Color - Principle of food chemistry

Color - Principle of food chemistry

Color is important to many foods, both

those that are unprocessed and those that are

manufactured Together with flavor and

tex-ture, color plays an important role in food

acceptability In addition, color may provide

an indication of chemical changes in a food,

such as browning and caramelization For a

few clear liquid foods, such as oils and

bev-erages, color is mainly a matter of

transmis-sion of light Other foods are opaque—they

derive their color mostly from reflection

Color is the general name for all sensations

arising from the activity of the retina of the

eye When light reaches the retina, the eye's

neural mechanism responds, signaling color

among other things Light is the radiant

energy in the wavelength range of about 400

to 800 nm According to this definition, color

(like flavor and texture) cannot be studied

without considering the human sensory

sys-tem The color perceived when the eye views

an illuminated object is related to the

follow-ing three factors: the spectral composition of

the light source, the chemical and physical

characteristics of the object, and the spectral

sensitivity properties of the eye To evaluate

the properties of the object, we must

stan-dardize the other two factors Fortunately,

the characteristics of different people's eyes

for viewing colors are fairly uniform; it is nottoo difficult to replace the eye by someinstrumental sensor or photocell that can pro-vide consistent results There are several sys-tems of color classification; the mostimportant is the CIE system (CommissionInternational de 1'Eclairage—InternationalCommission on Illumination) Other systemsused to describe food color are the Munsell,Hunter, and Lovibond systems

When the reflectance of different coloredobjects is determined by means of spectro-photometry, curves of the type shown in Fig-ure 6-1 are obtained White materials reflectequally over the whole visible wavelengthrange, at a high level Gray and black materi-als also reflect equally over this range but to

a lower degree Red materials reflect in thehigher wavelength range and absorb theother wavelengths Blue materials reflect inthe low-wavelength range and absorb thehigh-wavelength light

CIE SYSTEM

The spectral energy distribution of CIE

light sources A and C is shown in Figure 6-2 CIE illuminant A is an incandescent light

operated at 28540K, and illuminant C is the

same light modified by filters to result in a

Color

CHAPTER 6

Figure 6-1 Spectrophotometric Curves of

Col-ored Objects Source: From Hunter Associates

Lab., Inc.

spectral composition that approximates that

of normal daylight Figure 6-2 also shows

the luminosity curve of the standard observer

as specified by CIE This curve indicates

how the eyes of normal observers respond tothe various spectral light types in the visibleportion of the spectrum By breaking downthe spectrum, complex light types are re-duced to their component spectral lighttypes Each spectral light type is completelydetermined by its wavelength In some lightsources, a great deal of radiant energy is con-centrated in a single spectral light type Anexample of this is the sodium lamp shown inFigure 6-3, which produces monochromaticlight Other light sources, such as incandes-cent lamps, give off a continuous spectrum

A fluorescent lamp gives off a continuousspectrum on which is superimposed a linespectrum of the primary radiation produced

by the gas discharge (Figure 6-3)

In the description of light sources,

refer-ence is sometimes made to the black body.

This is a radiating surface inside a hollowspace, and the light source's radiation comesout through a small opening The radiation isindependent of the type of material the lightsource is made of When the temperature isvery high, about 600O0K the maximum ofthe energy distribution will fall about in themiddle of the visible spectrum Such energydistribution corresponds with that of daylight

on a cloudy day At lower temperatures, themaximum of the energy distribution shifts tolonger wavelengths At 3000° K, the spectralenergy distribution is similar to that of anincandescent lamp; at this temperature theenergy at 380 nm is only one-sixteenth ofthat at 780 nm, and most of the energy isconcentrated at higher wavelengths (Figure6-3) The uneven spectral distribution ofincandescent light makes red objects lookattractive and blue ones unattractive This iscalled color rendition The human eye hasthe ability to adjust for this effect

The CIE system is a trichromatic system;its basis is the fact that any color can be

Figure 6-2 Spectral Energy Distribution of

Light Sources A and C, the CIE, and Relative

Luminosity Function y for the CIE Standard

matched by a suitable mixture of three

mary colors The three primary colors, or

pri-maries, are red, green, and blue Any possible

color can be represented as a point in a

trian-gle The triangle in Figure 6-4 shows how

colors can be designated as a ratio of the three

primaries If the red, green, and blue values of

a given light type are represented by a, b, and

c, then the ratios of each to the total light are

given by a/(a + b + c), bl(a + b + c), and cl(a

+ b + c), respectively Since the sum of these

is one, then only two have to be known to

know all three Color, therefore, is

deter-mined by two, not three, of these mutually

dependent quantities In Figure 6-4, a color

point is represented by P By determining the

distance of P from the right angle, the

quanti-ties al(a + b + c) and bl(a + b + c) are found.

