Definition: Acetyl number is defined as the number of milligram of KOH required to completely neutralise the acetic acid obtained by saponification of 1 gm of fat after it has been acetylated. (The treatment of fat or fatty acid mixture with acetic anhydride results in acetylating of all alcoholic OH groups.)

Significance: The acetyl number is, thus, a measure of the number of OH groups in the fat. For example, castor oil has a high acetyl number (146) because of high content of a hydroxyl acid.