Quick Search
Introduction
Magnesium hydroxide (Mg(OH)₂), also known as milk of magnesia, is a weakly basic, white solid with very low solubility in water. In food processing, it is used as a pH regulator, as a color stabilizer, and as a dietary source of magnesium. It is also used as an antacid in over-the-counter medicines. In foods, magnesium hydroxide is added to certain canned vegetables to maintain green color, to mineral water to adjust pH and add magnesium, and to some dairy products.
Unlike sodium or potassium hydroxides, magnesium hydroxide is not caustic because of its low solubility. It is considered safe and gentle. It is also used as a food additive (E528) in the EU.
CAS Number
1309-42-8
Synonyms
Milk of magnesia (suspension), Magnesium hydrate, Brucite (mineral), E528.
Topic Headings
1. Role in color stabilization of canned green vegetables (paragraph form)
Green vegetables (peas, beans, spinach, asparagus) contain chlorophyll, which is sensitive to acid. During canning or cooking, the natural plant acids and the acidic canning brine can cause chlorophyll to degrade to pheophytin, which is olive-brown and unappetizing. Magnesium hydroxide, being a base, neutralizes these acids and raises the pH of the brine to near neutral or slightly alkaline (pH 7.0-7.5). At this pH, chlorophyll remains green. Moreover, the magnesium ion (Mg²⁺) in magnesium hydroxide can replace the central magnesium atom of chlorophyll if it has been lost, restoring the green color. This is known as "regreening." Typical usage is 0.01-0.05% of the canning brine. However, excessive alkalinity can cause the vegetables to become slippery or develop off-flavors. Therefore, magnesium hydroxide is often used in combination with calcium hydroxide or other buffers.
2. Physical and chemical properties (list form)
• Appearance — White powder or crystalline solid.
• Odor — Odorless.
• Molecular weight — 58.32 g/mol.
• Melting point — 350°C (decomposes to magnesium oxide and water).
• Solubility in water — 0.00064 g/100 mL at 25°C (very low); forms a suspension (milk of magnesia).
• Solubility in other solvents — Insoluble in ethanol; soluble in dilute acids.
• pH (saturated suspension) — Approximately 10.5 (weakly alkaline).
• Density — 2.36 g/cm³.
• Hygroscopicity — Slightly hygroscopic; absorbs CO₂ from air to form magnesium carbonate.
• Flame retardant — Decomposes endothermically, releasing water vapor.
3. Food applications and typical usage levels (list form)
• Canned green vegetables (peas, beans, asparagus) — 0.01-0.05% of brine; maintains green color and provides magnesium.
• Mineral water (magnesium enrichment) — 0.01-0.1% (as magnesium) to achieve desired mineral content; contributes to water alkalinity.
• Dairy products (processed cheese, yogurt) — As a pH buffer and magnesium source.
• Cocoa and chocolate processing — Occasionally used as an alkalizing agent (less common than KOH or NaOH).
• Food supplements (dietary magnesium) — In tablets or powders; provides non-acidifying magnesium.
• Antacid preparations (non-food but OTC medicine) — Suspensions for heartburn relief.
• Sugar refining — As a clarifying aid.
4. Regulatory status and safety (paragraph form)
Magnesium hydroxide is approved as a food additive (E528) in the EU for use as a pH regulator, color stabilizer, and magnesium source. In the USA, it is GRAS under 21 CFR 184.1428 (as a direct food additive). JECFA has not set an ADI because of low toxicity and because magnesium is an essential nutrient. The tolerable upper intake level (UL) for supplemental magnesium is 350 mg/day for adults (for magnesium hydroxide, the hydroxide component is not toxic). In food uses, the amounts are far lower. The main safety concern is laxative effect: magnesium hydroxide is the active ingredient in milk of magnesia, a laxative. At doses above 1 gram of Mg(OH)₂, most individuals experience diarrhea. In canned vegetables, the residual magnesium is very low (tens of milligrams per serving), well below laxative threshold. In mineral water, magnesium hydroxide is dissolved by carbonic acid to form soluble magnesium bicarbonate, which is well-tolerated. Individuals with kidney disease should avoid excessive magnesium intake. Overall, magnesium hydroxide is considered safe and is actually beneficial as a magnesium source.
