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Introduction
Aluminum sodium sulfate, commonly known as sodium alum or SAS, is a double salt with the formula NaAl(SO₄)₂·12H₂O. It is used in food processing primarily as a firming agent and as an acidic component in baking powders. The compound provides aluminum ions for cross-linking pectin and also provides acidity through hydrolysis.
Sodium alum is a white crystalline solid that is highly soluble in water. It has been used for decades in pickling, baking, and water treatment. However, like other aluminum compounds, its use has declined due to health concerns about dietary aluminum intake. It is still permitted in many countries but with usage limits.
CAS Number
10102-71-3 (dodecahydrate). Anhydrous: 20917-56-2.
Synonyms
Sodium alum, Aluminum sodium sulfate dodecahydrate, Sodium aluminum sulfate, SAS, Soda alum, E521 (in EU, though rarely assigned).
Topic Headings
1. Role as an acidic component in baking powder (paragraph form)
Baking powders contain an alkali (sodium bicarbonate) and one or more acidulants that release carbon dioxide when hydrated. Aluminum sodium sulfate is a slow-acting acidulant that reacts with bicarbonate only at elevated temperatures (above 50°C). In double-acting baking powders, SAS provides the "second action" during baking, producing additional gas after the initial reaction from fast-acting acids (like monocalcium phosphate). The reaction: NaAl(SO₄)₂ + 3NaHCO₃ → Al(OH)₃ + 2Na₂SO₄ + 3CO₂↑. The slow release ensures that gas is generated throughout the baking process, giving baked goods (cakes, muffins, pancakes) a fine, uniform crumb and good volume. Typical baking powder contains 10-30% sodium alum. The aluminum hydroxide produced as a byproduct remains in the baked good and contributes to a slightly bitter aftertaste, which is a drawback compared to other acidulants.
2. Physical and chemical properties (list form)
• Appearance — White crystalline powder, granules, or colorless octahedral crystals.
• Odor — Odorless.
• Molecular weight — 458.28 g/mol (dodecahydrate).
• Melting point — 61°C (loses water of hydration at 61-100°C; decomposes at higher temperatures).
• Solubility in water — 51 g/100 mL at 20°C (highly soluble).
• Solubility in other solvents — Insoluble in ethanol.
• pH (5% aqueous solution) — 3.5-4.5 (acidic).
• Density — 1.68 g/cm³.
• Hygroscopicity — Slightly efflorescent (loses water crystals in dry air).
• Crystal water — 12 molecules of water of crystallization.
3. Food applications and usage levels (list form)
• Baking powder (double-acting) — 10-30% of baking powder formulation; provides slow CO₂ release during baking.
• Pickled vegetables (cucumbers, peppers) — 0.1-0.3% of brine; acts as a firming agent similar to aluminum sulfate.
• Firming agent for canned fruits and vegetables — Used in some commercial products (declining use).
• Water treatment (coagulant) — In some drinking water facilities.
• Cheese processing (historical) — Used as a firming agent in cheese curds; largely replaced by calcium chloride.
4. Regulatory status and safety considerations (paragraph form)
Sodium alum is permitted in the EU under Directive 95/2/EC with maximum aluminum limits: 50 mg/kg in pickled vegetables, 20 mg/kg in bakery products (from leavening agents). The FDA considers aluminum sodium sulfate as GRAS for use in baking powder and pickling, but it must be listed on ingredient labels. The safety concerns are the same as for other aluminum salts: low absorption but potential accumulation in bone and brain. The EFSA TWI for aluminum is 1 mg/kg body weight. A typical slice of cake made with alum-based baking powder contains approximately 5-10 mg aluminum. For a 70 kg adult, the TWI is 70 mg, so occasional consumption is safe, but daily intake from multiple aluminum-containing foods could exceed the limit. Many modern baking powders use sodium acid pyrophosphate (SAPP) or monocalcium phosphate (MCP) instead of SAS to avoid aluminum residues. In the USA, aluminum-free baking powders are widely available. Individuals with chronic kidney disease should avoid products with alum because of reduced aluminum excretion.
