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Introduction
Aluminum ammonium sulfate, commonly known as ammonium alum, is a double salt with the formula NH₄Al(SO₄)₂·12H₂O. It is used in food processing similarly to other alums: as a firming agent for pickles, as a clarifying agent, and in some baking powders. Compared to potassium alum, ammonium alum has the advantage of leaving no metallic aftertaste in some applications because the ammonium ion is volatile and can be driven off by heating.
Ammonium alum is a white crystalline solid that is highly soluble in water. Its use in food has declined in recent decades due to the shift away from aluminum-containing additives. However, it is still found in some commercial pickling products and in traditional recipes.
CAS Number
7784-26-1 (dodecahydrate). Anhydrous: 10138-58-6.
Synonyms
Ammonium alum, Aluminum ammonium sulfate dodecahydrate, Ammonium aluminum sulfate, Burnt ammonium alum (heated form), E523 (in EU).
Topic Headings
1. Unique property: volatility of ammonium ion (paragraph form)
Unlike potassium alum or sodium alum, ammonium alum contains the ammonium ion (NH₄⁺). When heated to temperatures above 100°C, the ammonium ion decomposes to ammonia gas and water vapor, leaving behind aluminum sulfate and then aluminum oxide. In baking applications, this means that ammonium alum leaves less inorganic residue than other alums. For example, in baking powders, after the carbon dioxide release reaction, the ammonium alum produces Al(OH)₃ and (NH₄)₂SO₄; upon further heating during baking, (NH₄)₂SO₄ decomposes to ammonia and SO₃, both of which volatilize. The result is a baked product with lower residual aluminum and no off-flavors from sulfate salts. This property historically made ammonium alum a preferred acidulant for certain leavening systems, especially for products where a clean taste is desired. However, ammonia release can produce a slight ammoniacal odor in the oven, which is undesirable in some bakeries.
2. Physical and chemical properties (list form)
• Appearance — White crystals or crystalline powder; forms octahedral crystals.
• Odor — Odorless when dry; slight ammonia odor when heated or in humid conditions.
• Molecular weight — 453.33 g/mol (dodecahydrate).
• Melting point — 94°C (loses water of hydration); decomposes above 200°C releasing ammonia.
• Solubility in water — 12.5 g/100 mL at 20°C; 105 g/100 mL at 100°C.
• Solubility in other solvents — Insoluble in ethanol.
• pH (5% aqueous solution) — 4.0-5.0 (less acidic than other alums).
• Density — 1.64 g/cm³.
• Hygroscopicity — Slightly efflorescent; stable under normal conditions.
• Crystal water — 12 molecules.
3. Food applications and usage levels (list form)
• Pickled vegetables — 0.1-0.3% of brine; firming agent; sometimes preferred for reduced aftertaste.
• Baking powder (specialty formulations) — 10-20% of baking powder; slow-acting acidulant with low residual solids.
• Flour treatment — In some countries, used to improve dough characteristics for certain baked goods.
• Wine and juice fining — 0.01-0.03% as a clarifying agent.
• Water treatment (non-food) — As a coagulant in drinking water and wastewater treatment.
• Dyeing mordant (textile industry) — Not food-related but historically significant.
4. Regulatory status, safety, and limitations (paragraph form)
Ammonium alum is permitted as E523 in the EU with the same aluminum limits as other alums: 50 mg Al/kg in pickled vegetables, 20 mg Al/kg in bakery products. In the USA, it is GRAS as a firming agent and as a component of baking powder, but usage is low. The EFSA TWI for aluminum (1 mg/kg body weight) applies. The ammonium ion component raises additional considerations: individuals with urea cycle disorders or liver disease cannot properly metabolize ammonia, and even small amounts of ammonium salts could lead to hyperammonemia. However, typical food uses involve such small amounts (tens of milligrams per serving) that the ammonium load is negligible compared to protein metabolism. Heating during baking drives off most of the ammonia, further reducing exposure. In pickled vegetables, where no heating occurs, some ammonia remains. The odor and taste of ammonium alum are slightly different from potassium alum, and some consumers detect a "medicinal" note. For these reasons, ammonium alum is the least common of the three food-grade alums in modern food processing. Most commercial pickles use calcium chloride or no firming agent, and baking powders have moved to aluminum-free alternatives.
