10 Colorants

Pag e 651 10 Colorants J. H. VON ELBE University of Wisconsin—Madison, Madison, Wisconsin STEVEN J. SCHWARTZ* North Carolina State University, Raleigh, North Carolina 10.1. Introduction 651 10.2. Pigments in Animal and Plant Tissue 653 10.2.1. Heme Compounds 653 10.2.2. Chlorophyll 659 10.2.3. Carotenoids 673 10.2.4. Flavonoids and Other Phenols 681 10.2.5. Betalaines 697 10.3. Food Colorants 703 10.3.1. Regulatory Aspects 703 10.3.2. Properties of Certified Dyes 708 10.3.3. Use of Certified Dyes 711 10.3.4. Colors Exempt from Certification 713 10.3.5. Use of Colors Exempt from Certification 717 Bibliography 718 References 718 10.1 Introduction To understand colorants in foods some terms need to be defined. Color refers to human perception of colored materials—red, green, blue, etc. A colorant is any chemical, either natural or synthetic, that imparts color. Foods have color because of their ability to reflect or emit different quantities of energy at wavelengths able to stimulate the retina in the eye. The energy *Present affiliation: The Ohio State University, Columbus, Ohio. Pag e 652 range to which the eye is sensitive is referred to as visible light. Visible light, depending on an individual's sensitivity, encompasses wavelengths of approximately 380–770 nm. This range makes up a very small portion of the electromagnetic spectrum (Fig. 1). In addition to obvious colors (hues), black, white, and intermediate grays are also regarded as colors. Pigments are natural substances in cells and tissues of plants and animals that impart color. Dyes are any substances that lend color to materials. The term dye is commonly used in the textile industry. In the U.S. food industry, a dye is a food-grade water- soluble colorant certified by the U.S. Food and Drug Administration (FDA). These specific dyes are referred to as “certified colors,” and each one is assigned an FD&C number. The FD&C designation means that the dye may be used in foods, drugs, and cosmetics. Added to the approved list of certified colors are the FD&C lakes. Lakes are dyes extended on a substratum and they are oil dispersible. The dye/substratum combination is achieved by adsorption, coprecipitation, or chemical reaction. The complex involves a salt of a water-soluble primary dye and an approved insoluble base stratum. Alumina is the only approved substratum for preparing FD&C lakes. In addition, there are other dyes or lakes approved for use in other countries, where specifications are established by the European Economic Community (EEC) or the World Health Organization (WHO). Colorants exempt from certification may also be used. These are natural pigments or substances synthesized, but identical to the natural pigment. A classification of colorants and an example within each class are given in Table 1. Color and appearance are major, if not the most important, quality attributes of foods. It is because of our ability to easily perceive these factors that they are the first to be evaluated by the consumer when purchasing foods. One can provide consumers the most nutritious, safest, and most economical foods, but if they are not attractive, purchase will not occur. The consumer also relates specific colors of foods to quality. Specific colors of fruits are often associated with maturity—while redness of raw meat is associated with freshness, a green apple may be judged immature (although some are green when ripe), and brownish-red meat as not fresh. Color also influences flavor perception. The consumer expects red drinks to be strawberry, raspberry, or cherry flavored, yellow to be lemon, and green to be lime flavored. The impact of color on sweetness perception has also been demonstrated. It should also be noted that some substances such as b-carotene or riboflavin are not only colorants but nutrients as well. It is clear FIGURE 1 Electromag netic spectrum. Pag e 653 TABLE 1 Classification of Colorants Colorant Example A Certified 1. Dye FD&C Red No. 40 2. Lake Lake of FD&C Red No. 40 B Exempt from certification 1. Natural pig ments Anthocyanin, juice concentrate, annatto extract 2. Synthetic (nature identical) b-Carotene therefore that color of foods has multiple effects on consumers, and it is wrong to regard color as being purely cosmetic. Many food pigments are, unfortunately, unstable during processing and storage. Prevention of undesirable changes is usually difficult or impossible. Depending on the pigment, stability is impacted by factors such as the presence or absence of light, oxygen, heavy metals, and oxidizing or reducing agents; temperature and water activity; and pH. Because of the instability of pigments, colorants are sometimes added to foods. The purpose of this chapter is to provide an understanding of colorant chemistry—an essential prerequisite for controlling the color and color stability of foods. 