Determination of higher carbonyl compounds in used frying fats by HPLC of DNPH derivatives

Abstract.

In current analytical practice, simple methods are required for assessing the quality of a deep-frying fat after some time of use. Therefore, a new procedure was developed for the fast and selective determination of higher carbonyl compounds, i.e., those triglycerides resulting from the oxidation and/or oxidative cleavage of double bonds in unsaturated fatty acids. For analysis, a fat sample is dissolved in a 1-butanol/toluene mixture containing 6-undecanone as an internal standard. Aldehydes and ketones present are allowed to react with 2,4-dinitrophenylhydrazine (DNPH) in acidic solution for at least 1 h at room temperature. The mixture is injected directly onto a reversed phase HPLC column that is eluted with a very steep gradient between methanol and tert-butylmethyl ether (TBME) and is fitted with a UV detector set at 370 nm. In this way, the DNPH derivatives of all higher carbonyl compounds are eluted as one single peak. The result is calculated as milliequivalents of carbonyls per kg of fat (meq kg^–1). The method takes a minimum of time and reagents and only requires the usual equipment.