2000 mg/kg 5000 mg/kg 2200 mg/kg 1500 mg/kg 20000 mg/kg 3500 mg/kg
GMP = good manufacturing practices; UHT = ultrahigh temperature. Source: Adapted from JECFA (2001).2
hydroxides provide colors in the range of red, yellow, and black. In 1984, iron oxide sold in the United States amounted around 50 x 106 kg. Almost all iron oxide pigments meet the FDA requirements for plastics, pigments, or inks in contact with food; only some meet the requirements for cosmetics. As a class, iron oxides are second only to titanium dioxide in volume consumed. They show excellent heat and light stability.13
Iron oxides are obtained by three different processes. According to all legislation they are defined as inorganic pigments and are used as food additives: iron oxide red (other names are pigment red 101, bright red iron oxide, Indian red, Turkey red, and ferrite red, among others); ferrite yellow, also known as synthetic ocher, ferrite oxide monohydrate, iron oxide yellow, or pigment yellow 42; and iron oxide black.
Several manufacturing processes produce synthetic red iron oxides. Some of these are (1) calcination of ferrous sulfate, (2) precipitation of red iron oxide, (3) calcination of yellow iron oxide, and (4) calcination of black iron oxide.14
1. Calcination of ferrous sulfate (copperas red iron oxide). Iron sulfate (cop-pera) is calcinated to Fe2O3 (coppera red). The process is carried out in two stages:
6FeSO4 • H2O + 1.5O2 —^ Fe2O3 + 2Fe2(SO4)3 + 6H2O
Calcined Fe2O3 is washed, dried, milled, and packed. In this process, a range of colors from light to dark is produced. The particle shape is spheroidal.
2. Precipitated red iron oxide. Red iron oxide is produced by nucleation in which a seed of iron oxide is used in an aqueous solution of ferrous salt and scrap steel. Particle size controls the shade of the finished oxide; nucleation is stopped at the proper shade level and the precipitated red iron oxide is filtered and dried. The particle is rhombohedral and a soft dispersible.
3. Red iron oxide produced from yellow iron oxide. Yellow iron oxide is the calciner feedstock:
Particle shape is derived from the yellow iron oxide and is acicular.
4. Red iron oxide produced from black iron oxide. Red iron oxide is produced by calcination of synthetic black iron oxide:
The particle shape is spheroidal and a wide range of colors is available.
Iron red oxides do not exhibit the color purity or the high chroma of the organic pigments but their price is one quarter to one tenth lower. On the other hand, the water content of ferrite yellow (Fe2O3xH2O) is ~10.1%. It is produced from selected iron ores (ochers and raw siennas) and by three different procedures:
1. Penniman-Zoph or scrap process. This process uses scrap steel and a ferrous salt. The first step is the formation of the yellow iron oxide nuclei of seed as described by the following reactions:
2 22 seed 2
The yellow iron oxide seed is transferred to a vessel that contains scrap and dilute ferrous salt solution. In the presence of oxygen the ferrous sulfate is converted to iron oxide. In the reaction, sulfuric acid is produced, which reacts with the iron of the vessel to regenerate the ferrous sulfate:
The reaction is maintained until the crystals reach the proper size for the color shade desired. Pigment suspension is removed from the tank, and the product is washed, dried, milled, and packed. 2. Direct precipitation process. Seed is formed as described above and transferred to a reaction vessel that contains a solution of a ferrous salt, oxygen, and alkali. Alkali is used to neutralize the sulfuric acid produced, allowing the reaction to continue:
H2SO4NH4OH->(NH4)2SO4 + H2O
The main difference from the Penniman process is that in this process iron scrap is not used in the crystal-growing step. 3. Aniline process. Metallic iron is used to reduce mononitrobenzene and aniline, and iron oxide is formed:
Iron oxide black is also known as ferrous ferric oxide or iron (II,III) oxide. Its formula is FeOFe2O3 and is prepared from iron (II) salt solutions by a single- or two-phase precipitation process or by the Laux process by nitrobenzene reduction with metallic iron. In the two-stage process (1) iron salt (e.g., sulfate) is alkalized to precipitate the iron, and the solution is oxidized to convert the precipitate to goethite; (2) the solution is further alkalized to precipitate more iron, and the precipitate is allowed to react with goethite to form iron oxide black. In addition, in the second stage, metal (e.g., copper, nickel, aluminum, chromium, titanium, zirconium, or vanadium at 0.05 to 5%) is added to obtain a pigment with high color intensity. The global reaction can be written as follows:15
FeSO4 + NaOH + O2 Metal > FeO • Fe2O3 + Na2SO4 + H2SO4
The precipitated iron oxide black pigment is dried and ground to a fineness that depends on its use.
The Laux process is analogous to the aniline process described above, but conditions are modified to obtain a product with a magnetite structure. In the nitrobenzene reduction process, pigments are subjected to a heat treatment of 200 to 800°C under a nonoxidizing atmosphere, which allows controlled conditions of oxidation that are necessary to obtain the iron oxide black.
Iron oxides are insoluble in most solvents but usually soluble in hydrochloric acid. They are permitted by the FDA at levels not exceeding 0.25% by weight in fish pastes and pet foods and are also permitted in pharmaceutical and cosmetological applications.6 This pigment is the preferred choice when products are manufactured by severe heat treatments.1 Codex Alimentarius Commission2 has regulated the use of iron oxide for water-based flavored drinks, including "sport" or "electrolyte" beverages and particulated beverages at 100 mg/kg.
Silver, gold, and aluminum. These metals are used as a finely divided powder or leaf for confectionery items and cake decorations in Middle and Far Eastern countries.1 However, the use of these metals in foods is not approved by the FDA.3,6
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