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protective effect was largely related to less ferment-

able and more insoluble fibers, especially wheat bran.

However, a recent prospective study found a reduced

risk of distal colon adenoma with increasing intake of

fiber from fruit but not cereals or vegetables. This

result suggests that soluble fiber is more protective.

0019 Studies in humans have not yet provided a convin-

cing answer as to whether or not fiber is truly protect-

ive. In the Health Professionals Follow-up Study, a

large prospective cohort trial, dietary fiber was in-

versely associated with a risk of developing colorectal

adenomas in men, and all sources of fiber were asso-

ciated with a decreased risk of adenoma. However,

data from the Nurses’ Health Study, a parallel study

in women by the same investigators, failed to indicate

any protective effect of dietary fiber on the develop-

ment of colorectal cancer. A subsequent analysis of

the latter cohort, with 10 additional years of follow-

up and more thorough adjustment for confounding

factors, also showed no protective effect in women

with regard to adenomas or cancer. These large

prospective epidemiologic studies raise the question

as to whether the protective effect of dietary fiber

against colorectal cancer may be a gender-specific

phenomenon.

0020 A few human intervention trials have demon-

strated that wheat-bran supplementation favorably

alters a number of biomarkers related to the risk of

colorectal cancer, including fecal mutagenecity, fecal

secondary bile acids, and rectal cell proliferation.

However, two recent well-performed trials which

utilized the most widely accepted intermediary bio-

marker of colon cancer, development of adenomatous

polyps, have both yielded negative results: neither a

daily dietary supplement containing 13.5 g of wheat

bran fiber nor a low-fat, high-fiber diet over 3–4 years

reduced the incidence of recurrent adenomas. The

results of these two recent studies strongly suggest

that dietary supplementation of fiber is not an effect-

ive chemopreventive strategy for the prevention of

colorectal adenoma recurrence. Nevertheless, the

results of these two trials should not be construed as

definitive evidence that fiber is ineffective: the follow-

up in these trials was relatively brief, and the research

question that was addressed (adenoma prevention) is

related, but not identical, to the question of cancer

prevention.

Antioxidant Nutrients

0021 The antioxidant micronutrients, including vitamin A,

carotenoids, vitamin C, vitamin E, selenium, zinc,

copper, and manganese, are part of the body’s defense

against free radicals and reactive oxygen species.

These antioxidants are thought to possibly convey

protective effects against a number of chronic degen-

erative diseases through a host of different mechan-

isms. In some animal studies, vitamins A, C, and E

have been shown to have a direct inhibitory effect on

tumor growth and damage to both cellular mem-

branes and DNA. However, the epidemiologic litera-

ture for the protective effect of antioxidant vitamins

in colorectal cancer has little in the way of a consen-

sus. Although early clinical intervention trials sug-

gested a possible preventive role for vitamin E, two

recent prospective randomized trials with a more

definitive endpoint (adenoma recurrence) have shown

no benefit.

0022Selenium is an essential trace mineral found in

cereal grains and seafood. Epidemiologic studies in-

dicate an increased incidence of colorectal cancer in

humans in geographic regions where selenium levels

are low in the soil and a lower mortality rate of color-

ectal cancer in areas with high soil selenium levels.

The selenium content of foods is determined by the

soil in which the food is grown, so assessment of

individual dietary intakes using a food content data-

base is of limited value. Results from one study indi-

cate that patients with low plasma selenium levels are

at four times the risk for colorectal adenomas. Selen-

ium may alter carcinogen metabolism, affect immune

function, protect against oxidant stress, inhibit cell

proliferation, and impair tumor metabolism. The

most compelling data to data are from a randomized,

placebo-controlled intervention trial, where daily

supplementation of 200 mg of selenium reduced the

incidence of colorectal cancer by 60%.

Folate

0023Findings from animal and epidemiologic studies indi-

cate that decreased dietary intake of folate or reduced

serum or RBC folate levels increases the incidence of

colorectal cancer and adenomas. This relationship is

underscored in patients with ulcerative colitis, where

folate deficiency is commonly induced by increased

requirements related to the high turnover of the

colonic epithelium, reduced oral intake of nutritious

diets, and reduced intestinal absorption resulting

from competitive inhibition of folate absorption due

to sulfasalazine. Two case-control studies found that

folate intake may protect against neoplasia in ulcera-

tive colitis, and the overwhelming consensus of over

15 epidemiologic studies in the general population

suggests folate is equally protective in sporadic colo-

rectal cancer. Results from a recent follow-up of the

Nurses’ Health Study confirmed that colorectal

cancer risk was reduced with increased folate intake,

as had previously been shown with adenoma recur-

rence. Interestingly, the study suggested that more

1546 COLON/Cancer of the Colon

than 10 years of increased intake were necessary to

observe a protective effect. Folate deficiency also has

been considered as an important factor in alcohol-

related enhancement of rectal carcinogenesis because

alcohol alters normal folate metabolism in a variety

of ways. Although several large multicenter trials are

underway to definitively determine the chemopreven-

tive role of folate supplementation in colorectal

carcinogenesis, dietary folate intake appears to be

inversely associated with the risk of developing colo-

rectal adenomas and cancer.

