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Introduction
Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a highly caustic alkaline solid. It is produced by heating calcium carbonate (limestone) to high temperatures (above 825°C), driving off carbon dioxide. In food processing, calcium oxide is used as a processing aid, primarily in sugar refining and in the preparation of calcium hydroxide (slaked lime) by reaction with water. It is also used in nixtamalization (corn processing) in some traditional methods, though calcium hydroxide is more common.
Calcium oxide reacts violently with water, releasing large amounts of heat and forming calcium hydroxide. Therefore, it is usually handled as an intermediate rather than added directly to foods. It is considered a food additive (E529) in the EU.
CAS Number
1305-78-8
Synonyms
Quicklime, Burnt lime, Unslaked lime, Calx, Lime (when context indicates CaO), E529.
Topic Headings
1. Use in sugar refining (paragraph form)
In the production of cane and beet sugar, raw juice contains various impurities (organic acids, proteins, color compounds) that must be removed. Calcium oxide is added to the juice to raise the pH and precipitate impurities. Specifically, calcium oxide reacts with water to form calcium hydroxide, which then reacts with carbon dioxide (from flue gas or combustion) to form calcium carbonate. The calcium carbonate precipitates as fine particles, trapping impurities by adsorption and coprecipitation. This process is called carbonatation. After filtration, the purified sugar solution is evaporated and crystallized. The use of calcium oxide is critical for producing white refined sugar. It is also used to adjust pH in beet sugar processing, as beets have high levels of invert sugars and organic acids. Typical usage levels are 0.1-0.5% by weight of juice. After processing, residual calcium is removed by filtration and ion exchange; final sugar contains less than 0.01% calcium.
2. Physical and chemical properties (list form)
• Appearance — White to grayish-white lumps, granules, or powder (color depends on impurities).
• Odor — Odorless.
• Molecular weight — 56.08 g/mol.
• Melting point — 2613°C.
• Boiling point — 2850°C.
• Reaction with water — Vigorously exothermic: CaO + H₂O → Ca(OH)₂ + 65.2 kJ/mol.
• Solubility — Reacts with water to form Ca(OH)₂, which is sparingly soluble; insoluble in organic solvents.
• pH (suspension in water) — 12.4 (from Ca(OH)₂ formed).
• Density — 3.34 g/cm³.
• Hygroscopicity — Highly hygroscopic; absorbs atmospheric moisture and CO₂ to form Ca(OH)₂ and CaCO₃.
3. Food applications and typical usage levels (list form)
• Sugar refining (cane and beet) — 0.1-0.5% of juice; used in carbonatation and pH adjustment.
• Nixtamalization of corn (traditional method) — Quicklime is sometimes used instead of slaked lime; reacts with water to generate heat and Ca(OH)₂ in situ.
• Water treatment (municipal) — Used to soften water and adjust pH (not direct food additive but affects drinking water).
• Production of calcium hydroxide for food use — Most food-grade calcium hydroxide is made from calcium oxide.
• pH regulator in some fruit and vegetable processing — Less common because of its violent reaction with water.
• Processing aid in cheese making (historical) — Used to adjust milk pH before rennet addition.
4. Regulatory status and safety (paragraph form)
Calcium oxide is approved as a food additive (E529) in the EU for use as a pH regulator and processing aid. In the USA, it is GRAS under 21 CFR 184.1210 (as a direct food additive). However, it is rarely used directly in food because of its corrosive nature; typically, it is first slaked to calcium hydroxide. The ADI is "not specified" by JECFA. Safety: calcium oxide is extremely caustic and causes severe burns to skin, eyes, and mucous membranes. Inhalation of dust can cause respiratory tract burns. In food processing, workers handling quicklime must use full protective equipment (goggles, face shield, rubber gloves, dust mask). When slaking quicklime with water, steam and heat are released; proper ventilation is required. For the consumer, final processed foods (e.g., refined sugar) contain negligible residues. In nixtamalization, any residual quicklime is converted to calcium hydroxide and then neutralized by washing and cooking; the final masa is safe. Because of its hazards, most food plants prefer to purchase calcium hydroxide directly rather than handling quicklime.