The quantity cl(a + b + c) is then found, by

first extending the horizontal dotted line

through P until it crosses the hypotenuse at Q

and by then constructing another right angle

triangle with Q at the top All combinations

of a, b, and c will be points inside the

trian-gle

The relative amounts of the three primariesrequired to match a given color are called the

WAVELENGTH NM Figure 6-3 Spectral Energy Distribution of Sunlight (S), CIE Illuminant (A), Cool White Fluorescent

Lamp (B), and Sodium Light (N)

tristimulus values of the color The CIE

pri-maries are imaginary, because there are no

real primaries that can be combined to match

the highly saturated hues of the spectrum

In the CIE system the red, green, and blue

primaries are indicated by X, Y 9 and Z The

amount of each primary at any particular

wavelength is given by the values J, y, and z.

These are called the distribution coefficients

or the red, green, and blue factors They

rep-resent the tristimulus values for each chosen

wavelength The distribution coefficients for

the visible spectrum are presented in Figure

6-5 The values of y correspond with the

luminosity curve of the standard observer

(Figure 6-2) The distribution coefficients

are dimensionless because they are the

num-bers by which radiation energy at each

wave-length must be multiplied to arrive at the X,

y, and Z content The amounts of X, Y, and Z

primaries required to produce a given colorare calculated as follows:

/ = spectral energy distribution of

illu-minant

R = spectral reflectance of sample

dh = small wavelength interval

jc, y, ~z = red, green, and blue factors

The ratios of the primaries can beexpressed as

The quantities x and y are called the

chroma-ticity coordinates and can be calculated foreach wavelength from

Figure 6-5 Distribution Coefficients JC, y, and z

for the Visible Spectrum Source: From Hunter

Associates Lab., Inc.

WAVELENGTH (NANOMETERS)

jc = xf(x + y + z)

y= y/(x + y + z)

z=l-(x + y)

A plot of jc versus y results in the CIE

chro-maticity diagram (Figure 6-6) When the

chromaticities of all of the spectral colors are

placed in this graph, they form a line called

the locus Within this locus and the line

con-necting the ends, represented by 400 and 700

nm, every point represents a color that can be

made by mixing the three primaries The

point at which exactly equal amounts of each

of the primaries are present is called theequal point and is white This white pointrepresents the chromaticity coordinates of

illuminant C The red primary is located at jc

= 1 and y = O; the green primary at x = O and

y = 1; and the blue primary at x = O and y = O.

The line connecting the ends of the locusrepresents purples, which are nonspectralcolors resulting from mixing various amounts

of red and blue All points within the locusrepresent real colors All points outside thelocus are unreal, including the imaginary pri-

maries X, Y, and Z At the red end of the

locus, there is only one point to represent thewavelength interval of 700 to 780 nm This

Figure 6-6 CIE Chromaticity Diagram

means that all colors in this range can be

simply matched by adjustment of luminosity

In the range of 540 to 700 nm, the spectrum

locus is almost straight; mixtures of two

spectral light types along this line segment

will closely match intervening colors with

little loss of purity In contrast, the spectrum

locus below 540 nm is curved, indicating that

a combination of two spectral lights along

this portion of the locus results in colors of

decreased purity

A pure spectral color is gradually diluted

with white when moving from a point on the

spectrum locus to the white point P Such a

straight line with purity decreasing from 100

to O percent is known as a line of constant

dominant wavelength Each color, except the

purples, has a dominant wavelength The

position of a color on the line connecting the

locus and P is called excitation purity (p e )

and is calculated as follows:

x w and y w are the chromaticity coordinates

of the achromatic source

x p and y p are the chromaticity coordinates

of the pure spectral color

Achromatic colors are white, black, and

gray Black and gray differ from white only

in their relative reflection of incident light

The purples are nonspectral chromatic

col-ors All other colors are chromatic; for

exam-ple, brown is a yellow of low lightness and

low saturation It has a dominant wavelength

in the yellow or orange range

A color can be specified in terms of the

tri-stimulus value Y and the chromaticity

coor-dinates x and y The Y value is a measure of

luminous reflectance or transmittance and isexpressed in percent simply as 7/1000.Another method of expressing color is interms of luminance, dominant wavelength,and excitation purity These latter are roughlyequivalent to the three recognizable attrib-utes of color: lightness, hue, and saturation.Lightness is associated with the relativeluminous flux, reflected or transmitted Hue