0024 The mechanism(s) by which folate depletion en-

hances colorectal carcinogenesis have not been

clearly defined, although several candidate mechan-

isms have been proposed. Folate is an essential factor

in a number of critical cellular metabolic pathways

that involve the transfer of one-carbon groups, in-

cluding the de novo biosynthesis of deoxyribonucleo-

tides and many biological methylation processes.

Therefore, folate deficiency could contribute to DNA

damage, instability of the chromatin, impairment of

DNA repair, or aberrant patterns of genomic, and/or

gene-specific DNA methylation, all of which are

phenomena thought to enhance carcinogenesis.

0025 Recently, the relationship of a common poly-

morphism (C677T) of the methylenetetrahydrofolate

reductase (MTHFR) gene (homozygous in 10–15%

of the US population) and the risk of colorectal cancer

has been examined. MTHFR catalyzes the irrevers-

ible conversion of 5,10 methylenetetrahydrofolate to

5-methyltetrahydrofolate. Men with this homozy-

gous mutation have half the risk of colorectal cancer

than do homozygous wild-type or heterozygous geno-

types. Among men with adequate folate levels, a

threefold decrease in risk was observed, but protec-

tion associated with the mutation was largely absent

in men with a low systemic folate status in the

Physician’s Health Study. It has been suggested

that the cancer protective effect of the MTHFR

mutation is related to the increased availability of

5,10 methyleneTHF, and therefore increased ease

of nucleotide synthesis. One might speculate that,

under low but not high folate conditions, the avail-

ability of 5-methyltrahydrofolate for biological meth-

ylation constitutes a more critical determinant of

whether the cell is pushed down the pathway towards

neoplasia.

Calcium, Vitamin D, and Dairy Foods

0026 Many epidemiologic studies suggest an inverse rela-

tionship between the risk of colorectal cancer and

dietary calcium and/or vitamin D intake. However,

the results of epidemiologic research have been incon-

sistent, even regarding adenomas.

0027Human clinical trials have been rather promising,

although the magnitude of the protective effects have

been rather modest. A recent blinded study demon-

strated that the administration of low-fat dairy foods,

which can double the calcium intake, significantly

improved indices of proliferation and differentiation

markers in the colonic mucosa. Furthermore, calcium

carbonate supplementation (3 g (1200 mg of elemen-

tal calcium) daily) provided a significant reduction

(* 15%) in the risk of recurrent colorectal adenomas

as well as reduced numbers of adenomas in a random-

ized, double-blind trial recently reported by the

Calcium Polyp Prevention Study Group. The effect

of calcium supplementation was independent of ini-

tial dietary fat and dietary calcium intake.

0028The underlying mechanisms are not clear, but the

principal hypothesis is that calcium may precipitate

fatty acids and bile acids that are potentially toxic to

the colorectal epithelium. Both calcium administra-

tion and dairy food administration diminish concen-

trations of bile acids and fatty acids in the aqueous

fraction of the feces, and this is accompanied by a

significant decrease in in vitro cytotoxicity. Animal

studies suggest that supplemental calcium can retard

the hyperproliferative stimulus of dietary fat, bile

salts, and fatty acids.

Iron

0029Iron, a necessary nutrient, has been associated with

increased cancer risk in humans. In vitro and in vivo

data suggest that iron may be capable of mutagenic

effects mediated through free-radical generation or

tumor promotion through nutritional mechanisms.

As a transition metal, iron possesses loosely bound

electrons that are capable of participating in lipid

peroxidation reactions. Such reactions are thought

to lead to DNA damage and, in some cases, neoplasia.

0030Prospectively gathered data from a large study sug-

gest that the risk of colon cancer increases in parallel

with iron intake throughout the colon for both men

and women. A nested case control study similarly

showed an increasing trend of risk of cancer of the

proximal colon with increasing total iron intake.

However, increased body iron stores were not related

to the development of colorectal cancer. Even though

increased iron intake is related to increased red-meat

consumption, the putative relationship between iron

intake and colorectal cancer risk remains rather ten-

tative at this point that requires further confirmation.

Heterocyclic Amines

0031Heterocyclic amines (HCAs) are the carcinogenic and

mutagenic chemicals formed from cooking muscle

COLON/Cancer of the Colon 1547

meats such as beef, pork, fowl, and fish. HCAs form

when amino acids and creatine react at high cooking

temperatures and are formed in greater quantities

when meats are overcooked or blackened. Seventeen

different HCAs resulting from the cooking of muscle

meats have been identified, which may pose human

cancer risk.

0032 HCAs develop a powerful carcinogenic potential

when activated by cytochrome P450 1A2-mediated

oxidation of the amino group, which is followed by

acetylation or sulfation to form direct-acting reactive

mutagens that attack key elements in DNA. When

fed to rats, several of these HCAs can induce

cancer of the mammary glands, colon, and pancreas.

