Inorganic

As mentioned previously, U.S. regulation of colorants in foods started before 1900, whereas the European Union (EU) did not begin regulation until 1950. It was also evident that legislation appeared only after serious health problems occurred from foods pigmented with inorganic pigments: pickles colored green with copper sulfate; cheese colored with vermilion (HgS) and red lead (Pb3O4); tea leaves colored with copper arsenite, lead chromate, and indigo; and candies and other confectioneries pigmented with lead chromate, red lead, vermilion, and white lead (lead carbonate).1

Today, some inorganic compounds are still used as colorants in foods (e.g., titanium dioxide, carbon black), while others are mainly used because of their other properties, although they also contribute to color. For example, calcium carbonate is added as an anticaking agent, acidity regulator, emulsifier, and stabilizer but it also contributes to color. Magnesium chloride and magnesium hydroxide are also food additives that contribute to food color.2,3

Titanium dioxide (TiO2). Titanium dioxide is the most important white pigment used in world commerce. In 1999, production was estimated in 4 Mt/year. This pigment has been used in a wide range of applications including paint, inks, plastics, rubber, paper, and textiles, and a small percentage of the global production is used in foods and pharmaceuticals. Titanium is the ninth most abundant element present in the Earth's crust. Abundance is greater than zinc (Zn), copper (Cu), lead (Pb), and tin (Sn) combined. In fact, after the tremendous growth of its commercial exploitation in the 1950s and 1960s, titanium displaced the older, more toxic, and less efficient white pigments.4 TiO2 is an intensely white pigment; it exhibits excellent stability toward light, oxidation, pH, and microbiological attack.1,3,5

In nature, TiO2 is found in three crystalline forms: anatase, brookite, and rutile. The major world reserves of titanium-bearing ores are located in New Zealand, Norway, and Brazil, but the purest TiO2 is obtained from Australia, Africa, and Asia. However, the color additive is obtained from synthesis to avoid admixture with other substances.

Commercial TiO2 pigments are produced by either the sulfate or chloride process. In the sulfate process, the TiO2 ore is reacted with sulfuric acid; then the product is hydrolyzed to obtain a hydrated oxide. This is calcined (900°C) to produce the pigmentary TiO2.4,5

FeOTiO2 + 2H2SO4 — TiOSO4 + FeSO4 + 2H2O TiOSO4 + 2H2O — TiO(OH)2 + H2SO4 TiO(OH)2 TiO2 + H2O

The chloride process was developed in 1920 but not commercially applied until the late 1950s. Here ore reacts with gaseous chlorine in the presence of coke to obtain liquid titanium tetrachloride; the product is distilled and oxidized in the vapor phase to produce pigmentary titanium dioxide.4

TiO2 + C + 2Cl2 — TiCl4 + CO2 TiCl4 + O2 — TiO2 + 2Cl2

Different modifications are applied to obtain a TiO2 with the balance of properties required. The characteristics of titanium dioxide used for foodstuff, pharmaceutical, and cosmetic products are established by the FDA in the United States, and similar regulations are established in most other parts of the world. And as can be expected, only grades of the highest purity are permitted. Uncoated anatase pigments, with the lowest levels of impurities, are generally used; these pigments have higher TiO2 content, between 95 and 99%.14

In the United States, TiO2 is approved for use in general for coloring foods, but the quantity added must not exceed 1% by weight of the food.6 The principal use is in sugar-panned confectionery to give an opaque white finish or as background to other colors. The approval for drugs and cosmetics is in accordance with GMP (Good Manufacturing Practices).15 TiO2 is virtually insoluble in all common solvents but soluble in mineral acids (HCl and H2SO4); its commercial presentations are in water- and oil-dispersible forms.3

JECFA has not established an acceptable daily intake (ADI); thus, TiO2 is permitted at GMP. In addition, TiO2 is used to color confections, cheese, icings, tableted drug products, and a variety of cosmetics, as shown in Table 5.125 Today, the importance of whiteness in the sensory characteristics of products such as fat-free milk and marine products has been clearly established. Particularly, the introduction of new pigmenting agents in place of TiO2 has been proposed but without success.7 It has been shown that xanthan gum at 0.05/kg stabilizes the dispersion of TiO2 providing the best whitening characteristic and without undesirable effects.8 In addition, TiO2 has been proposed as a barrier film, by coating glass containers to protect sensitive foods from light; the film is prepared by adding 20 g of TiO2 per kg of polymer and is used to protect a caramel-containing product (aquavit) that is degraded by light.9

Carbon black. Carbon black is composed of a fixed content of carbon (80 to 96%) and volatile compounds and its particles are of colloidal size (20 to 65 pm). It is produced by the controlled combustion of natural gas followed by ignition against a mobile surface of cold iron where carbon black is deposited; conditions

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