is associated with the sense of redness, lowness, blueness, and so forth Saturation isassociated with the strength of hue or the rel-ative admixture with white The combination

yel-of hue and saturation can be described aschromaticity

Complementary colors (Table 6-1) areobtained when a straight line is drawn

through the equal energy point P When this

is done for the ends of the spectrum locus,the wavelength complementary to the 700 to

780 point is at 492.5 nm, and for the 380 to

410 point is at 567 nm All of the lengths between 492.5 and 567 nm are com-plementary to purple The purples can bedescribed in terms of dominant wavelength

wave-by using the wavelength complementary toeach purple, and purity can be expressed in amanner similar to that of spectral colors

Table 6-1 Complementary Colors

Wavelength Complementary (nm) Color Color

An example of the application of the CIE

system for color description is shown in

Fig-ure 6-7 The curved, dotted line originating

from C represents the locus of the

chromatic-ity coordinates of caramel and glycerol

solu-tions The chromaticity coordinates of maple

syrup and honey follow the same locus Three

triangles on this curve represent the

chroma-ticity coordinates of U.S Department of

Agri-culture (USDA) glass color standards for

maple syrup These are described as lightamber, medium amber, and dark amber Thesix squares are chromaticity coordinates ofhoney, designated by USDA as water white,extra white, white, extra light amber, lightamber, and amber Such specifications areuseful in describing color standards for a vari-ety of products In the case of the light amberstandard for maple syrup, the following values

apply: x = 0.486, y = 0.447, and T = 38.9

per-Figure 6-7 CIE Chromaticity Diagram with Color Points for Maple Syrup and Honey Glass Color

Standards

X

y

GLASS COLOR STANDARDS

A FOR MAPLE SYRUP

• FOR HONEY

cent In this way, x and y provide a

specifica-tion for chromaticity and T for luminous

transmittance or lightness This is easily

expressed as the mixture of primaries under

illuminant C as follows: 48.6 percent of red

primary, 44.7 percent of green primary, and

6.7 percent of blue primary The light

trans-mittance is 38.9 percent

The importance of the light source and

other conditions that affect viewing of

sam-ples cannot be overemphasized Many

sub-stances are metameric; that is, they may have

equal transmittance or reflectance at a certain

wavelength but possess noticeably different

colors when viewed under illuminant C

MUNSELL SYSTEM

In the Munsell system of color

classifica-tion, all colors are described by the three

attributes of hue, value, and chroma This

can be envisaged as a three-dimensional

sys-tem (Figure 6-8) The hue scale is based on

ten hues which are distributed on the

circum-ference of the hue circle There are five hues:

red, yellow, green, blue, and purple; they are

written as R, Y, G, B, and P There are also

five intermediate hues, YR, GY, BG, PB, and

RP Each of the ten hues is at the midpoint of

a scale from 1 to 10 The value scale is a

lightness scale ranging from O (black) to 10

(white) This scale is distributed on a line

perpendicular to the plane of the hue circle

and intersecting its center Chroma is a

mea-sure of the difference of a color from a gray

of same lightness It is a measure of purity

The chroma scale is of irregular length, and

begins with O for the central gray The scale

extends outward in steps to the limit of purity

obtainable by available pigments The shape

of the complete Munsell color space is

indi-cated in Figure 6-9 The description of a

color in the Munsell system is given as //,

VIC For example, a color indicated as 5R

Figure 6-8 The Munsell System of Color

Clas-sification

2.8/3.7 means a color with a red hue of 5R, avalue of 2.8, and a chroma of 3.7 All colorsthat can be made with available pigments arelaid down as color chips in the Munsell book

