A hydroxytricarboxylic acid, general formula C6H8O7. Citric acid (see illustration) and its salts are widely used because they are nontoxic, safe to handle, and easily biodegraded. They are universally accepted, without restriction on level, as food ingredients, including acceptance by the Joint FAO/WHO Expert Committee on Food Additives and the U.S. Food and Drug Administration. Because of their food use, specifications for purity are included in the Food Chemicals Codex (except diammonium citrate). In addition, specifications for citric acid, sodium citrate, and potassium citrate are included in the U.S. Pharmacopeia and many pharmacopeia throughout the world.
Citric acid is available primarily as anhydrous material but also as the monohydrate. The major commercial salts are sodium and potassium, with calcium, diammonium, and ferric ammonium (complex) also available.
Citric acid occurs in relatively large quantities in citrus fruits. It also occurs in other fruits, in vegetables, and in animal tissues and fluids either as the free acid or as citrate ion (Table 1). It is an integral part of the Krebs (citric acid) cycle involving the metabolic conversion of carbohydrates, fats, and proteins in most living organisms. See also: Citric acid cycle
|Dissociation constants||K1 8.2 x 10−4|
|K2 1.8 x 10−5|
|K3 3.9 x 10−7|
|Solubility, g/100 ml at 25°C|
Key properties of anhydrous citric acid are summarized in Table 2. Citric acid is a relatively strong organic acid (see dissociation constants), and is very soluble in water. See also: Acid and base; Carboxylic acid
|Product||Percent citric acid|
Essentially all commercial citric acid is produced by fermentation. Processes employed are surface or submerged fermentation by mold (Aspergillus niger) and submerged fermentation by yeast (Candida guilliermondii, C. lipolytica), using a variety of substrates including sucrose, molasses, corn syrup, enzyme-treated starch, and normal paraffins. Citric acid is recovered from the fermentation broth by solvent extraction or more commonly by precipitation as calcium citrate, followed by treatment with sulfuric acid to convert the calcium citrate to calcium sulfate and citric acid. The calcium sulfate is removed by filtration, and the citric acid solution is further purified. Crystallization of citric acid from a hot aqueous solution (above the transition temperature of 36.6°C) yields anhydrous citric acid; crystallization from a cold solution yields the monohydrate. Although total chemical syntheses for citric acid have been published, they have never achieved commercial success. See also: Fermentation; Industrial microbiology
Citric acid is widely used in the food and pharmaceutical industries. In foods it is used primarily to produce a tart taste and to complement fruit flavors in carbonated beverages, beverage powders, fruit-flavored drinks, jams and jellies, candy, sherbets, water ices, and wine. It is also used to reduce pH in certain canned foods to make heat treatment more effective, and in conjunction with antioxidants to chelate trace metals and retard enzymatic activity. See also: Food manufacturing; pH
In pharmaceuticals, citric acid provides the acid source in effervescent tablets in addition to being used to adjust pH, impart a tart taste, and chelate trace metals. It is also used as a blood anticoagulant. See also: Pharmaceutical chemistry
Citric acid, because of its low toxicity, relative noncorrosiveness, and biodegradability, is also being used for applications normally reserved for the strong mineral acids. These include preoperational and operational cleaning of iron and copper oxides from boilers, nuclear reactors, and heat exchangers; passivation of stainless steel tanks and equipment; and etching of concrete floors prior to coating. It is also used as a dispersant to retard settling of titanium dioxide slurries and as a sequestering and pH control agent in the textile industry. See also: Textile chemistry
Since citric acid is a tribasic acid, it can form acid and neutral salts, and will buffer over a broad pH range. The salts which are readily available commercially are trisodium citrate (dihydrate and anhydrous), tripotassium citrate (monohydrate), calcium citrate (tetrahydrate), diammonium citrate (anhydrous), and ferric ammonium citrate. See also: Buffers (chemistry); Salt (chemistry)
Trisodium citrate dihydrate, the salt which is most widely used, is a stable white crystal or granule. It is used as an emulsifier in processed cheese products where it prevents fat separation, imparts good meltdown properties, and produces slices with proper flexibility. In other dairy products, such as ice cream, whipping cream, and evaporated milk, it acts as a stabilizer and viscosity control agent. The sharpness of high-acid beverages is mellowed with sodium citrate, and it is used as a buffer to control pH in jams and jellies, gelatin desserts, and many pharmaceutical preparations. A large industrial use is as a detergent builder since it is rapidly biodegradable, environmentally acceptable, and can chelate calcium, magnesium, and other metal ions. Sodium citrate is also used as a water conditioner, set-retardant for cement, buffer, and scrubbing agent to remove sulfur dioxide from stack gases or process tail gases. In most applications the dihydrate is used since it is less expensive. Occasionally, certain dry formulations require the use of the anhydrous salt. See also: Cheese; Detergent
Potassium citrate is a white crystalline granule or powder which readily absorbs moisture from the air. It is often used as a substitute for sodium citrate in special food products where sodium ion is undesirable. Calcium citrate is a stable, white, free-flowing powder used as a source of calcium either for nutritional purposes or for functional purposes (that is, as a firming agent). Ferric ammonium citrate is a complex salt, of undetermined structure, composed of iron, ammonia, and citric acid. Product containing 14.5–16% iron is green in color, and product containing 14.5–18.5% iron is brown. Both absorb moisture readily and are affected by light. They are used as a nutrient source of iron in food and in pharmaceutical syrups and elixirs. Diammonium citrate is a white granule or powder which is stable in air and used in dry formulations for metal cleaning.