Reduction of HCAs in food can be accomplished

by avoiding high-temperature cooking, addition

of high-antioxidant-containing foods, or addition of

soy protein. Nevertheless, there is no definitive

evidence that the degree to which meat is cooked is

a determinant of colorectal cancer risk.

Alcohol

0033 A large number of epidemiologic studies have exam-

ined the association between the risk of colorectal

cancer and alcohol consumption. The results are not

entirely consistent. Although there are studies show-

ing either increased risk or no association with alco-

hol intake, there are essentially no studies that show a

reduced risk with higher intake.

0034 Alcohol, particularly beer, consumption has been

linked most convincingly with rectal cancer. How-

ever, there is no strong evidence to suggest that one

source of alcohol (e.g., beer) is more associated with

risk than alcohol as a whole. Studies of colorectal

adenomas, which are colorectal cancer precursors,

also have demonstrated an effect for alcohol. The

mechanism for an increased risk with alcohol is not

known for sure, but the carcinogenicity of acetalde-

hyde as well as folate and methionine depletion

are plausible explanations. Although alcohol is not

a direct-acting carcinogen, beer, wines and spirits

contain at least 1200 different compounds, such as

aldehydes, phenols, and amines, many of which are

thought to be related to carcinogenesis.

Tea

0035 Tea is grown in about 30 countries and, next to water,

is the most widely consumed beverage in the world.

Black tea is consumed primarily in Western countries

and in some Asian countries, whereas green tea is

consumed primarily in China, Japan, India, and a

few countries in North Africa and the Middle East.

Most of the studies showing the preventive effects of

tea have been conducted with green tea; only a few

studies have assessed the usefulness of black tea.

Green tea contains polyphenolic compounds, which

include flavanoids, flavandiols, flavonoids, and

phenolic acids. In black teas, the major polyphenols

are theaflavin and thearubigin.

0036()-Epigallocatechin-3-gallate (EGCG) is the pri-

mary component of green tea, accounting for 40% of

the total polyphenolic mixture. Numerous in vitro,

human, and animal studies have identified these anti-

oxidant polyphenols, in particular EGCG, as cancer-

chemopreventive agents. EGCG inhibits cancer cell

growth and represses the catalytic activities of several

P450 enzymes such as P450 1A and 2B1. Repression

of phase I enzymes involved in cancer initiation, and

enhancement of enzymes that play a role in carcino-

gens detoxification, are thought to guard against

carcinogenesis.

0037Animal studies show that tea consumption protects

against colon cancer induced by chemical carcino-

gens. A cross-sectional study showed a 30% lower

risk of adenomatous polyps of the sigmoid colon in

persons consuming five or more cups of green tea per

day. However, human studies on tea drinking and

colorectal cancer are still inconclusive.

Soy

0038Epidemiologic data suggest that populations that

regularly consume soy foods have a lower incidence

of colon cancer than populations that do not. Soy is

abundant in isoflavones such as genistein and daid-

zein, which, because of their structural similarity

to estrogen and/or their biological activity, are

considered to be phytoestrogens. Several other

cancer-related properties have also been reported for

soy-derived genistein, such as antioxidant activity,

an antipromotional effect, inhibition of tyrosine

kinase, inhibition of cell-cycle progression and

growth and inhibition of angiogenesis. Studies in

laboratory animals have further shown that a soy-

rich diet inhibits chemically induced carcinogenesis.

However, animal studies of the effects of single

phytochemicals from soy on intestinal cancer have

yielded conflicting results. The relationship between

soy intake and the risk of colonic cancer still needs

further investigation.

Dietary Recommendations

0039Diet is important in preventing colorectal cancer.

With our current knowledge regarding the dietary

risk factors of colorectal cancer, the following sugges-

tions are offered: (1) avoid overweight (BMI > 26), (2)

reduce total fat intake to less than 25–30% of total

1548 COLON/Cancer of the Colon

calories and saturated fat to less than 10% of

total calories, (3) eat five or more servings of fresh

vegetables and fruits daily, (4) eat other foods from

plant sources, such as breads, cereals, grain products,

rice, pasta, or beans several times each day, (5) choose

foods low in fat, (6) replace red meat with chicken,

fish, nuts, and legumes, (7) eat processed meat in

moderation, (8) eat low-fat dairy products, (9) if

you drink, keep alcohol consumption moderate (< 2

drinks per day), and (10) consider taking a multiple

vitamin containing folic acid, particularly if alcohol is

consumed daily. These recommendations are based

on the guidelines by the American Cancer Society

and currently accepted dietary modulations for the

prevention of colorectal cancer (Tables 1 and 2).

0040Daily supplementation with fiber (20–30 g day

1

calcium (1200 mg day

1

), folate (400 mg day

1

), fish

oils, and selenium (200 mg day

1

) have not been

proven by prospective randomized control trials

to prevent cancer but have considerable potential

(Table 1).