White

Blue

Red

Saturation Chroma

HUNTER SYSTEM

The CIE system of color measurement is

based on the principle of color sensing by the

human eye This accepts that the eyes contain

three light-sensitive receptors—the red, green,

and blue receptors One problem with this

system is that the X, Y, and Z values have no

relationship to color as perceived, though a

color is completely defined To overcome this

problem, other color systems have been

sug-gested One of these, widely used for food

colorimetry, is the Hunter L, a, fo, system The

so-called uniform-color, opponent-colors color

scales are based on the opponent-colors

theory of color vision In this theory, it is

assumed that there is an intermediate

signal-switching stage between the light receptors in

the retina and the optic nerve, which

trans-mits color signals to the brain In this

switch-ing mechanism, red responses are compared

with green and result in a red-to-green color

dimension The green response is compared

with blue to give a yellow-to-blue color

dimension These two color dimensions are

represented by the symbols a and b The third

color dimension is lightness L, which is linear and usually indicated as the square orcube root of K This system can be repre-sented by the color space shown in Figure

non-6-10 The L, a, b, color solid is similar to the

Munsell color space The lightness scale iscommon to both The chromatic spacing isdifferent In the Munsell system, there are thepolar hue and chroma coordinates, whereas in

the L, a, b, color space, chromaticity is defined by rectangular a and b coordinates.

CIE values can be converted to color values

by the equations shown in Table 6-2 into L, a,

b, values and vice versa (MacKinney and

Lit-tle 1962; Clydesdale and Francis 1970) This

is not the case with Munsell values These areobtained from visual comparison with colorchips (called Munsell renotations) or frominstrumental measurements (called Munsellrenotations), and conversion is difficult andtedious

The Hunter tristimulus data, L (value), a (redness or greenness), and b (yellowness or

blueness), can be converted to a single color

Figure 6-10 The Hunter L, a, b Color Space Source: From Hunter Associates Lab., Inc.

L=O BLACK BLUE

RED

YELLOW

GRAY GREEN

L 100

WHITE

function called color difference (AE) by

using the following relationship:

AE = (AL)2 + (Afl)2 + (Ab) 2

The color difference is a measure of the

dis-tance in color space between two colors It

does not indicate the direction in which the

colors differ

LOVIBOND SYSTEM

The Lovibond system is widely used for

the determination of the color of vegetable

oils The method involves the visual

compar-ison of light transmitted through a glass

cuvette filled with oil at one side of aninspection field; at the other side, coloredglass filters are placed between the lightsource and the observer When the colors oneach side of the field are matched, the nomi-nal value of the filters is used to define thecolor of the oil Four series of filters areused—red, yellow, blue, and gray filters Thegray filters are used to compensate for inten-sity when measuring samples with intensechroma (color purity) and are used in thelight path going through the sample The red,yellow, and blue filters of increasing inten-sity are placed in the light path until a matchwith the sample is obtained Vegetable oilcolors are usually expressed in terms of red

Source: From Hunter Associates Lab., Inc.

Table 6-2 Mathematical Relationship Between Color Scales

To Convert To L, a, b To X%, Y, Z% ToY,x,y

From

From

From

and yellow; a typical example of the

Lovi-bond color of an oil would be Rl.7 Y17 The

visual determination of oil color by the

Lovi-bond method is widely used in industry and

is an official method of the American Oil

Chemists' Society Visual methods of this

type are subject to a number of errors, and

the results obtained are highly variable A

study has been reported (Maes et al., 1997)

to calculate CIE and Lovibond color values

of oils based on their visible light

transmis-sion spectra as measured by a

spectropho-tometer A computer software has been

developed that can easily convert light

trans-mission spectra into CIE and Lovibond color

indexes

GLOSS

In addition to color, there is another

impor-tant aspect of appearance, namely gloss

Gloss can be characterized as the reflecting

property of a material Reflection of light can

be diffused or undiffused (specular) In

spec-ular reflection, the surface of the object acts

as a mirror, and the light is reflected in a

highly directional manner Surfaces can range

from a perfect mirror with completely

specu-lar reflection to a surface reflecting in a

com-pletely diffuse manner In the latter, the light

from an incident beam is scattered in all

directions and the surface is called matte

FOOD COLORANTS

The colors of foods are the result of natural

pigments or of added colorants The natural

pigments are a group of substances present in

animal and vegetable products The added

colorants are regulated as food additives, but

some of the synthetic colors, especially

ca-rotenoids, are considered "nature identical"

and therefore are not subject to stringent icological evaluation as are other additives(Dziezak 1987)

tox-The naturally occurring pigments embracethose already present in foods as well asthose that are formed on heating, storage, orprocessing With few exceptions, these pig-ments can be divided into the following fourgroups:

