature of melanin has been exploited by PhotoProtective Technologies (United States) in making a variety
of eye wear that filter out colors to reduce the risks of macular
degeneration and cataracts (Anon 2005). Melanin has also been
incorporated in a hair dye, Melancor-NH, for darkening gray
hair in humans.
O
OH
Melanins and Health
Figure 37.8. Structures of some common betalains.
betalains have anticancer properties by virtue of their antioxidant and free radical scavenging behavior. They have been
shown to inhibit redox state alteration induced by cytokines
to protect the endothelium layer that line the internal surfaces of
blood vessels. The content of betalains in samples may be measured by the HPLC and LC-MS/MS analyses (Stintzing et al.
2006).
MELANINS
Properties and Functions
Melanins are dark brown and black pigments found in plants,
animals, and microorganisms. They are formed via the enzymatic oxidation of phenolic compounds, for example, tyrosine,
through several intermediates like DOPA and quinones, followed
Melanin deficiency in humans has been linked with certain abnormalities and diseases. Examples include oculocutaneous albinism that is characterized by reduced levels of the pigment in
the eyes, hair, and skin, and ocular albinism that affects both eye
pigmentation and visual acuity (Peracha et al. 2008). Melanin
deficiency has also been linked to deafness (Cable et al. 1994)
and Parkinson’s disease (Nicolaus 2005).
Measurement of Melanins
Melanin measurements may be carried out by spectrofluorometry (Rosenthal et al. 1973) by measuring the fluorescence induced by excitation at 410 nm and emission at 500 nm, or by
spectrophotometric measurement of the rate of melanochrome
production from 5,6-dihydroxyindole-2-carboxylic acid and 5,6dihydroxyindole systems at 540 and 500 nm (Blarzino et al.
1999). Melanin in tissues may also be measured based on spectrophotometry, HPLC, and electron spin resonance spectroscopy
(Ito 2006, Hu et al. 1995).
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TANNINS
Properties and Functions
Tannins are a heterogeneous group of water-soluble polyphenolic compounds present in many plants. The term “tannin”
originated from the ancient practice of using plant extracts to
“tan” leather, thus the name “tanning” is used to describe the
process. This process makes use of the capacity of tannins to
form cross-links with other biomolecules such as proteins or
polysaccharides to “toughen” the textures of otherwise soft materials. Tannins occur mainly in tea, coffee, immature fruits,
leaves, and the bark of trees where their function is believed
to be one of defense. They have an astringent and bitter taste
that is unpleasant to prospective consumers. The best known
members of this family of compounds include tannic acid and
its breakdown products (ellagic acid, gallic acid, and propyl gallate; Fig. 37.9). Tannic acid is used as a food additive. Foods
known to be high in tannins include pomegranates, persimmons,
cranberries, strawberries and blueberries, hazelnuts, walnuts,
smoked meats and smoked fish, spices (e.g., cloves, tarragon,
thyme, and cinnamon), most legumes, and beverages such as
wine, cocoa, chocolate, and tea.
Tannins are classified into two main classes, (1) the hydrolyzable and (2) the condensed (nonhydrolyzable) tannins. The hydrolyzable tannins are those that on heating with dilute mineral
acids or bases form carbohydrates and phenolic acids (i.e., gallic or egallic acid). The condensed tannins form phlobaphenes
(e.g., phloroglucinol) when heated with dilute acids or bases. It
OH
HO
Gallic acid
OH
O
OH
O
HO
O
HO
OH
O
Ellagic acid
717
is the condensed tannins that are more important in alcoholic
beverages such as wines.
Some tannins such as tannic acid and propyl gallate display
antimicrobial activity toward foodborne bacteria, aquatic bacteria and off-flavor-producing microorganisms, e.g., tannic acid
reduces staphylococcal activity (Akiyama et al. 2001). Thus,
they can also be used in food processing to extend the shelf-life
of some food products.
Tannins and Health
Some of the health benefits of tannins include their antiinflammatory, antiseptic, and astringent properties. They also
are effective against diarrhea, irritable bowel syndrome, and
skin disorders. However, tannins are metal ion chelators and
their binding to metal irons make these mineral elements not
readily bioavailable. High intake of tannins could hamper the
absorption of minerals such as Fe to result in anemia. They can
also cause protein precipitation to interfere with the uptake of nutrients in some ruminants. Foods with high levels of tannins are
judged to be of poor nutritional value, because they reduce feed
intake, protein digestibility, feed efficiency and growth rates,
among other things (Chung et al. 1998). These observations
have been linked to tannins decreasing the efficiency in converting absorbed nutrients into new body components (Chung
et al. 1998). The intake of foods high in tannins such as herbal
teas is also thought to increase the incidence of cancers (e.g.,
esophageal cancer) and this has led to the suggestion that these
compounds may be carcinogenic, but other reports attribute the
apparent carcinogenic activity observed to constituents associated with tannins and not the tannins per se (Chung et al. 1998).