See also: Alcohol: Properties and Determination;

Amines; Calcium: Properties and Determination;

tbl0001 Table 1 Dietary components that may prevent colorectal cancer

Food or nutrient Proposed mechanism Common diet source Estimated effect-

ive dose

Calcium

Reduces bile acid/fatty-acid-induced irritation in

colon as well as proliferation

Dairy products, dried beans, kale,

broccoli

1200 mg day

1

Vitamin D Reduces cell proliferation and DNA synthesis,

modulates signal transduction pathway, induces

apoptosis, promotes repair of membranes and

DNA

Fortified milk, salmon, mackerel, fortified

cereals, egg yolks

400 IU day

1

Selenium

Reduces risk of excessive oxidative DNA damage

and cell toxicity, induces apoptosis, reduces

proliferation, restores immune function

Meats, fish, grains, brazil nuts 200 mg day

1

o-3 fatty acids Inhibit arachidonic acid metabolism and oxidative

reactions, reduce proliferation

Tuna, salmon, sardines, mackerel, and

lake trout

3–4 g day

1

Folate

Corrects imbalance of DNA synthesis and DNA

methylation, enhances DNA repair

Fortified cereals, citrus fruits and juices,

asparagus, spinach, baked beans

400 mg day

1

Fiber Increases stool bulk, binds with bile acids and other

potential carcinogens, reduces fecal pH and

anaerobic fecal flora, ferments to short-chain

fatty acids

Whole grains, legumes, wheat bran,

nuts, barley, beans

20–30 g day

1

Vegetables

and fruits

Block the formation of carcinogens, induce a

multiplicity of solubilizing and activating

enzymes, suppress DNA and protein synthesis

5 more servings

day

1

Tea Antioxidant activity, represses phase I enzymes

involved in cancer initiation, and enhances

enzymes involved in carcinogen detoxification

Green tea 5 more cups

day

1

Soy Antioxidant activity, antipromotional activity,

inhibits cell-cycle progression and growth, and

inhibits angiogenesis

Soy bean, tofu, soy milk

Existing data are particularly compelling.

tbl0002 Table 2 Dietary components that may enhance the risk of colorectal cancer

Food or nutrient Proposed mechanism Common diet source

Animal fat, calories,

and red meat

Increasing DNA damage and/or promoting

carcinogenesis

Beef, lamb, pork

Heterocyclic amines Direct-acting reactive mutagens that attack key

elements in DNA

Meats cooked at very high

temperature (fry, broil, barbecue)

Alcohol Folate and methionine depletion and carcinogenicity of

acetaldehyde, phenol, and amines

Beer, wine, spirit

Iron DNA damage by lipid peroxidation reactions and

mutagenic effects through free-radical generation

Red meat

Existing data are particularly compelling.

COLON/Cancer of the Colon 1549

Cholecalciferol: Properties and Determination; Energy:

Intake and Energy Requirements; Epidemiology; Fats:

Requirements; Iron: Properties and Determination; Meat:

Nutritional Value; Soy (Soya) Beans: Dietary Importance;

Tea: Chemistry

Further Reading

Alberts DS, Martinez ME, Roe DJ et al. (2000) Lack of

effect of a high-fiber cereal supplement on the recurrence

of colorectal adenomas. New England Journal of

Medicine 342: 1156–1162.

Baron JA, Beach M, Mandel JS et al. (1999) Calcium sup-

plements for the prevention of colorectal adenomas.

New England Journal Medicine 340: 101–107.

Choi SW and Mason JB (2000) Nutritional chemopreven-

tion of colorectal cancer. Clinical Perspectives in Gastro-

enterology 3: 259–266.

Franceschi S, Parpinel M, Vecchia CL et al. (1998) Role of

different types of vegetables and fruit in the prevention

of cancer of the colon, rectum, and breast. Epidemiology

9: 338–341.

Lipkin M, Reddy B, Newmark H and Lamprecht SA (1999)

Dietary factors in human colorectal cancer. Annual

Review of Nutrition 19: 545–586.

Nelson RL, Persky V and Turyk M (1999) Determination

of factors responsible for the declining incidence of

colorectal cancer. Diseases of the Colon and Rectum

42: 741–752.

Schatzkin A, Lanza E, Corle D et al. (2000) Lack of effect of

a low-fat, high-fiber diet on the recurrence of colorectal

adenomas. New England Journal of Medicine 342:

1149–1155.

Slattery ML, Edwards SL, Boucher KM, Anderson K and

Caan BJ (1999) Lifestyle and colon cancer: An assess-

ment of factors associated with risk. American Journal

of Epidemiology 150: 869–877.

Potter JD (1999) Colorectal cancer: Molecules and popula-

tions. Journal of the National Cancer Institute 91:

916–932.

COLORANTS (COLOURANTS)

Contents

Properties and Determination of Natural Pigments

Properties and Determinants of Synthetic Pigments

Properties and Determination

of Natural Pigments

A Lea, The University, White Knights, Reading, UK

B Henry, Phytone Ltd, Burton-on-Trent, UK

Background

0001 Natural colors in foodstuffs may arise in basically

three ways. They may be present originally in the

plant or animal tissue from which the food is derived

(e.g. chlorophyl in green vegetables), they may be

generated during processing (e.g., brown Maillard

products in fried onions), or they may be added

deliberately (e.g., b-carotene in orange squash). This

article considers the major pigments in these three

groups. (See Browning: Nonenzymatic; Carotenoids:

Occurrence, Properties and Determination; Chloro-

phyl.)

