COLOR CHEMISTRY

IV. COLOR CHEMISTRY

The perceptual property of color is induced by two broad mechanisms: either white light is selectively interacted with by matter and thus decomposed into its constituent wave- lengths, or else nonwhite light is directly emitted by some source (Nassau, 1987). Nassau (1987) proceeds to list fifteen specific physicochemical mechanisms whereby color is pro- duced; from the standpoint of the food scientist, however, those mechanisms involving electron transitions between molecular orbitals are the most important. These molecular orbital transitions are largely responsible for the color associated with organic compounds, whether synthetic or natural in origin.

It is the bond conjugation within the organic molecule which is responsible for color; delocalization of the π-bonding electrons lowers their excitation energies, allowing them to absorb light (Nassau, 1987). Extensive conjugation, or the presence of electron donor and acceptor groups within the molecule, serves to shift the absorption of light to the lower energies (that is, longer wavelengths) comprising the visible spectrum (wavelengths of 400 to 750 nm). Those wavelengths absorbed will be dependent on the existence of molecular orbital levels separated by energy, hc/ λ, where h is Planck’s constant, c is the velocity of light, and λ the wavelength of the absorbed radiation; the incident radiation will

cause an electron to shift to the higher energy (⫹ hc/λ) orbital. This harmless interaction of photon with electron defines what wavelengths are visible to us; lower energy light ( λ⬎

750 nm) induces only small vibrational changes (at most perceived as heat) whereas higher energy light ( λ ⬍ 400 nm) will ionize matter (that is, knock out electrons completely) (Nassau, 1987).

Note, however, what we properly see as color are not the wavelengths of light ab- sorbed, but rather the remainder of the incident light reflected (or in the case of transparent objects or solutions, the remainder of light transmitted) following absorption (Pavia et al., 1979). These perceptual colors are termed ‘‘complementary’’ to those wavelengths ab- sorbed (see Table 8).

A. Colorants Subject to Certification

The certified colors discussed previously are all organic dyes, and may be grouped into the following five classes based upon general chemical structure: monoazo (FD&C Yellow

Table 8 Relationship Between Absorbed Color and Observed (Complementary) Color

Wavelength Color of

Color of

absorbed (nm) absorbed light

observed light

400 Violet

Yellow

450 Blue

Orange

500 Blue-green

Red

530 Yellow-green

Red-violet

550 Yellow

Violet

600 Orange-red

Blue-green

700 Red

Green

Figure 1 Monoazo colorants.

No. 6, FD&C Red No. 40, Citrus Red No. 2; see Fig. 1), pyrazolone (FD&C Yellow No.

5, Orange B; see Fig. 2), triphenylmethane (FD&C Blue No. 1, FD&C Green No. 3; see Fig. 3 ), indigoid (FD&C Blue No. 2; see Fig. 4 ), and xanthene (FD&C Red No. 3; see Fig. 5 ) (Marmion, 1979). Tables 9 and 10 summarize their respective chemical properties and stabilities. As is evident from the tables, the dyes have varying degrees of stability dependent upon their chemical structure. The monoazo and pyrazolone structures are sub-

ject to SO 2 decolorization through HSO 3 ⫺ addition to the nitrogens, resulting in the color- less hydroazo sulfonic acids (von Elbe and Schwartz, 1996); although data were not avail- able for Citrus Red No. 2 and Orange B, their structures indicate that they too would be

Figure 3 Triphenylmethane colorants.

Figure 4 Indigoid colorant FD&C Blue No. 2.

Figure 5 Xanthene colorant FD&C Red No. 3.

Table 9 Chemical Data and Properties of the U.S. Certified Colorants Solubility (g/100 mL, 25 °C) a

Chemical

Molecular

FDA nomenclature

Gly Glycol FD&C Yellow No. 6

classification

Empirical formula

weight

EtOH

20.0 2.2 FD&C Red No. 40

Monoazo

C 16 H 9 N 4 O 9 S 2 Na 3 452.36

19.0 IN

22.0 0.01 3.0 1.5 Citrus Red No. 2

Monoazo

C 18 H 14 N 2 O 8 S 2 Na 2 496.42

VSS VSS FD&C Yellow No. 5

Monoazo

C 18 H 16 N 2 O 3 308.34

IN

VSS

18.0 7.0 Orange B

Pyrazolone

C 16 H 9 N 4 O 9 S 2 Na 3 534.36

20.0 IN

na na FD&C Blue No. 1

Pyrazolone

C 22 H 16 N 4 O 9 S 2 Na 2 590.49

22.0 b na

20.0 0.15 20.0 20.0 FD&C Green No. 3

Triphenylmethane

C 37 H 34 N 2 O 9 S 3 Na 2 792.84

20.0 0.01 20.0 20.0 FD&C Blue No. 2

Triphenylmethane

C 37 H 34 O 10 N 2 S 3 Na 2 808.84

1.0 0.1 FD&C Red No. 3

Indigoid

C 16 H 8 N 2 O 8 S 2 Na 2 466.35

1.6 IN

20.0 20.0 a H 2 O—water EtOH—ethanol, Gly—glycerine, Glycol—propylene glycol, IN—insoluble, VSS—very slightly soluble, na—data not available.

Xanthene

C 20 H 6 O 5 I 4 Na 2 879.86

9.0 IN

b At 77 °C. Source : NAS/NRC (1971); Marmion (1979).