Some of these observations contradict findings alluding to the
anticarcinogenic behavior of tannins (Nepka 2009), or their capacity to reduce the mutagenicity of certain compounds (Chen
and Chung 2000). The anticarcinogenic and antimutagenic activities of tannins and related compounds have been attributed
to their polyphenolic nature that enables them to behave as antioxidants to check lipid peroxidation (Hong et al. 1995) and
the damaging effects of reactive O2 species (Nakagawa and
Yokozawa 2002) in biological tissues. Tannins are also reported
to reduce blood clotting, blood pressure, and serum lipid levels
(Akiyama et al. 2001). They also induce liver necrosis (Goel and
Agrawal 1981), and modulate immunoresponses (Marzo et al.
1990).
OH
O
QUINONES
Properties and Functions
OH
HO
Propyl gallate
O
HO
O
Figure 37.9. Structures of some common tannins.
Quinones are a class of heterocyclic organic compounds with
structures like cyclic conjugated diones. They have been described as secondary metabolites and are found in several organisms including flowering plants, bacteria, fungi, and to a
small extent in animals like insects, sea urchins, corals, and
other marine animals (Thompson 1991). The compounds display a plethora of colors from yellow to orange to intense
purple to black. Examples of quinones include anthraquinone,
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are involved in electron transfer reactions between photosystems
1 and 2 in photosynthesis.
O
O
1,2-Benzoquinone
O
1,4-Benzoquinone
O
O
1,2-Napthaquinone
O
O
OH
Lawsone
Quinones and Health
Quinones and related compounds (e.g., tertiary butyl hydroquinone or TBHQ) have antioxidant properties and the terpenes of hydroquinone are thought to be more potent than
the more common antioxidants such as α-tocopherol (DelgadoVargas and Paredes-López 2003). Some other quinones like the
napthoquinones have antibacterial activity and can inhibit various strains of aerobic and anaerobic microorganisms (Didry
et al. 1986, Riffel et al. 2002). Some have also been shown to
have anticancer or antitumor activities (Smith 1985). For example, dietary intake of certain quinone derivatives was reported
to play a role in the prevention of breast cancer (Zhang et al.
2009). Ironically, certain quinones and their derivatives have
also been shown to be mutagenic or carcinogenic (Zhang et al.
2009). In this regard, it is reported that extended exposure to
some of these compounds increases the incidence of breast cancer. Some metabolic products of quinones are also known to be
toxic, and the toxicity appears to arise from either the arylation of sulfhydryls compounds or the production of free radicals
and/or reactive O2 species (Smith 1985).
O
O
XANTHONES
OH
OH
Alizarin
O
Properties and Functions
Xanthones are a group of heat stable organic compounds with
9H-xanthen-9-one (xanthone) as the core structure (Fig. 37.11).
Xanthones occur naturally in tropical plants and about two
Figure 37.10. Structures of some common quinones.
O
1,2-benzo- and 1,4-benzo- quinones and naphthaloquinones
(Fig. 37.10). Because of their high coloring power, quinones
are used as dyes.
Quinones are used for the commercial scale production of hydrogen peroxide (H2 O2 ) by the hydrogenation of anthraquinones
to hydroquinones, and subsequent transfer of hydrogen (H2 ) to
O2 (Schreyer et al. 1972, Drelinkiewicz and Waksmundzka-Góra
2006).
For example:
Xanthone
(a.k.a. 9H-xanthen-9-one)
O
OH
Xanthydrol
O
2-ethyl-9,10-anthraquinone + H2
→ 2-ethyl-9,10-anthrahydroquinone
dihydroanthraquinone + O2 → anthraquinone + H2 O2
Quinones can also react and form conjugates with amino
acids, and the products formed can impact the flavor and color
of foods (Bittner 2006).
In nature, some quinones serve as important constituents of
biomolecules (e.g., vitamin K1 ); some like ubiquinone and plastoquinone in mitochondria participate in electron transfer reactions in aerobic respiration, while plastoquinone in chloroplasts
H3C
O
Mangostin
OH
O
HO
O
OH
Figure 37.11. Structures of some common xanthones.
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hundred known compounds in this group have been identified.
Well-known examples are α-mangostin, β-mangostin, garcinone
B, and garcinone E from the pericarp of mangosteen fruit. Xanthones are said to enhance the body’s immune system and also
impart other human health benefits such as promoting healthy
cardiovascular or respiratory systems, supporting cartilage and
joint function (Weecharangsan et al. 2006). Other health benefits
attributed to xanthones include their capacity to reduce: allergies, cholesterol levels, inflammation, skin disorders and fatigue,
and they also curb susceptibility to infections and help maintain
gastrointestinal health (Weecharangsan et al. 2006). They have
antioxidant properties and they do this by deactivating free radicals in the body. In this regard, they are said to be even more
potent antioxidants than vitamins E and C.
Some applications of xanthones include their use as insecticide and it has been used in this regard against moth eggs and
larvae. Xanthone in the form of xanthydrol is also used to verify
urea levels in the blood.
OTHER PIGMENTS
Apart from the pigments described above, there are other lesser
known natural pigments that are also found in various living organisms. Examples of these include the blue and red chromoproteins (phycocyanin and phycoerythrin) found in the blue-green
algae and cyanobacteria. There are also other pigments such as
flavins and pterins.
