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Introduction
Aluminum sulfate (Al₂(SO₄)₃) is an inorganic salt used primarily in food processing as a firming agent for pickled vegetables and as a processing aid in water treatment. It is also used in baking powder formulations (though less common today) and as a clarifying agent in sugar refining. In pickling, aluminum ions cross-link pectin in vegetable cell walls, maintaining crispness.
Aluminum sulfate is highly soluble in water and hydrolyzes to form acidic solutions. This acidity also contributes to its functionality in pickling brines. However, concerns about aluminum bioavailability and potential neurotoxicity have led to reduced use in food applications, with calcium salts (calcium chloride, calcium lactate) often preferred as alternatives.
CAS Number
10043-01-3 (anhydrous). Also 17927-65-0 (hexadecahydrate), 7784-31-8 (octadecahydrate).
Synonyms
Aluminium sulphate, Sulfuric acid aluminum salt (3:2), Cake alum, Filter alum, Papermaker's alum.
Topic Headings
1. Mechanism of firming action in pickled vegetables (paragraph form)
The crispness of pickled cucumbers, peppers, and other vegetables depends on the integrity of pectin in the middle lamella of cell walls. During pickling, acidic brines (acetic acid or lactic acid) can solubilize pectin, leading to softening. Aluminum sulfate dissociates in water to form Al³⁺ ions and SO₄²⁻ ions. Al³⁺ ions act as polyvalent cations that form ionic bridges between negatively charged carboxyl groups on adjacent pectin molecules. This cross-linking stabilizes the cell wall structure and prevents pectin solubilization, even at low pH. The effect is similar to calcium firming (using calcium chloride), but aluminum ions are more effective at low pH because they do not precipitate as phosphates or carbonates as easily as calcium. However, the use of aluminum in pickling has declined because of health concerns. In brines with pH 3.0-3.5, aluminum sulfate at 0.1-0.3% (by weight of brine) maintains cucumber firmness for 6-12 months of storage.
2. Physical and chemical properties (list form)
• Appearance — White to off-white crystalline powder or lumps (anhydrous); hydrated forms are also white.
• Odor — Odorless.
• Molecular weight — 342.15 g/mol (anhydrous).
• Melting point — 770°C (decomposes before melting).
• Solubility in water — 31.2 g/100 mL at 0°C; 36.4 g/100 mL at 20°C; 89.0 g/100 mL at 100°C (highly soluble).
• Solubility in other solvents — Insoluble in ethanol.
• pH (5% aqueous solution) — 2.5-3.5 (strongly acidic due to hydrolysis of Al³⁺).
• Density — 2.67 g/cm³ (anhydrous).
• Hygroscopicity — Hygroscopic; absorbs moisture from air.
• Stability — Stable; decomposes at high temperatures to aluminum oxide and sulfur trioxide.
3. Food applications (historical and current) (list form)
• Pickled cucumbers and other vegetables — 0.1-0.3% of brine; maintains crispness during fermentation and storage (declining use).
• Baking powder (historical) — Used as the acidic component with sodium bicarbonate; replaced by sodium aluminum phosphate and monocalcium phosphate.
• Sugar refining (clarification) — 0.01-0.05% as a flocculant to remove impurities from cane and beet sugar juices.
• Water treatment (non-food but relevant) — Used as a coagulant in municipal drinking water treatment; residual aluminum may remain in finished water.
• Egg white clarification — Used in some juice and wine fining processes.
4. Safety, regulatory status, and concerns (paragraph form)
JECFA has not established a specific ADI for aluminum sulfate but evaluates aluminum compounds collectively. The provisional tolerable weekly intake (PTWI) for aluminum is 2 mg/kg body weight (JECFA 2011). EFSA established a tolerable weekly intake (TWI) of 1 mg/kg body weight for aluminum. Aluminum is not easily absorbed from the gastrointestinal tract (approximately 0.1-1% absorption), but absorbed aluminum accumulates in bone and brain and has been associated with neurotoxicity in high doses. Long-term exposure has been linked to Alzheimer's disease in some epidemiological studies, though causality is not proven. Therefore, food use of aluminum-containing additives is restricted in many countries. EU maximum levels: 50 mg/kg (as aluminum) in pickled vegetables; 10 mg/kg in cheese; 20 mg/kg in some bakery products. In the USA, aluminum sulfate is GRAS when used as a firming agent in pickles, but its use is voluntarily limited by most manufacturers. The FDA requires labeling of aluminum-containing additives in foods. Individuals with kidney disease are advised to limit aluminum intake because reduced renal excretion leads to accumulation and potential bone disease (osteomalacia) and neurological symptoms.
