What is the main acid component in vinegar?

Concept. Organic acids are told apart by the functional group they carry and by the natural process that concentrates them in a given material; a carboxylic acid, for instance, is any compound carrying the –COOH group. Acids reach a material by very different routes: mineral acids are manufactured industrially from inorganic feedstock, plant polyphenolic acids are extracted from galls and leaves, fruit acids build up inside ripening fruit, and the acids of fermented foods are produced by microbes acting on the food’s own raw material. Microbial routes are themselves varied: lactic-acid bacteria convert sugar directly into lactic acid, whereas an aerobic two-stage route has yeast first convert sugar into ethanol and then acetic-acid bacteria of the genus Acetobacter oxidise that ethanol in the presence of air into the corresponding two-carbon carboxylic acid. Because each acid has its own route, identifying the acid present in a food usually means identifying how that food was made.

Applying it here. Vinegar is the product of exactly that two-stage fermentation, so the acid it carries is the two-carbon carboxylic acid acetic acid, CH3COOH (IUPAC name ethanoic acid). Ordinary table vinegar is a dilute aqueous solution containing roughly 4-8% acetic acid by volume, and this acid supplies its sharp sour taste, its pungent smell and its preservative action. The pure, water-free compound is called glacial acetic acid.

Cross-check against the other acids. Each of the other acids is produced by a different route and accumulates in a different material:

Only the fermentation-derived carboxylic acid fits the way vinegar is made, so the main acid component of vinegar is acetic acid.