1 tetrapyrrole compounds: chlorophylls,hemes, and bilins

2 isoprenoid derivatives: carotenoids

3 benzopyran derivatives: anthocyaninsand flavonoids

4 artefacts: melanoidins, caramels

The chlorophylls are characteristic of greenvegetables and leaves The heme pigmentsare found in meat and fish The carotenoidsare a large group of compounds that arewidely distributed in animal and vegetableproducts; they are found in fish and crusta-ceans, vegetables and fruits, eggs, dairyproducts, and cereals Anthocyanins and fla-vonoids are found in root vegetables andfruits such as berries and grapes Caramelsand melanoidins are found in syrups andcereal products, especially if these productshave been subjected to heat treatment

Figure 6-11 Schematic Representation of the

Heme Complex of Myoglobin M = methyl, P =

propyl, V = vinyl Source: From C.E Bodwell

and RE McClain, Proteins, in The Sciences of

Meat Products, 2nd ed., I.E Price and B.S

Sch-weigert, eds., 1971, W.H Freeman & Co.

In the heme pigments, the nitrogen atoms are

linked to a central iron atom The color of

meat is the result of the presence of two

pig-ments, myoglobin and hemoglobin Both

pigments have globin as the protein portion,

and the heme group is composed of the

por-phyrin ring system and the central iron atom

In myoglobin, the protein portion has a

molecular weight of about 17,000 In

hemo-globin, this is about 67,000—equivalent to

four times the size of the myoglobin protein

The central iron in Figure 6-11 has six

coor-dination bonds; each bond represents an

electron pair accepted by the iron from five

nitrogen atoms, four from the porphyrin ring

and one from a histidyl residue of the globin

The sixth bond is available for joining with

any atom that has an electron pair to donate

The ease with which an electron pair is

donated determines the nature of the bond

formed and the color of the complex Other

factors playing a role in color formation arethe oxidation state of the iron atom and thephysical state of the globin

In fresh meat and in the presence of gen, there is a dynamic system of three pig-ments, oxymyoglobin, myoglobin, and met-myoglobin The reversible reaction with oxy-gen is

oxy-Mb + O2 ^ MbO2

In both pigments, the iron is in the ferrousform; upon oxidation to the ferric state, thecompound becomes metmyoglobin Thebright red color of fresh meat is due to thepresence of oxymyoglobin; discoloration tobrown occurs in two stages, as follows:

Red Purplish red Brownish

Oxymyoglobin represents a ferrous covalentcomplex of myoglobin and oxygen Theabsorption spectra of the three pigments areshown in Figure 6-12 (Bodwell and McClain1971) Myoglobin forms an ionic complexwith water in the absence of strong electronpair donors that can form covalent com-plexes It shows a diffuse absorption band inthe green area of the spectrum at about 555

nm and has a purple color In metmyoglobin,the major absorption peak is shifted towardthe blue portion of the spectrum at about 505

nm with a smaller peak at 627 nm The pound appears brown

com-As indicated above, oxymyoglobin andmyoglobin exist in a state of equilibriumwith oxygen; therefore, the ratio of the pig-ments is dependent on oxygen pressure Theoxidized form of myoglobin, the metmyo-globin, cannot bind oxygen In meat, there is

a slow and continuous oxidation of the hemeGlobin

pigments to the metmyoglobin state

Reduc-ing substances in the tissue reduce the

met-myoglobin to the ferrous form The oxygen

pressure, which is so important for the state

of the equilibrium, is greatly affected by

packaging materials used for meats The

maximum rate of conversion to

metmyoglo-bin occurs at partial pressures of 1 to 20 nm

of mercury, depending on pigment, pH, and

temperature (Fox 1966) When a packaging

film with low oxygen permeability is used,

the oxygen pressure drops to the point where

oxidation is favored To prevent this,

Lan-drock and Wallace (1955) established that

oxygen permeability of the packaging film

must be at least 5 liters of oxygen/square

meter/day/atm

Fresh meat open to the air displays thebright red color of oxymyoglobin on the sur-face In the interior, the myoglobin is in thereduced state and the meat has a dark purplecolor As long as reducing substances arepresent in the meat, the myoglobin willremain in the reduced form; when they areused up, the brown color of metmyoglobinwill predominate According to Solberg(1970), there is a thin layer a few nanome-ters below the bright red surface and justbefore the myoglobin region, where a defi-nite brown color is visible This is the areawhere the oxygen partial pressure is about1.4 nm and the brown pigment dominates.The growth of bacteria at the meat surfacemay reduce the partial oxygen pressure to

