Synthetic

1. General Information

Since the earliest written record of the use of dyestuffs in China (2600 b.c.), a long story of use of natural colorants has been accumulated. Until the mid-19th century, all dyes were obtained from plant (leaves, roots, fruits) or animal extracts. The textile industry developed the use of such natural dyes as cochineal, turmeric, wood madder, and henna. However, the discovery of mauve, the first synthetic dyestuff, by William Henry Perkin in 1856 was a breakthrough for development of the color industry. The synthesis was carried out while Perkin searched for a cure for malaria; he was working with the coal tar derivative aniline and after an accidental aniline oxidation a derivative with dyeing capabilities was obtained. Thereafter, Perkin established the first factory of organic synthetic dyes to produce mauve. In addition, experiments were carried out to discover more dye products from aniline and other coal tar derivatives.16

In 1860, the discovery of the diazotization and coupling reaction by the German Peter Griess was the next major breakthrough for development of the color industry. Griess observed that aromatic amines (R-NH2) could be readily converted to diaz-onium compounds (R-N=NCl) under appropriate conditions. Those were years of intense research and other dyes were introduced: magenta (1858-1859), methyl violet (1861), and malachite green (1877), among others. Manufacture of organic pigments in the United States began in 1895 with the production of para red, lithol red, toluidine red, and hansa yellow.16 These efforts also focused on laboratory synthesis of natural dyes, and alizarine was the first of these products in 1868, followed by indigo in 1880. In 1888, Armstrong proposed the "quinone theory" when it was found that the best-known colorants had the quinone group in their structure. This theory was the base for the proposal of the resonance of double bonds as the main cause of color in organic synthetic pigments, establishing the importance of conjugated double bonds in colorant structure. The last assumption was suggested by Bury in 1835 with doebner violet:10

The economic importance of the color industry is clearly reflected by the large number of synthesized compounds; as many as 700 colorants are currently available to industry.3

As discussed above, the first comprehensive legislation on colors appeared in 1906. Later, the 76th Food Inspection Decision, July 13, 1907, marked the appearance of certified colorants. In that year, only 16 of the 80 colorants offered to the food industry were considered safe. Between 1907 and 1914, the U.S. industry of certifiable colors grew consistently but was limited by a strong dependence on imported raw materials. With the start of World War I in 1914, the importation of raw materials for certified color production was interrupted and U.S. color industrials introduced new intermediates and other coal tar derivatives with purity established by the U.S. Department of Agriculture. By 1937, approximately 270,000 kg of food certifiable colorants was produced. However, the approved colors suffered several limitations. They were oil insoluble, with a short range of hues and other inconvenient physical properties. Consequently, it was not rare that new certifiable colors were introduced: tartrazine, sudan I, butter yellow, yellow AB, and yellow OB, which are oil soluble; plus fast green, guinea green B, ponceau SX, sunset yellow, and brilliant blue.1017

In the early 1900s, one of the main problems with organic colorants was bleeding; and in the 1920s, Alfred Siegel of Du Pont solved the bleeding problem by precipitating the dye with divalent alkaline earth or transition metals. Precipitation with these metals produces a significant reduction in water solubility and consequently bleeding. Another approach to reduce bleeding was the use of phosphotung-stic, phosphomolybdic, or phosphotungstomolybdic acid, ascribed to Imerhiser and to Beyer in 1917, as precipitating agents. It was shown that careful selection of the precipitation reaction can almost entirely eliminate the problems of bleeding associated with residual dyestuff in the pigment.

Pigmented lakes are the oldest class of organic pigments, although strictly they are not entirely organic, as they result from the precipitation of an organic dye onto an inorganic substrate (most often alumina hydrate). The name lake arose because the first organic pigment to be absorbed on an inorganic substrate was obtained from resin of the lac insect (Laccifer lacca). The first synthetic dyes used in lake production were pigment scarlet and xylidine ponceau. After the 1938 Enactment, only certified colorants were approved for use in foods, drugs, and cosmetics (FD&C). By then, 18 colors had been approved for use in foods, with the introduction of orange SS and the red XO. These pigments were designed as FD&C colorants (certifiable colorants used to pigment food, drugs, and cosmetics) and only coal tar derivatives were permitted.3

In 1950, two events were decisive in regulation of certifiable colorants: (1) several children became sick after consumption of popcorn and candies pigmented with high colorant levels, and (2) the FDA carried out a new toxicity evaluation of colorants, implementing stringent conditions (higher doses and longer durations). After this evaluation, FD&C orange No. 1, FD&C orange No. 2, and FD&C red No. 32 were taken off the list of permitted colorants. All the events associated with certifying colorants led to the appearance of the "Delaney clause," and approval of the products became expensive and time-consuming. Consequently, some colorants listed as "provisional" were taken off after permission expired and only the colorants with the highest economic feasibility survived. Other delisted colors in the United States were FD&C yellows No. 1, 2, and 4; FD&C violet No. 1; and FD&C reds No. 2 and 4. Moreover, of the seven colorants recommended by Hesse, only two remain in the list of approved colorants (FD&C red No. 3 and FD&C blue No. 2). FDA certified tartrazine in 1970 and sunset yellow obtained its final FDA approval in 1986. The general restrictions of quality imposed for the FD&C colorants were less than 0.001% of lead (Pb), less than 0.00014% of arsenic (As) as As2O3, and only traces of other heavy metals.3101819

By 1916, the color industry was concentrated in Germany and only after the onset of World War II was its position as a world supplier of dyes lost. The United States emerged as number one in the production and exportation of dyes. Today, Western Europe is the leader in exportation of organic synthetic colorants. Government regulation of food colorants is complex and consequently legislation differs around the world. As an example, some colorants are permitted by the United States and WHO (allura red, brilliant blue, fast green) but not by the EU; others are permitted in the EU and WHO (carmoisine, ponceau 4R, patent blue V) but not in the United States; while still other pigments have wide acceptability around the world (sunset yellow, tartrazine, indigotine, and erythrosine) (Table 4.2). Some of the colorants used today in foods are presented in Figure 5.1.20 Many other colorants are permitted only as D&C (drug and cosmetic) colorants, such as D&C red 9, D&C red 7, D&C orange 4, and D&C orange 17.2122

2. Reactions in the Production of Pigments

The raw materials in the manufacture of dyes are organics and inorganics. The most used organic compounds are aromatic and sometimes heterocyclic (Figure 5.2). Coal

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