Naturally Occurring Pigments

0002The range of compounds responsible for the natural

colors of foodstuffs is surprisingly limited. Plants are

the primary sources, yielding anthocyanins, carote-

noids, and chlorophyls as the major groupings, with

limited contributions from compounds such as beta-

nins and curcumin.

Anthocyanins

0003The anthocyanins are responsible for most of the red,

purple, and blue colors in plant foodstuffs. The skins

of red apples, plums, and grapes are rich in antho-

cyanins, as are strawberries, rhubarb stalks, and red

cabbage leaves. Chemically, most anthocyanins are

based on the six common benzopyrilium aglycones

(anthocyanidins) shown in Figure 1. These aglycones

are not stable, and the 250 or so anthocyanins which

are known in nature reflect the fact that the aglycone

is stabilized by substitution with different sugars at

different positions. Substitution at the 3 position is

1550 COLORANTS (COLOURANTS)/Properties and Determination of Natural Pigments

most widespread, but 5 and 7 substitution is also

common. The substituent sugars may be simple

(e.g., glucose, galactose) or complex and unusual

(e.g., rutinose, sambubiose). The sugars themselves

are sometimes acylated with other complex organic

acids – grapes, for instance, contain significant quan-

tities of the caffeic and p-coumaric acid conjugates of

malvidin 3-glucoside. The polar nature of anthocya-

nins means that they are soluble only in water or

polar organic solvents – they are not associated with

lipid in the plant cells and they are not soluble in oils

or in organic solvents such as chloroform or hexane.

0004 The color chemistry of anthocyanins is complex.

The benzopyrilium structure can exist in four major

equilibrium forms depending upon pH (Figure 2).

The flavylium salt is the most stable and intensely

colored, predominating at low pH. Hence the well-

known observation that red cabbage pigment is in-

tense and stable when cooked in the presence of acid,

but rapidly turns blue and degrades at a pH near or

above neutrality. The blue color is due to the gener-

ation of the quinoidal base (which is stable under

nitrogen, even at pH 10), which rapidly degrades in

the presence of oxygen.

0005The perceived color of anthocyanins is also highly

dependent on copigmentation effects. In the concen-

trated environment of intact plant cells, the proximity

of pectin, metal ions, organic acids, and other poly-

phenols act to stabilize and to augment anthocyanin

color to a remarkable degree. Hence the skins of red

grapes seem intensely dark blue or black but, when

the pigment is extracted by ethanol during wine-

making, the native anthocyanin is revealed as bright

red. The visible absorbance maximum of isolated

anthocyanins is somewhat dependent on structure,

moving from 506 nm (orange) for pelargonidin de-

rivatives to 534 nm (purple-red) for malvidin, delphi-

nidin, and petunidin. A very few anthocyanins are

naturally blue, even in the extracted state, due

to internal copigmentation with their own sugar-

acylating groups.

0006Even at low pH, the anthocyanins are not stable

indefinitely, and they tend to polymerize with other

polyphenols or reactive carbonyls. After only a few

months’ aging, therefore, very little monomeric

anthocyanin remains in most red wines and the spe-

cific E

520

drops dramatically. The overall visual chro-

mophore is only slightly affected by polymerization,

however, giving a color shift from purple to red. In

these circumstances, tristimulus colorimetry is a far

more accurate measure of perceived color than is a

spot measurement at 520 nm.

Carotenoids

0007The carotenoids are a range of lipid-associated pig-

ments widely present in higher plants and algae. They

are responsible for many of the orange-yellow colors

of fruits and vegetables such as red peppers, toma-

toes, carrots, and bananas. They are also responsible

for the yellow skin ground color of apples such as

Golden Delicious. The structure of carotenoids is

more diverse than that of anthocyanins and does not

fall into such well-defined groups. The basic unit is a

hydrocarbon structure and those consisting of

carbon and hydrogen only are collectively called

carotenes, whilst those containing oxygen are called

xanthophylls. Some carotenoids are acylic (e.g., lyco-

pene), though the majority contain a six-membered

ring at either one end or both ends (e.g., b-carotene).

Apocarotenoids are those compounds where an

end group has been removed from the normal C

structure.

0008A few carotenoids appear in animal flesh, such as

salmon and shrimp, which acquire them via their diet.

The dark blue of the live lobster is a carotenoid–

protein complex which is destroyed when the animal

is boiled and only the pink carotenoid then remains.