Figure 6-12 Absorption Spectra of Myoglobin, Oxymyoglobin, and Metmyoglobin Source: From

C.E Bodwell and RE McClain, Proteins, in The Sciences of Meat Products, 2nd ed., I.E Price and

B.S Schweigert, eds., 1971, W.H Freeman & Co.

below the critical level of 4 nm

Microor-ganisms entering the logarithmic growth

phase may change the surface color to that

of the purplish-red myoglobin (Solberg

1968)

In the presence of sulfhydryl as a reducing

agent, myoglobin may form a green pigment,

called sulfmyoglobin The pigment is green

because of a strong absorption band in the

red region of the spectrum at 616 nm In the

presence of other reducing agents, such as

ascorbate, cholemyoglobin is formed In this

pigment, the porphyrin ring is oxidized The

conversion into sulfmyoglobin is reversible;

cholemyoglobin formation is irreversible,

and this compound is rapidly oxidized to

yield globin, iron, and tetrapyrrole

Accord-ing to Fox (1966), this reaction may happen

in the pH range of 5 to 7

Heating of meat results in the formation of

a number of pigments The globin is

dena-tured In addition, the iron is oxidized to the

ferric state The pigment of cooked meat is

brown and called hemichrome In the

pres-ence of reducing substances such as those

that occur in the interior of cooked meat, the

iron may be reduced to the ferrous form; the

resulting pigment is pink hemochrome

In the curing of meat, the heme reacts with

nitrite of the curing mixture The

nitrite-heme complex is called nitrosomyoglobin,

which has a red color but is not particularly

stable On heating the more stable

nitrosohe-mochrome, the major cured meat pigment is

formed, and the globin portion of the

mole-cule is denatured This requires a

tempera-ture of 650C This molecule has been called

nitrosomyoglobin and nitrosylmyoglobin,

but Mohler (1974) has pointed out that the

only correct name is nitric oxide myoglobin

The first reaction of nitrite with myoglobin is

oxidation of the ferrous iron to the ferric

form and formation of MetMb At the same

time, nitrate is formed according to the lowing reaction (Mohler 1974):

fol-4MbO2 + 4NO2- + 2H2O -»

4MetMbOH + 4NO3" + O2During the formation of the curing pigment,the nitrite content is gradually lowered; thereare no definite theories to account for thisloss

The reactions of the heme pigments inmeat and meat products have been summa-rized in the scheme presented in Figure 6-13(Fox 1966) Bilin-type structures are formedwhen the porphyrin ring system is broken

Chlorophylls

The chlorophylls are green pigmentsresponsible for the color of leafy vegetablesand some fruits In green leaves, the chloro-phyll is broken down during senescence andthe green color tends to disappear In manyfruits, chlorophyll is present in the unripestate and gradually disappears as the yellowand red carotenoids take over during ripen-ing In plants, chlorophyll is isolated in thechloroplastids These are microscopic parti-cles consisting of even smaller units, calledgrana, which are usually less than one micro-meter in size and at the limit of resolution ofthe light microscope The grana are highlystructured and contain laminae betweenwhich the chlorophyll molecules are posi-tioned

The chlorophylls are tetrapyrrole pigments

in which the porphyrin ring is in the dihydroform and the central metal atom is magne-

sium There are two chlorophylls, a and b,

which occur together in a ratio of about 1:25

Chlorophyll b differs from chlorophyll a in

that the methyl group on carbon 3 is replacedwith an aldehyde group The structural for-

mula of chlorophyll a is given in Figure

6-14 Chlorophyll is a diester of a dicarboxylic

acid (chlorophyllin); one group is esterified

with methanol, the other with phytyl alcohol

The magnesium is removed very easily by

acids, giving pheophytins a and b The action

of acid is especially important for fruits that

are naturally high in acid However, it

appears that the chlorophyll in plant tissues

is bound to lipoproteins and is protected

from the effect of acid Heating coagulates

the protein and lowers the protective effect

The color of the pheophytins is olive-brown

Chlorophyll is stable in alkaline medium

The phytol chain confers insolubility in

water on the chlorophyll molecule Upon

hydrolysis of the phytol group, the

water-sol-uble methyl chlorophyllides are formed Thisreaction can be catalyzed by the enzymechlorophyllase In the presence of copper orzinc ions, it is possible to replace the magne-sium, and the resulting zinc or copper com-plexes are very stable Removal of the phytolgroup and the magnesium results inpheophorbides All of these reactions aresummarized in the scheme presented in Fig-ure 6-15

