Citric acid is a naturally occurring weak acid found in citrus and other fruits, as well as some vegetables. It is a component of the metabolic process in organisms called the citric acid (or Krebs) cycle. Manufactured from the fermentation of fungi, molasses, and other carbohydrates, it has a white crystalline form. Citric acid and its compounds are used as preservatives and flavouring in foods or beverages, industrial detergents, and chelating agents -- stabilisers -- in cosmetics and pharmaceuticals.
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Intolerance to citric acid occurs when the body is unable to digest it and reacts by causing abdominal cramps, eczema, rashes, and constipation. Unlike a citrus allergy, which occurs immediately after ingestion, intolerance takes time to develop and depends on the amount of food eaten. Lemons and grapefruit have the highest concentrations of citric acid among fruit and vegetables.
Tooth enamel erosion can occur with an excessive intake of citric acid through eating fruit or processed foods. The most important hazard posed by citric acid for human health is skin and eye irritation, or respiratory problems if inhaled. Waiters and bakers may be prone to dermatitis because of citric acid exposure. Laboratory personnel conducting experiments on the substance should wear protective covering for the eyes, mouth, and hands. Scientists do not believe that citric acid is a carcinogen or reprotoxic (harmful for reproduction) substance.
Citric acid is combustible, producing water vapour and carbon dioxide gas. Powdered citric acid melts and burns at 153 degrees Celsius (307 degrees Fahrenheit). It spontaneously decomposes to carbon dioxide and water vapour at 175 degrees Celsius (350 degrees Fahrenheit). At this point, the gaseous molecules expand very rapidly and can create an explosion. Industrial storage tanks of citric acid have exploded when ignited by electrical sparks.
Citric acid occurs extensively in nature and is biodegradable in soil, surface, and waste waters. However, large spills of it from industrial premises can cause oxygen depletion. It can also be toxic to fish. At concentrations of 884 parts per million and a pH of 4, it is fatal to goldfish. At 120 ppm citric acid is fatal to daphnia. It is used widely in pesticides and fertilisers, and to control pests such as coqui (or Caribbean) tree frogs. This application produces a phytotoxic (harmful) effect on plants, causing dark spots and burns on leaves, and a bleaching effect on flowers.
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- University of Hawaii at Manoa: Citric acid phytotoxicity observations