Butter, cheese, and egg yolks contain carotenoids

which derive ultimately from the vegetable diet of

Anthocyanidins: R

= OH, R

= OH

Pelargonidin (Pg) H H

(Cy) OH H

(Pn)

OCH

(Dp) OH OH

(Pt)

OCH

(Mv)

OCH

Cyanidin

Peonidin

Delphinidin

Petunidin

Malvidin

Anthocyanins: Pg. Cy. Pn. Dp. Pt. Mv with

= O-Sugar or O-acylated sugar

= OH or O-glucose

Anion

−

fig0001 Figure 1 The common anthocyanins and anthocyanidins. Re-

produced from Colours: Properties and Determination of Natural

Pigments, Encyclopaedia of Food Science, Food Technology and Nu-

trition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Aca-

demic Press.

COLORANTS (COLOURANTS)/Properties and Determination of Natural Pigments 1551

the animals producing them. In animals, many

carotenoids act as vitamin A precursors, so have

additional value in addition to their properties as

pigments. (See Retinol: Physiology.)

0009 Although some 500 or so carotenoids are known in

nature, very few of them are water-soluble – those

that are have achieved a disproportionate importance

as food colors, as described later. Their general water-

insolubility means that they tend to be more stable in

processed foods than do anthocyanins, since they

remain attached to cellular fragments. Thus, the

carotenoid pigments of canned tomatoes are more

stable than the anthocyanins of canned strawberries.

Nevertheless, carotenoids are still prone to degrad-

ation once freed from their cellular matrix, and are

particularly susceptible to light and oxygen.

Chlorophyls and Heme Pigments

0010 Chlorophyls are amongst the most noticeable of all

plant pigments due to their central photosynthetic

role. Like carotenoids, they are water-insoluble and

are associated with the lipid portion of plant cells in

specialized organelles (the chloroplasts). The bright

green of native chlorophyl in plants is determined by

the presence of a central magnesium atom in a large

porphyrin ring. When green-leaf vegetables are

cooked under normal (slightly acid) conditions, this

magnesium is easily lost to form the dull green pheo-

phytins. Although this process can be prevented by

the use of alkali during cooking and the green color

thereby retained, this is no longer a common practice

since it leads to oxidation and loss of vitamin C. The

green color can also be retained and stabilized if the

central magnesium atom is replaced by copper. (See

Chlorophyl.)

0011The bright red heme pigments, characteristic of

animal blood and therefore of fresh meat, contain

similar porphyrin ring structures, but the central

atom is iron which shifts the chromophore into the

red region. Like chlorophyl, the heme pigments are

soon degraded when meat is cooked in the normal

way. If preserved in the presence of nitrite, however,

as in traditional curing, the nitrosohemoglobin stabil-

izes the pigment and a red color is retained. (See

Curing.)

Other Plant Pigments

0012Although most orders of higher plants can synthesize

anthocyanins to provide red and purple shades, the

Centrospermae lack this ability and synthesize in-

stead a group of nitrogenous pigments known as the

betalains. These occur in both red forms (betanins)

and yellow forms (vulgaxanthins) (Figure 3). The

betalains are water-soluble and are generally stable

(a)

OGI

−H

: Flavylium cation (red)

OGI

fig0002 Figure 2 Equilibrium forms of anthocyanins. (a) Quinoidal base (blue); (b) carbinol pseudobase (colorless); (c) chalcone (colorless).

Reproduced from Colours: Properties and Determination of Natural Pigments, Encyclopaedia of Food Science, Food Technology and Nu-

trition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

1552 COLORANTS (COLOURANTS)/Properties and Determination of Natural Pigments

at moderately acid pH, although heat, light, and

oxygen will hasten their decomposition. Although of

restricted natural distribution, the importance of

betalains, particularly the red betanins, is enhanced

by their presence in a number of important food crops

such as amaranth, chard, and beetroot.

0013 The yellow tubers of turmeric (Curcuma longa) are

well known as both a spice and a coloring material

for food when dried and ground into a powder. The

pigment, known as curcumin, has the extended con-

jugated structure shown in Figure 4. This is subject to

keto/enol equilibrium and the pigment converts to an

unstable orange form above pH 7.

Pigments Generated During Processing

0014 Pigments generated during processing or cooking are

amongst the least well defined in terms of chemical

structure. They fall into two broad groups – those

produced by enzymatic browning (principally from

natural phenolics and polyphenoloxidase) and those

produced by (nonenzymatic) Maillard reactions

between reducing sugars and amino compounds.

Enzymatic Browning

0015Colors of this sort are generated by the action of

polyphenoloxidase or peroxidase enzymes on natural

phenolic substrates. The development of color in a

freshly cut apple is a typical example. The principal

substrates involved are the catechins or procyanidins,

although in older literature the phenolic acids are

often mentioned. It is now known that phenolic

acids do not themselves brown, although they may

play a part in coupled oxidation systems which allow

the procyanidins to generate the color. The pigments

derived from procyanidins have no specific l

max

the visible region of the spectrum, although those

derived from simpler phenolics such as catechins

may initially display yellow-orange chromophores

about 400 nm before further polymerization occurs.

0016In some food systems, such as apple juice or white

wine, only a limited amount of color development is

desirable before the product becomes unacceptably

dark. In these cases, color formation may be inhibited

by antioxidants such as sulfite or by a drop in pH

which reduces enzyme activity. In other foods, such as

black tea or cocoa, the development of color during

processing is critical to the product and is also associ-

ated with flavor development processes. The tea

system has been well studied and is known to gener-

ate two types of phenolic pigment from catechins.