In addition to those reactions describedabove, it appears that chlorophyll can bedegraded by yet another pathway Chichesterand McFeeters (1971) reported on chloro-phyll degradation in frozen beans, whichthey related to fat peroxidation In this reac-tion, lipoxidase may play a role, and no

Figure 6-13 Heme Pigment Reactions in Meat and Meat Products ChMb, cholemyoglobin (oxidized

porphyrin ring); O 2 Mb, oxymyoglobin (Fe +2 ); MMb metmyoglobin (Fe +3 ); Mb, myoglobin (Fe +2 ); MMb-NO 2 , metmyoglobin nitrate; NOMMb, nitrosylmetmyoglobin; NOMb, nitrosylmyoglobin; NMMb, nitrimetmyoglobin; NMb, nitrimyoglobin, the latter two being reaction products of nitrous acid

and the heme portion of the molecule; R, reductants; O, strong oxidizing conditions Source: From J.B Fox, The Chemistry of Meat Pigments, J Agr Food Chem., Vol 14, no 3, pp 207-210, 1966,

American Chemical Society.

hemochrome

Nitrosyl-Denatured Globin Hemichrome

FRESH CURED

pheophytins, chlorophyllides, or

pheophor-bides are detected The reaction requires

oxygen and is inhibited by antioxidants

Carotenoids

The naturally occurring carotenoids, with

the exception of crocetin and bixin, are

tet-raterpenoids They have a basic structure of

eight isoprenoid residues arranged as if two

20-carbon units, formed by head-to-tail

con-densation of four isoprenoid units, had

joined tail to tail There are two possible

ways of classifying the carotenoids The first

system recognizes two main classes, the

car-otenes, which are hydrocarbons, and thexanthophylls, which contain oxygen in theform of hydroxyl, methoxyl, carboxyl, keto,

or epoxy groups The second system dividesthe carotenoids into three types (Figure 6-16),acyclic, monocyclic, and bicyclic Examplesare lycopene (I)—acyclic, y-carotene (II)—monocyclic, and a-carotene (III) and p-car-otene (IV)—bicyclic

The carotenoids take their name from the

major pigments of carrot (Daucus car old).

The color is the result of the presence of asystem of conjugated double bonds Thegreater the number of conjugated doublebonds present in the molecule, the further themajor absorption bands will be shifted to the

Figure 6-14 Structure of Chlorophyll a (Chlorophyll b differs in having a formyl group at carbon 3).

Source: Reprinted with permission from J.R Whitaker, Principles of Enzymology for the Food Sciences,

1972, by courtesy of Marcel Dekker, Inc.

region of longer wavelength; as a result, the

hue will become more red A minimum of

seven conjugated double bonds are required

before a perceptible yellow color appears

Each double bond may occur in either cis or

trans configuration The carotenoids in foods

are usually of the all-trans type and only

occasionally a mono-cis or di-cis compound

occurs The prefix neo- is used for

stereoiso-mex.1 with at least one cis double bond The

prefix pro- is for poly-a's carotenoids The

effect of the presence of cis double bonds on

the absorption spectrum of p-carotene is

shown in Figure 6-17 The configuration has

an effect on color The all-trans compounds

have the deepest color; increasing numbers

of cis bonds result in gradual lightening of

the color Factors that cause change of bonds

from trans to cis are light, heat, and acid.

In the narrower sense, the carotenoids arethe four compounds shown in Figure 6-16—a-, p-, and y-carotene and lycopene—poly-ene hydrocarbons of overall composition

C40H56 The relation between these and tenoids with fewer than 40 carbon atoms is

caro-shown in Figure 6-18 The prefix apo- is

used to designate a carotenoid that is derivedfrom another one by loss of a structural ele-ment through degradation It has been sug-gested that some of these smaller carotenoidmolecules are formed in nature by oxidativedegradation of C40 carotenoids (Grob 1963).Several examples of this possible relation-ship are found in nature One of the bestknown is the formation of retinin and vita-min A from p-carotene (Figure 6-19).Another obvious relationship is that of lyco-pene and bixin (Figure 6-20) Bixin is a food

Figure 6-15 Reactions of Chlorophylls

chlorin purpurins

phytol

chlorophyllase chlorophyll

strong acid acid

phytol