JOURNAL OF
CHEMICAL EDUCATION
Nutritive Significance of Some Natural Colors in Foods ONE of the chief functions of naturallv occurrine colors in foods is esthetic in character, giving pleasure and satisfaction to the eye and encouraging the appetitr in that way. Pleasing col. ors in foods often stimulate digestion, increasing their nutritive appeal. Our restaurants and eating places are well aware that warm colors in the surroundings are effective in sharpening the appetite. As an example, a midwestern cafeteria is said t o have doubled its sales of salads by serving them an green plates instead of white plates, creating the illusion of larger and greener salads. However, other functions of color related to biologic phenomenon are beginning to be appreciated, particularly the relation of certain naturally occurring pigments t o specific nutritive factors and t o clinical medicine. Several such naturally occurring pigments are of interest because of their activity in sexual reproduction, especially for certain lower forms of life. In the present discussion only natural colors that are related to foods or their metabolism will be discussed, with special note of their nutritional sienificance. The artificial addition of dves ~ ~ - or ~ medical "~ t o foods is acomprehensive and controversial subject and will not be considered herein as a part of the discussion. I n certain instances f w d may be colored naturally by the method of processing, as by smoking, toasting, curing, or pickling. Thus, the toasting of bread, roasting of coffee and peanuts, caramelization of certain sugar products, and the reddish color of ham imuarted bv the action of nitrites an uork durine- cure. are a few examples of many food4 that ire specially treated for ronsumer acceptance. I n moqt methorls of processing the acidity or alkalinity of the product and lrngrh of hcat treatment are import a n t in the production of color. The scope and variety of the carotenoids occurring in nature is only now beginning t o be fully appreciated. Such chromatic comoounds as the carotenoids for carotinoids) form a lame - and importarlr group widely distributed in foods, and are interesting because certain n~embrrs,i. e., the carotmcs and kryptoxanthin. possess vitamin A activity. I n fact, the vitamin A content of most fwds exists as an activity of a provitamin rather than as that of the preformed vitamin. Like many naturally occurring pigments, the carotenes derive their scientific name from a substance, i.c., carrots, from which they were fmt isolated. I n foods the color of carotenes is yellow as in butter, whereas in oranges it is deeper as an orange yellow. Certain carotenoid pigments contain oxygen and are known as xanthophylls. Because of their close structural relationship, the xanthophylls are occasionally confused with the carotenes. At least m e member of t h e group, kryptoxanthin, bas vitamin A activity, while most of the others are inactive in this respect. The yellow color of xanthophylls is evident in egg yolks, yellow corn, yellow flowers, and in the yellow plumage of canaries. Many green plant materials contain xanthophyll, the yellow color of which is usually masked by the intense green color of chlorophyll. Lycopene, a major pigment of tomatoes and watermelons, is also closely associated with the carotenes but apparently possesses no vitamin A activity. Other carotenoids without vitamin A effect are astaxanthin, chiefly responsible for the change in lobsters from dark green t o red on boiling, a related camtenaid in crabs and shrimps, and a carotenoid that forms a protein complexcharacteristic of the pink color of salmon flesh. Several naphthoquinones have received renewed attention following the discovery that some of them exhibit vitamin K or antihemorrhagic activity. The naphthoquinones are found in the leaves. wood, and roots of certain shrubs or trees, walnut shells. and pathogenic bacteria, such as the tubercle bacillus. They exhibit a variety of colorsyellow, red, brown, and violet. ~~
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The flavones, a class of compounds separate and distinct from the flavins, possess a member, hesperitin or hesperidin, found in citrus fruits, that is related to the chemistry of vitamin P . This vitamin is so called because of its association with the permeability of the capillaries. Another flavone, quercetin, is one of the yellow pigments found in anion skins, corn, beans, and apples. The oxidation of a colorless aqueous solution of thiamine (vitamin BI) t o thiochrome, which possesses a blue fluorescence, is conveniently used nowadays, in estimation of the vitamin in various foodstuffs. ~~Over 10 years ago a fluorescent yellow-green pigment was identified and termed lactofladn (vitamin BIor G ) because of its occurrence in milk. Riboflavin, the modern name for the vitamin. is now employed as a generic term for several flavin compounds having ribose as a constituent of the molecule. Several important respiratory enzymes contain riboflavin, one of the most famous being the "yellow respiratory enzyme" of Warburg and Christian. ~ ~ ~ ~ ~ . The pterins are yellow pigments widely distributed in nature. Xanthopterin, a well-known member of the group found in butt d y wings, appears to be related t o nutritional anemia. Many other naturally occuming pigments are without special nutritional significance. They are important because of their occurrence in various foodstuffsor because of their medicinal application. Chlorophyll, the pigment that gives growing vegetation its verdure, is employed medicinally in combatting infection. and t o increase the production of hemoglobin, e. g., in certain types of anemia characterized by deficiency of hemoglobin precursors. Hemoglobin, the red coloring matter of the blood, bas received much attention. Several degradation products of hemoglobin are important in metabolism. The porphyrins are such substances. Also throneh " elimination of iron bv the liver several hile pigments arc derived from pigments of the blood. For example, bilirubin. a reddish yellow pigment, may be oxidized to bilivcrdin, shtrh is green in color. Cytochromes arc other naturally occurring pigments that are found in vigorausly respiring tissues. Apparently they are universally distributed. During autumn several changes occur in foliage t o produce our gorgeous autumn landscapes. The green hues of chlorophyll are replaced by yellow and red tints, caused, respectively, by carotenoid and anthocyanin pigments. The anthocyanins qr anthocyans are chiefly responsible for the red, violet, or blue pigmentation of many fruits, plant tissues, and flowers. Several such fruits are cherries, cranberries, grapes, plums, strawberries, and tangerines. The anthocyanin, hetanin, occurs as a red coloring matter of the beet; also red cabbage and red radishes owe their color mainly t o this class of pigments. Little is known concerning t h e physiologic property of anthocyanins; their production seems associated with a high sugar content. Limes, lemons, and grapefruit are thought t o awe their color t o tannin comolexes. Melanin: the nredominatine- oiement .in the skin of negroes and a racial characteristic, is thought to he proleinaceous in character. Thc blackening of the hcarts of potatoes anrl of potato slices in air is caused by a n enzymic reaction that forms melanin. Onecannot fail to find how frequently nature uses a carotenoid, chlorouhvll. or anthocvanin.. and t o observe that these substances are aisoriated wthcertain known vitaminsand mrdirin*, or with metabolism. 1)oes this association sucgesl the lnse of pigments such as nnture uses for the nutritive improvement of foods' ~
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-Reprinted, 8 (1945).
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with permission, from Nutritional O b s m d o r y , 6