These are the theaflavins, which are bright orange-

red, and the thearubigins, which are dull brown.

Most oxidized phenolic pigments tend to become

browner and duller with time, particularly if further

nonspecific oxidation takes place. (See Antioxidants:

Synthetic Antioxidants.)

COOH

HOOC

Betalaine chromophore

Epimerization to

iso-betanin

occurs at position

marked

Betanin R' = Glucose

Betanidin R' = OH

Prebetanin

R' = Glucose 6-sulfate

Vulgaxanthin

I R' = NH

II R' = OH

COO

−

Betacyanin

(red)

Betaxanthin

(yellow)

COO

−

fig0003 Figure 3 The structure of betalains. Reproduced from Colours:

Properties and Determination of Natural Pigments, Encyclopaedia

of Food Science, Food Technology and Nutrition, Macrae R, Robin-

son RK and Sadler MJ (eds), 1993, Academic Press.

fig0004Figure 4 The structure of curcumin. R ¼ H or OCH

. Repro-

duced from Colours: Properties and Determination of Natural

Pigments, Encyclopaedia of Food Science, Food Technology and Nu-

trition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Aca-

demic Press.

COLORANTS (COLOURANTS)/Properties and Determination of Natural Pigments 1553

Nonenzymatic (Maillard) Browning

0017 This form of color development is usually associated

with heat and is a type of caramelization, in which the

temperature for degradation of a reducing sugar is

lowered by the presence of catalytic amino groups.

The sugar degrades to give aldehydic functions which

polymerize to give visible chromophores, intensifying

after further complex formation with amino groups.

Typical desirable examples include the browning of

fried onions, the roasting of meat, or the manufacture

of toffee confectionery.

0018 Almost inevitably, Maillard browning is associated

with flavor development. The pigments generated are

even less well defined than those from the phenolic

browning reaction. Although temperatures above



C will generate Maillard pigments rapidly in suit-

able systems, the reactions will also proceed slowly at

room temperature if the water activity is low enough.

Thus, apple juice concentrate held at 20



C for several

weeks shows discernible color development from

Maillard reactions between fructose and amino

acids (in addition to any enzymatic browning which

may have taken place earlier). Similarly, tinned con-

densed milk or dried milk powder develops color in

storage due to reaction between lactose and the lysine

e-amino groups in milk protein. Such slow Maillard

reactions are generally considered undesirable, in

contrast to the fast reactions which are under control

during cooking.

Natural Pigments Deliberately Added to

Foodstuffs

0019 Most of the natural pigments added to food are de-

rived from one of the categories described above. A

list of those permitted in the European Union is given

in Tables 1 and 2.

Blues and Reds

0020 Most anthocyanin extracts are derived from grape

skins as a byproduct of wine or grape-juice manufac-

ture. Some extracts are also available from black

carrot, elderberry, red cabbage, purple corn, and

blackcurrant. A particularly interesting extract is

that from red cabbage which contains highly copig-

mented anthocyanins of much bluer shade than most

fruit-derived anthocyanins. The extracts are generally

made using acidified water or alcohol, sometimes in

the presence of sulfite (which is later removed). The

extracts are then concentrated to a liquid or dried to a

powder. None of the extracts are pure compounds but

consist of a variety of anthocyanin structures com-

plete with polymeric degradation products.

0021Anthocyanin extracts are well suited to aqueous

systems but they cannot be directly used in fatty

foods or in foods of pH much greater than 4.5. They

are susceptible to copigmentation and complexation

effects – for instance, the presence of tin, iron, or

aluminum (as in canned fruits) can cause noticeable

blueing.

0022Native (monomeric) anthocyanins are reversibly

bleached by sulfite but are irreversibly degraded by

ascorbic acid. Polymeric or copigmented anthocya-

nins are generally more resistant to degradation and

are therefore particularly suited to use in soft drinks.

Other phenolic compounds such as procyanidin

tannins will tend to cause coprecipitation or haze

formation. The crude extracts of grape skin which

tbl0001Table 1 Natural and nature-identical colorants (and those of

natural origin) covered by European Union regulations

Colorant E number

Reds

Anthocyanins E163

Beetroot red E162

Cochineal, carmines E120

Orange-Yellow

Annatto E160(b)

b-apo-8

-carotenal E160(e)

Canthaxanthin E161(g)

Mixed carotenes and b-carotene E160(a)

Curcumin E100

Ethyl ester of b-apo-8

-carotenoic acid E160(f)

Lutein E161(b)

Lycopene E160(d)

Paprika extract E160(c)

Riboflavin, riboflavin-5-phosphate E101

Greens

Chlorophyls and chlorophyllins E140

Copper complexes of chlorophyls and chlorophyllins E141

Brown

Plain caramel E150(a)

Caustic sulfite caramel E150(b)

Ammonia caramel E150(c)

Sulfite ammonia caramel E150(d)

Black

Vegetable carbon E153

tbl0002Table 2 Inorganic colorants covered by European Union

regulations

Colorant E number

Calcium carbonate E170

Titanium dioxide E171

Iron oxides and hydroxides E172

Aluminum E173

Silver E174

Gold E175

1554 COLORANTS (COLOURANTS)/Properties and Determination of Natural Pigments

are high in tannins may also react with food pro-

teins such as gelatin. Anthocyanins, particularly

those derived from grapes or black carrot, have

found considerable application in providing a natural

coloring for soft drinks where they are stable at

low pH and moderately resistant to light and pasteur-

ization.

