A carboxylic acid /ˌkɑːrbɒkˈsɪlɪk/ is an organic compound that contains a carboxyl group (C(=O)OH). The general formula of a carboxylic acid is R–COOH, with R referring to the rest of the (possibly quite large) molecule. Carboxylic acids occur widely and include the amino acids (which make up proteins) and acetic acid (which is part of vinegar and occurs in metabolism).
Carboxylic acids are produced during fermentation by yeast. Further carboxylic acids can be produced by bacteria (Lactobacillus, Acetobacter, and others). These acids can be used to create Esters through enzymatic activity or Fischer Esterification.