A colorless, pungent liquid, CH₃COOH, melting at 16.7°C (62.1°F) and boiling at 118.0°C (244.4°F). Acetic acid is the sour principle in vinegar. Concentrated acid is called glacial acetic acid because of its readiness to crystallize at cool temperatures. Acetic acid in vinegar arises through an aerobic fermentation of dilute ethanol solutions, such as wine, cider, and beer, with any of several varieties of Acetobacter.