The Chemical Composition and Origin
Erythrosine, chemically known as FD&C Red No. 3, is an organoiodine compound and a specific type of xanthene dye. Its precise chemical name is disodium 2′,4′,5′,7′-tetraiodofluorescein. This name gives away a critical piece of its composition: it is a tetraiodinated derivative of fluorescein, meaning it is a fluorescein molecule with four iodine atoms attached. The presence of these iodine atoms accounts for a significant portion of its molecular weight, and it is the key chemical modification that gives the compound its vibrant pink-red color. The disodium salt form makes it highly soluble in water, which is a desirable trait for food and drug applications.
Unlike colorants sourced directly from nature, erythrosine is a man-made, or synthetic, product. It is primarily derived from petroleum resources through a controlled chemical process in a laboratory setting. This artificial origin is a common characteristic of many food colorings used in processed foods and pharmaceuticals today.
The Synthesis Process: A Step-by-Step Guide
The creation of erythrosine is a two-step chemical process that starts with simpler compounds and ends with the complex dye molecule. This careful synthesis ensures the final product is a consistent and stable coloring agent.
Step 1: Creating the Fluorescein Base
The synthesis begins with two primary starting materials: resorcinol and phthalic anhydride. These are simple organic molecules that, when heated together, undergo a condensation reaction.
- Heating: Resorcinol and phthalic anhydride are combined and subjected to heat.
- Condensation: Under these conditions, the two molecules join together, with the removal of a water molecule, to form a new, larger compound.
- Product: The result of this initial reaction is fluorescein, a yellow-green fluorescent compound that forms the molecular backbone of erythrosine.
Step 2: Iodination to Form Erythrosine
Once fluorescein has been created, it is modified in a crucial second step to become erythrosine.
- Reaction with Iodine: Fluorescein is reacted with iodine and potassium iodide.
- Substitution: This reaction, known as iodination, substitutes four hydrogen atoms on the fluorescein molecule with four iodine atoms.
- Color Change: This substitution is what transforms the yellow-green fluorescein into the bright red-pink erythrosine.
- Disodium Salt Conversion: To make it more soluble for commercial use, the erythrosine compound is often converted into its disodium salt form.
Uses and Regulatory Status
Erythrosine has been widely used for decades in a variety of products, from foods to cosmetics and pharmaceuticals. However, its regulatory status and future use are subjects of ongoing change and debate due to health concerns, particularly regarding potential links to cancer.
Comparison with Other Red Dyes
| Feature | Erythrosine (Red No. 3) | Allura Red (Red No. 40) |
|---|---|---|
| Chemical Type | Organoiodine, Xanthene dye | Azo dye |
| Synthesis | Produced via iodination of fluorescein | Made from coal tar and aromatic hydrocarbons |
| Color | Cherry-pink to red | Bright red |
| US Regulatory Status | Banned in foods and ingested drugs by the FDA, effective 2027/2028 | Remains approved by the FDA for most uses |
| California Status | Banned in foods, effective 2027 | Banned in foods sold in public schools |
| Key Concern | Potential link to cancer in animals | Linked to neurobehavioral problems in children |
The Phase-Out of Erythrosine
Due to the significant concerns surrounding erythrosine, particularly its potential carcinogenicity based on animal studies, regulatory bodies have acted to limit or ban its use. The US Food and Drug Administration (FDA) first banned the use of Red No. 3 in cosmetics and topical drugs in 1990. Most recently, in January 2025, the FDA announced a ban on its use in all food and ingested drugs, a decision that will be phased in over the next few years. States like California have also moved independently to ban the additive.
This regulatory shift is a direct response to longstanding scientific evidence and advocacy from public health groups. Manufacturers are now required to remove the substance from their products and are actively seeking alternative colorants. For consumers, this means fewer products containing this synthetic dye will be available in the future. As a result, many companies have already removed erythrosine from their products, leading to a significant drop in its use.
Conclusion
In conclusion, erythrosine is a synthetic, iodine-containing dye (specifically, a tetraiodofluorescein) made from the chemical synthesis of resorcinol and phthalic anhydride. The process involves creating a fluorescein precursor, which is then iodinated to produce the final red-colored compound. While it has been a common coloring agent in foods, drugs, and cosmetics for decades, regulatory decisions driven by health concerns, such as the recent FDA ban on its use in foods, have led to its systematic phase-out in many regions. Its synthetic nature and chemical composition, particularly the presence of iodine, distinguish it from other food colorings and have been at the center of its safety debate.
For more technical information on food additives, consult the database provided by the Food and Agriculture Organization (FAO).
