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Introduction
Aluminum potassium sulfate, commonly known as potassium alum or potash alum, is a double salt with the formula KAl(SO₄)₂·12H₂O. It has been used for centuries in food processing, medicine, and water purification. In food, it is used as a firming agent for pickles, as a clarifying agent for water and wine, and as a component in some baking powders.
Potassium alum forms large, colorless, octahedral crystals that are highly soluble in water. The astringent taste of alum comes from its ability to precipitate proteins. In pickling, it preserves the crisp texture of vegetables by cross-linking pectin. However, like all aluminum compounds, its use is declining due to health concerns.
CAS Number
7784-24-9 (dodecahydrate). Anhydrous: 10043-67-1.
Synonyms
Potassium alum, Potash alum, Aluminum potassium sulfate dodecahydrate, Alum (common name), Burnt alum (when heated), E522 (in EU).
Topic Headings
1. Traditional and current uses in food (paragraph form)
Potassium alum has a long history of use in food preservation and preparation. In pickling, it is added to brine to maintain crispness of cucumbers, watermelon rinds, and other vegetables. The mechanism involves Al³⁺ ions forming cross-links with pectin, similar to other aluminum salts. It is also used as a purifying agent in sugar refining and in the clarification of drinking water. In traditional recipes, alum is used to make candied fruits (especially cherries) firmer. Some Asian cuisines use alum in the preparation of certain noodles (e.g., Japanese udon or Chinese lamian) to improve the texture and chewiness of the dough, as alum interacts with gluten proteins. In baking powders, potassium alum serves as a slow-acting acidulant. It also finds use as a component in pickling spice blends as a "crisping agent." However, many home picklers now avoid alum because of concerns about aluminum and because calcium chloride provides similar crispness without aluminum.
2. Physical and chemical properties (list form)
• Appearance — Colorless, transparent, octahedral crystals or white crystalline powder.
• Odor — Odorless.
• Taste — Sweetish-astringent.
• Molecular weight — 474.39 g/mol (dodecahydrate).
• Melting point — 92°C (loses water of hydration; forms "burnt alum" above 200°C).
• Solubility in water — 14 g/100 mL at 20°C; 200 g/100 mL at 100°C (very soluble in hot water).
• Solubility in other solvents — Insoluble in ethanol, soluble in glycerol.
• pH (5% aqueous solution) — 3.5-4.5.
• Density — 1.75 g/cm³.
• Hygroscopicity — Slightly efflorescent; loses water slowly in dry air.
• Crystal water — 12 molecules.
3. Food applications with typical usage levels (list form)
• Pickled vegetables (cucumbers, onions, watermelon rind) — 0.1-0.5% of brine; firming agent; traditional recipes use 1/4 to 1/2 teaspoon per quart of brine.
• Candied fruits (cherries, citrus peel) — 0.1-0.2% in soaking solution; maintains fruit firmness during sugar infusion.
• Noodle production (Asian cuisines) — 0.05-0.1% by flour weight (kansui substitute in some recipes); improves chewiness and yellow color.
• Baking powder (double-acting) — 10-25% of baking powder; slow-acting acidulant.
• Wine and juice clarification — 0.01-0.05% as a fining agent; removes suspended solids by protein precipitation.
• Sugar refining — As a flocculant to remove impurities from raw sugar solutions.
4. Regulatory status and safety (paragraph form)
Potassium alum is listed as E522 in the EU and permitted with aluminum limits: maximum 50 mg Al/kg in pickled vegetables, 20 mg Al/kg in bakery wares from leavening agents, and 10 mg Al/kg in cheese. In the USA, FDA includes potassium alum in the GRAS list for use as a firming agent and as a component of baking powder, but it must be declared on the ingredient label. The same aluminum safety concerns apply: low oral bioavailability but accumulation in body tissues over time. EFSA's TWI for aluminum is 1 mg/kg body weight. A typical dill pickle spear may contain 2-5 mg aluminum from alum; a slice of cake with alum-based baking powder may contain 5-10 mg. For a 70 kg person, the weekly limit is 70 mg, so moderate consumption is acceptable. However, because aluminum is also present in other foods (tea, processed cheese, antiperspirants, cookware), cumulative intake can approach the limit. Many pickling recipes now recommend calcium chloride (Pickle Crisp®) instead of alum. Individuals with kidney impairment, Alzheimer's disease risk factors, or those on long-term antacid therapy (which increases aluminum absorption) should minimize intake.