Table 10 Stability Data for the U.S. Certified Colorants Stability to a

FDA nomenclature Original hue pH 3 pH 5 pH 7 pH 8 Light Heat Acid Base SO 2

FD&C Yellow No. 6 Reddish VG VG VG VG M VG b VG M F FD&C Red No. 40

Yellowish red VG VG VG VG VG VG c VG F VG Citrus Red No. 2

Orangish red

na na na na na FD&C Yellow No. 5 Lemon yellow VG VG VG VG G VG c VG P

F Orange B

M na na na na FD&C Blue No. 1

Orangish red

G G G F VG c VG VP G FD&C Green No. 3 Bluish green

Greenish blue

F F b VG VP G FD&C Blue No. 2

VP F c VP P VP FD&C Red No. 3

Deep blue

VP

Bluish pink

IN

IN

G c IN VP IN a Acid—10% acetic acid, Base—10% sodium hydroxide, SO 2 —250 mg/L, VG—very good, G—good, M—

VG VG P

moderate, F—fair, P—poor, VP—very poor, IN—insoluble, na—data not available. b To 205 °C.

c To 105 °C. Source : Marmion (1979); Newsome (1990); Rayner (1991).

bleached by SO 2 ). The notable exception is the monoazo FD&C Red No. 40; the electron donor groups on the aromatic rings apparently inhibit the nitrogens from serving as nucleo- philic sites, thus forestalling HSO 3 ⫺ addition. Note that the lack of sulfonic acid groups on Citrus Red No. 2 prevent it from being water soluble. The triphenylmethane colorants FD&C Blue No. 1 and FD&C Green No. 3 are very similar in structure, differing only in the presence of a hydroxyl group in FD&C Green No. 3. These dyes are highly subject to alkali decolorization due to the formation of colorless carbinol bases (von Elbe and Schwartz, 1996).

FD&C Blue No. 2 (indigoid) is highly susceptible to oxidation by ultraviolet light and fades rapidly (Newsome, 1990). FD&C Red No. 3, the sole xanthene, is also very unstable to light. As Noonan (1972) notes, however, this instability is product dependent, with FD&C Blue No. 2 performing well in candies and baked goods, and FD&C Red No.

3 performing well in retorted products. Pure dye concentration in these colorants is strictly controlled by the FDA. FD&C Blue Nos. 1 and 2, FD&C Green No. 3, and FD&C Red 40 must contain no less than 85% pure dye; Orange B, FD&C Red No. 3, and FD&C Yellow Nos. 5 and 6 must contain no less than 87% pure dye; and Citrus Red No. 2 must contain no less than 98% pure dye (21 C.F.R. §74, 1996). While the coloring power of a colorant, the tinctorial strength, is directly proportional to the pure dye concentration, variations of a few percent have little practical significance (von Elbe and Schwartz, 1996). Furthermore, the tinctorial strength is ultimately an intrinsic property of the dye’s chemical structure (i.e., the extinc- tion coefficient); the coloring power is thus best manipulated through optimization of such parameters as the physical form of the dye used and the carrying vehicle (Marmion, 1979).

The colorants listed in Table 9 all exhibit (with the exception of Citrus Red No. 2) some degree of water solubility. However, not all foodstuffs have an adequate moisture content to insure complete dissolution. Previously oil-soluble dyes were used, but these were found to present health hazards, and the last four oil-soluble dyes (FD&C Yellow Nos. 1, 2, 3, and 4) were delisted in 1959 (Noonan, 1972). Although nonaqueous solvents The colorants listed in Table 9 all exhibit (with the exception of Citrus Red No. 2) some degree of water solubility. However, not all foodstuffs have an adequate moisture content to insure complete dissolution. Previously oil-soluble dyes were used, but these were found to present health hazards, and the last four oil-soluble dyes (FD&C Yellow Nos. 1, 2, 3, and 4) were delisted in 1959 (Noonan, 1972). Although nonaqueous solvents

The tinctorial strength of lakes is not proportional to their dye content; particle size, however, greatly affects the color intensity, as the smaller the particle size, the more complete the dispersion and the greater the reflective surface area (Dziezak, 1987; von Elbe and Schwartz, 1996). Lakes have better light, chemical, and thermal stabilities than their associated dyes (due in part to their lower dye content) (Dziezak, 1987); however, the complexity of preparing the substratum and extending the dyes increases the cost of lakes (Meggos, 1984). Furthermore, the energy required to properly disperse the lakes is high; improper dispersion, however, will result in particle-clumping, which is perceptually evidenced as speckling (Dziezak, 1987).

B. Colorants Exempt from Certification

There has been an increasing demand on the part of consumers for natural-source color- ants, due in large part to the perception that foods thus colored are more wholesome and of better quality (Wissgott and Bortlik, 1996). However, natural-source colorants are far less stable to heat, light, or pH, and the colorants themselves may impart extraneous flavors (Moore, 1991). Furthermore, natural-source colorant production is not easily scaled up to meet industrial demand, even with in vitro techniques replacing whole-plant cultivation (Wissgott and Bortlik, 1996). This makes natural-source colors more expensive; red and yellow colorants may cost 100 times more than their synthetic counterparts to deliver the same tinctorial strength (Riboh, 1977).

While natural-source could refer to any colorant derived from animal, vegetable, or mineral sources, it is most often taken to mean ‘‘derived from plant sources.’’ Of the 356 colorant patents on natural sources filed in the years 1969 through 1984, 63% were of plant origin (Francis, 1987); of the twenty-two colorants exempt from certification for use in foods, thirteen are of plant origin and three are nature-identical (see Table 5 ). The non–plant-derived colorants, however, are not to be discounted; this group includes such important colorants as caramel, cochineal, Monascus derivates, and titanium dioxide. See Lee and Khng (this volume) for information regarding the chemistry and properties of the colorants exempt from certification.