Phycocyanin and Phycoerythrin
Phycocyanin is blue pigment found in the blue-green algae
and cyanobacteria, while phycoerythrin refers to red pigment
found in the red algae commonly referred to as rhodophytes.
These pigments are conjugated chromoproteins and they comprise of subunits each with a protein backbone that is covalently linked to open chain tetrapyrrole groups. Their molecular
weights range from 44,000 daltons (monomers) to 260,000 daltons (hexamers) (Boussiba and Richmond 1979), and they function as light-absorbing substances together with chlorophyll in
photosynthesis.
An example of useful red algae is the nori or Porphyra that is
consumed as food and is used as wraps for sushi and in several
other Japanese dishes. These pigments are produced commercially from Spirulina platensis. Spirulina are blue-green algae
and they grow well in warm climates and alkaline waters. The
two best known members of the Spirulina species are S. platensis and Spirulina maxima. S. platensis is cultivated in California
while S. maxima are cultivated in Mexico. Phycocyanin and related compounds are thought to have antiviral and anticancer
activities as well as immune system stimulatory ability. Phycocyanins also enhance the formation and development of red
blood cells to reduce the incidence of anemia (Mathew et al.
1995, Jensen and Ginsberg 2000, Mani et al. 2000, Jensen et al.
2001, Samuels et al. 2002, Shih et al. 2003), and also thought
to promote the development of healthy skins. Thus, it is used to
treat eczema and psoriasis and some other skin disorders. These
pigments occur together with carotenoids and chlorophylls in
719
the blue-green and red algae. Phycoerythrin, in particular, is believed to facilitate red seaweed subsistence at greater depths in
the ocean where hydrostatic pressures are high, unlike the other
seaweed species (e.g., brown and green algae) that subsist in
shallow waters.
The antioxidant, free radical scavenging, and antiinflammatory properties of phycocyanin, as well as antiviral
activity of the pigment have been demonstrated in studies with
laboratory animals (Bhat and Madyastha 2000). Other studies
have also revealed protective effects by phycocyanin against
neuronal damage and oxidative damage to DNA (Bhat and
Madyastha 2001, Piñero Estrada et al. 2001). Phycocyanin is
used as a natural food-coloring agent in food products such as
yoghurts, milk shakes and ice creams, beverages, desserts, and
also in cosmetic products. Phycocyanin is used in medicine to
facilitate selective destruction of atherosclerotic plaques or cancer cells by radiation with little or no damage to the surrounding
cells or tissue (Morcos and Henry 1989), while phycoerythrin is
used as fluorescent dyes for fluorescence activated cell scanning
analysis (Hardy 1986).
Other pigments include curcumin found in turmeric. Curcumin is an oil soluble pigment and tends to fade when exposed
to light, but it is heat stable. It is used to impart lemon yellow
colors in food products such as curry, soups, and confectionery.
Riboflavin (or vitamin B2), is a water-soluble and heat stable
compound with an intense yellow color. It is used to color and
fortify foods like dairy products, cereals, and dessert mixes.
Carmine (or carminic acid, C22 H20 O13 ) is a water-soluble pigment obtained from the female cochineal insect (Dactylopius
coccus). It is stable to heat and light, and resistant to oxidation;
and is used in alcoholic beverages and processed meat products.
It is also used as an antirepellant.
SUMMARY/CONCLUDING REMARKS
For some time, interest in synthetic food colors was high because
of the perceived disadvantages with natural pigments. These
disadvantages include inconsistency in the product quality and
yields from batch to batch, susceptibility to factors such as heat,
pH, light, and air, as well as the high cost. However, pressure now
seems to be increasing for less use of synthetic pigments as food
colorants. Consumers are showing higher preference for natural
pigments in foods for health safety reasons, and recent studies
published in various scientific journals, including the Lancet,
linked synthetic colors with hyperactivity in children (Harley
et al. 1978, McCann et al. 2007). In addition, food regulations
in Europe are promoting the use of natural pigments in foods
over their synthetic counterparts. And the United States Food
and Drug Administration has been petitioned to ban the use
of synthetic colors like Yellow 5 and 6, Red 3 and 40, Blue 1
and 2, Green 3, and Orange B in foods, and to require labels on
foods that contain these dyes. In addition to their coloring power,
some of these natural pigments have also been shown to have
more potent antioxidant and free radical scavenging capabilities
than the synthetic ones commonly used in foods (Frankel et al.
1996). It is estimated that the world market for food colors is in
excess of a billion dollars (United States) annually, and natural
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pigments account for about 30% of the market. It is predicted that
the market share of natural pigments will continue to grow and
surpass that of synthetic colors due to the increasing consumer
concerns with the latter.
Thus, what needs to be done is more research to address
the disadvantages with the commercial production and use of
natural food pigments. Sources of the major food colors (red,
blue, and yellow) must be identified; procedures to recover them
efficiently and in purer and more stable forms, such as enzymeassisted treatments (Kim et al. 2005) need to be developed; and
treatments and/or conditions to stabilize them such as use in
combination with antioxidants need to be exploited.
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