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Introduction
Sodium hydroxide (NaOH), also known as caustic soda or lye, is a strong base used in food processing for various purposes, including pH adjustment, peeling of fruits and vegetables, and in the production of certain foods such as olives, pretzels, and cocoa. It is also used as a cleaning agent in food processing equipment. In very dilute concentrations, it is safe and effective; in concentrated form, it is highly corrosive.
In food applications, sodium hydroxide is used to remove the bitter skins from fruits (such as grapes for maraschino cherries), to give pretzels their characteristic brown color and shiny crust, and to cure olives (removing bitterness). It is also used in the production of caramel color (Class III and IV) and in the refining of vegetable oils.
CAS Number
1310-73-2
Synonyms
Caustic soda, Lye, Soda lye, Sodium hydrate, White caustic, NaOH.
Topic Headings
1. Use in pretzel production: the "lye bath" (paragraph form)
Traditional German-style pretzels (Brezeln) are dipped in a dilute sodium hydroxide solution (typically 1-4% NaOH, about 0.25-1.0 M) before baking. This alkaline bath serves two purposes. First, it gelatinizes the starch on the surface of the dough, creating a smooth, shiny coating. Second, during baking, the strong base promotes the Maillard reaction between reducing sugars and amino acids, producing the characteristic deep brown-red color and distinct "pretzel" flavor. The alkalinity also neutralizes some of the surface acidity, giving a slightly bitter, savory note. After dipping, the pretzels are sprinkled with coarse salt and baked. The lye is neutralized during baking as CO₂ from the oven atmosphere forms carbonic acid and as the dough's internal acidity buffers the base. Modern food safety regulations require that the lye concentration be carefully controlled (typically 1-4%) and that excess lye is drained off before baking. Home recipes often substitute baking soda (sodium bicarbonate) solution, but the pH of baking soda (about 8.5) is much lower than lye (pH 13-14), resulting in a different color and texture.
2. Physical and chemical properties (list form)
• Appearance — White solid (pellets, flakes, or powder); also available as aqueous solution (typically 50% w/w).
• Odor — Odorless.
• Molecular weight — 40.00 g/mol.
• Melting point — 323°C.
• Boiling point — 1390°C.
• Solubility in water — 111 g/100 mL at 20°C (highly exothermic dissolution).
• Solubility in other solvents — Soluble in ethanol and methanol; insoluble in ether.
• pH (0.1% solution) — Approximately 13 (strongly alkaline).
• Density — 2.13 g/cm³ (solid); 1.53 g/cm³ for 50% solution.
• Hygroscopicity — Extremely hygroscopic; absorbs moisture and carbon dioxide from air, forming sodium carbonate.
• Corrosivity — Highly corrosive to skin, eyes, and metals (aluminum, zinc, tin).
3. Food applications and typical usage levels (list form)
• Pretzel production (lye bath) — 1-4% NaOH solution; dip for 5-30 seconds before baking.
• Olive curing (black olives) — 1-2% NaOH solution to remove oleuropein (bitter compound); followed by repeated water washing to remove residual lye.
• Fruit peeling (tomatoes, peaches, grapes for maraschino cherries) — 1-3% NaOH solution at 90-100°C for 30-120 seconds; loosens skins for mechanical removal.
• Cocoa and chocolate processing (alkalization or Dutch process) — 0.5-3% NaOH added to cocoa nibs or powder to raise pH to 6.5-8.0; darkens color and reduces acidity.
• Caramel color production (Class III and IV) — Used as a catalyst in caramelization reactions.
• Vegetable oil refining — As a neutralizing agent to remove free fatty acids (forming soap stock).
• Food equipment cleaning (CIP – clean-in-place) — 1-2% NaOH solutions at 70-85°C for degreasing.
• Corn and grain processing — To remove hulls and improve digestibility in some animal feed applications.
4. Safety, regulatory status, and handling (paragraph form)
Sodium hydroxide is classified as GRAS by the FDA for specific food uses under 21 CFR 184.1763 (as a pH control agent and processing aid). The FAO/WHO JECFA has not set an ADI because it is used as a processing aid and is either removed or neutralized in the final product. In foods where residues may remain (e.g., olives), the residual hydroxide is neutralized by acids in the food or by subsequent washing. EU regulations permit sodium hydroxide as a food additive (E524) for surface treatment of certain fruits and vegetables and as a pH regulator. Maximum residual levels: for olives, residual NaOH must be less than 0.1% (as sodium) after washing. For pretzels, no specific limit is set, but the surface pH is typically 7-8 after baking. Safety for workers: concentrated NaOH is extremely corrosive; protective gloves, goggles, and face shields are required when handling. In food processing, automated systems are used to minimize human contact. If ingested in concentrated form, sodium hydroxide causes severe burns to the mouth, esophagus, and stomach; emergency medical treatment is required. For consumers, properly prepared foods using sodium hydroxide are safe because the alkali is neutralized or diluted to harmless levels.