0023 Betanin extracts are prepared from beetroot juice

which may be fermented to remove sugar and then

dried to a powder. Although generally stable between

pH 3 and 7, they are adversely affected by heat, light,

oxygen, high water activity, and sulfite. However

they have found considerable application in frozen

and short-shelf-life dairy products such as icecream

and yogurt, and in dry-mix desserts.

0024 The traditional red color cochineal is derived

from the dried South American insect (Coccus cacti)

and consists of a polyhydroxy anthroquinone pig-

ment known as carminic acid. Its aluminum lake,

known as carmine, is produced as an insoluble

powder which can be converted to a water-soluble

product by dissolution into dilute alkali. It is very

stable to heat, light, and oxygen and finds application

in a wide range of foodstuffs, including meat and

dairy products, sugar confectionery, and certain soft

drinks.

Orange and Yellow

0025 The carotenoids are generally oil-soluble and there-

fore well suited for coloring fatty foods such as mar-

garine and dairy spreads. Natural sources include

paprika, lutein from Tagetes erecta (Aztec marigold),

carotenes from palm oil, and lycopene from tomato.

Natural carotenes may be incorporated into an oil-in-

water emulsion to facilitate their dissolution into soft

drinks. A commonly used extract is annatto, which is

obtained from the seeds of a tropical shrub, Bixa

orellana. This consists mainly of bixin, the fat-soluble

methyl ester of a carotenoid carboxylic acid known

as norbixin, which is a particularly intense colorant

providing distinctive colors at dose levels as low as

10 p.p.m. It can be rendered completely water-soluble

by alkaline hydrolysis to yield the sodium or potas-

sium salts. These readily complex with protein and

are therefore ideally suited as colorants for smoked

fish, cheese, and other dairy products. Norbixin can

precipitate out at low pH and thus an annatto prep-

aration formulated to color cheese cannot be used to

color a soft drink where special-application forms of

norbixin are required. Bixin, however, is not affected

by pH.

0026 Another traditional water-soluble carotenoid is

crocin, an ester of the dicarboxylic acid crocetin,

which is the pigment from saffron (the stigmas of

Crocus sativus). This pigment is also extracted from

the seeds of certain oriental Gardenia spp.

0027Synthetic but ‘nature-identical’ carotenoids are

also available as food colorants, principally b-

carotene and b-apocarotenal. These are generally

used in oil-based systems but may also be made

water-dispersible by physical means (emulsification

and encapsulation), and thus are commonly used for

coloring cloudy-orange soft drinks. Canthaxanthin

may be incorporated into the diet of farmed salmon

to replace the carotenoids which wild fish would

naturally consume, thereby insuring a pink flesh.

Synthetic carotenoids and natural paprika and lutein

extracts are also used in poultry feed to insure either

that egg yolks are of the required color or that the

skin of the chicken is of a suitable shade of yellow.

0028Curcumin, a deodorized turmeric extract, also pro-

vides a useful yellow pigment, although its applica-

tion is limited by its sensitivity to light and its pH

dependence. New application forms, however, now

allow for its use in toppings and fillings for flour

confectionery as well as its more common use in

dairy products, desserts, sugar confectionery, soups,

and sauces. It has found considerable application,

together with annatto, in providing the yellow shade

required for vanilla icecream and certain cookies

(biscuits) and cakes.

Greens

0029Chlorophyl extracts are prepared using organic solv-

ents from sources such as grass and lucerne leaves.

The green extracts may be used to color fatty foods

directly, but are more usually converted into their

copper complexes to give a brighter green, followed

by preparation of water-soluble sodium or potassium

salts. In this form they are used to color sugar confec-

tionery, frozen desserts, and dairy products.

Browns and Blacks

0030These colors are principally obtained by the use of

synthetic caramels and carbon blacks. Four types of

synthetic caramel are recognized, which differ in their

solubilities and isoelectric points. This affects their

stability towards such factors as protein binding

(in beers) and alcohol-solubility (in spirits). The ap-

propriate caramel must therefore be chosen for the

product in hand. Class IV caramels E150(d) are used

throughout the world to color cola beverages.

Recently, ‘natural’ caramels based on malt extracts

have also become available. All these colors are based

on Maillard browning reactions. Brown pigments

based on enzymatic oxidation have not generally

been used for addition to food systems, principally

COLORANTS (COLOURANTS)/Properties and Determination of Natural Pigments 1555