acetic acid

acetic acid, CH₃CO₂H, colorless liquid that has a characteristic pungent odor, boils at 118°C;, and is miscible with water in all proportions; it is a weak organic carboxylic acid (see carboxyl group). Glacial acetic acid is concentrated, 99.5% pure acetic acid; it solidifies at about 17°C; to a crystalline mass resembling ice. Acetic acid is the major acid in vinegar; as such, it is widely used as a food preservative and condiment. For industrial use concentrated acetic acid is prepared from the oxidation of acetaldehyde. Acetic acid is also a product in the destructive distillation of wood. It reacts with other chemicals to form numerous compounds of commercial importance. These include cellulose acetate, used in making acetate rayon, nonflammable motion-picture film, lacquers, and plastics; various inorganic salts, e.g., lead, potassium, and copper acetates; and amyl, butyl, ethyl, methyl, and propyl acetates, which are used as solvents, chiefly in certain quick-drying lacquers and cements. Amyl acetate is sometimes called banana oil because it has a characteristic banana odor.

Most important carboxylic acid (CH₃COOH). Pure (“glacial”) acetic acid is a clear, syrupy, corrosive liquid that mixes readily with water. Vinegar is its dilute solution, from fermentation and oxidation (see oxidation-reduction) of natural products. Its salts and esters are acetates. It occurs naturally as a metabolic intermediate in body fluids and plant juices. Industrial production is either synthetic, from acetylene, or biological, from ethanol. Industrial chemicals made from it are used in printing and as plastics, photographic films, textiles, and solvents.

Learn more about acetic acid with a free trial on Britannica.com.

Acetic acid bacteria are bacteria that derive their energy from the oxidation of ethanol to acetic acid during respiration. They are Gram-negative, aerobic, rod-shaped bacteria.

The acetic acid bacteria are found in nature where ethanol is being formed as a result of yeast fermentation of sugars and plant carbohydrates. They can be isolated from the nectar of flowers and from damaged fruit. Other good sources are fresh apple cider and unpasteurized beer which has not been filter sterilized. In these liquids the acetic acid bacteria grow as a surface film due to their aerobic nature and active motility. Vinegar is produced when acetic acid bacteria act on alcoholic beverages such as wine.

Some genera, such as Acetobacter, can eventually oxidize acetic acid to carbon dioxide and water using Krebs cycle enzymes. Other genera, such as Gluconobacter, don't further oxidize acetic acid, as they do not have a full set of Krebs cycle enzymes.

Some acetic acid bacteria, notably Acetobacter xylinum, are known to synthesize cellulose, something normally only done by plants.

As these bacteria produce acid, they are unusually acid tolerant, growing well below pH 5.0, although the pH optimum for growth is 5.4-6.3.