Aldehydes, derived from the secondary oxidation of fat matrix, react with the p-anisidine determining a variation in the absorbance, measured at 366 nm. Anisidine value is expressed as AnV (Anisidine value) following AOCS (Cd 18-90) the reference method. Anisidine value test is used to assess the secondary oxidation of oil or fat, which is mainly imputable to aldehydes and ketones, and is therefore able to tell the oxidation “history” of an oil or a fat.