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Citric acid is a vital component of the citric acid cycle, or Krebs cycle.
During this sequence of reactions one acetate unit is converted to two equivalents of carbon dioxide giving rise to a molecule of adenosine triphosphate (ATP), the main energy source in cells. Citric acid occurs abundantly in citrus fruits, although commercial synthesis is by fermentation of sugar , glucose or molasses. It is used in food as an antioxidant as well as enhancing the effect of other antioxidants, and also as an acidity regulator. Present in virtually all plants, it was first isolated in 1784 from lemon juice, by the Swedish chemist Carl Wilhelm Scheele, and has been used as a food additive for over 100 years. Citric acid is available in forms of anhydrous primarily and in monohydrate, the crystallized form from water.

Chemical formula: C₆H₈O₇                
Rel. mol. weight: 192.13
CAS No.: 77- 92 -9
EINECS No.: 201-069-1
E-No.: E330

Density: 1.665 g/cm ³ (18 °C)
Solubility in water 1330 g/l (20 °C)
Boiling point ~153 °C (decomposition)
Bulk density ~560 kg/m³
Flash point 345 °C

Chemical formula: C₆H₈O₇ . H₂O
Rel. mol. weight: 210.14
CAS  No.: 5949-29-1
EINECS No.: 201-069-1
E-No.: E330

Density: 1.54 g/cm ³ (20 °C)
Solubility in water 1630 g/l (20 °C)
Boiling point  ~135-152 °C
Bulk density ~800 - 1000 kg/m³

Citric Acid is colourless crystals or a white, crystalline powder. Very soluble in water, freely soluble in ethanol, sparingly soluble in ether. Citric acid is a weak organic acid found in citrus fruits. It is a good, natural preservativeand is also used to add an acidic (sour) taste to foods and soft drinks. In biochemistry, it is important as an intermediate in the citric acid cycle and therefore occurs in the metabolism of almost all the living organisms. It also serves as an environmentally friendly cleaning agent and acts as an antioxidant.
Citric acid exists in a variety of fruits and vegetables, but it is most concentrated in lemons and limes, where it can comprise as much as 8% of the dry weight of the fruit.
Citric acid's chemical formula is C6H8O7 . Its structure is reflected in its exact chemical name 2-hydroxypropane-1,2,3-tricarboxylic acid. The acidity of citric acid results from the three carboxyl groups COOH which can lose a proton in solution. If this happens, the resulting ion is the citrate ion. Citrates make excellent buffers for controlling the pH of acidic solutions. Citrate ions form salts called citrates with many metal ions.