10.2 Pigments in Animal and Plant Tissue 10.2.1 Heme Compounds Heme pigments are responsible for the color of meat. Myoglobin is the primary pigment and hemoglobin, the pigment of blood, is of secondary importance. Most of the hemoglobin is removed when animals are slaughtered and bled. Thus, in properly bled muscle tissue myoglobin is responsible for 90% or more of the pigmentation. The myoglobin quantity varies considerably among muscle tissues and is influenced by species, age, sex, and physical activity. For example, pale-colored veal has a lower myoglobin content than red-colored beef. Muscle-to-muscle differences within an animal also are apparent, and these differences are caused by varying quantities of myoglobin present within the muscle fibers. Such is the case with poultry, where light-colored breast muscle is easily distinguished from the dark muscle color of leg and thigh muscles. Listed in Table 2 are the major pigments found in fresh, cured, and cooked meat. Other minor pigments present in muscle tissue include the cytochrome enzymes, flavins, and vitamin B12. 10.2.1.1 Myoglobin/hemoglobin Structure of Heme Compounds Myoglobin is a globular protein consisting of a single polypeptide chain. Its molecular mass is 16.8 kD and it is comprised of 153 amino acids. This protein portion of the molecule is known as globin. The chromophore component responsible for light absorption and color is a porphyrin known as heme. Within the porphyrin ring, a centrally located iron atom is complexed with four tetrapyrrole nitrogen atoms. Thus, myoglobin is a complex of globin and heme. The heme porphyrin is present within a hydrophobic pocket of the globin protein and bound to a histidine residue (Fig. 2). The centrally located iron atom shown possesses six coordination sites, four of which are occupied by the nitrogen atoms within the tetrapyrrole ring. The fifth coordination site Pag e 654 TABLE 2 Major Pig ments Found in Fresh, Cured, and Cooked Meat Pig ment Mode of formation State of iron State of hematin nucleus State of g lobin Color 1. Myog lobin Reduction of metmyog lobin; Fe2+ Intact Native Purplish-red deoxyg enation of oxymyog lobin 2. Oxymyog lobin Oxyg enation of myog lobin Fe2+ Intact Native Brig ht red 3. Metmyog lobin Oxidation of myog lobin, Fe3+ Intact Native Brown oxymyog lobin 4. Nitric oxide myog lobin Combination of myog lobin with Fe2+ Intact Native Brig ht red (pink) (nitrosomyog lobin) nitric oxide 5. Nitric oxide metmyog lobin Combination of metmyog lobin with Fe3+ Intact Native Crimson (nitrosometmyog lobin) nitric oxide 6. Metmyog lobin nitrite Combination of metmyog lobin with Fe3+ Intact Native Reddish-brown excess nitrite 7. Globin myohemochromog en Effect of heat, denaturing ag ents Fe2+ Intact (usually bound to Denatured Dull red on myog lobin, oxymog lobin; denatured protein other than (usually irradiation of g lobin g lobin) detached) hemichromog en (table continued on next page) Pag e 655 (table continued from previous page) Pig ment Mode of formation State of iron State of hematin nucleus State of g lobin Color 8. Globin myohemichromog en Effect of heat, denaturing ag ents Fe3+ Intact (usually bound to Denatured Brown on myog lobin, oxymog lobin, denatured protein other than (usually (sometimes metmyog lobin, hemochromog en g lobin) detached) g reyish) 9. Nitric oxide myohemochromog en Effect of heat, denaturing ag ents Fe2+ Intact Denatured Brig ht red (pink) on nitric oxide myog lobin 3+ 10. Sulfmyog lobin Effect of H2S and oxyg en on Fe Intact but one double bond Native Green myog lobin saturated 11. Metsulfmyog lobin Oxidation of sulfmyog lobin Fe3+ Intact but one double bond Native Red saturated 12. Choleg lobin Effect of hydrog en peroxide on Fe2+ or Fe3+ Intact but one double bond Native Green myog lobin or oxymyog lobin; effect saturated of ascorbine or other reducing ag ent on oxymyog lobin 13. Nitrihemin Effect of larg e excess nitrite and Fe3+ Intact but reduced Absent Green heat on 5 14. Verdohaem Effect of reag ents as in 7–9 in Fe3+ Porphyrin ring opened Absent Green excess 15. Bile pig ments Effect of reag ents as in 7–9 in larg e Fe absent Porphyrin ring destroyed Chain Absent Yellow or excess of porphyrins colorless From Lawrie (61) courtesy of Perg ammon Press, New York Pag e 656 FIGURE 2 Tertiary structure of myog lobin. (Ref. 35.) is bound by the histidine residue of globin, leaving the sixth site available to complex with electronegative atoms donated by various ligands. Hemoglobin consists of four myoglobins linked together as a tetramer. Hemoglobin, a component of red blood cells, forms reversible complexes with oxygen in the lung. This complex is distributed via the blood to various tissues throughout the animal where oxygen is absorbed. It is the heme group that binds molecular oxygen.

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