pubs.acs.org/JAFC Published on Web 05/22/2009 © 2009 American Chemical Society J. Agric. Food Chem. 2009, 57, 5391–5400 5391 DOI:10.1021/jf9008795 MALDI-TOF Mass Spectrometry Profiling of Polar and Nonpolar Fractions in Heated Vegetable Oils GIANLUCA PICARIELLO,* ,† ANTONELLO PADUANO, ‡ RAFFAELE SACCHI, ‡ AND FRANCESCO ADDEO* ,†,‡ † Istituto di Scienze dell’Alimentazione, Consiglio Nazionale delle Ricerche, Via Roma 64, I-83100 Avellino, Italy, and ‡ Dipartimento di Scienza degli Alimenti, Universit a di Napoli “Federico II”, Parco Gussone, I-80055 Portici (NA), Italy Triacylglycerol oxidation of thermally stressed (6 h at 180 °C, simulating deep-frying conditions) edible vegetable oil (sunflower and olive) was studied using matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF MS). Chromatographic separation of the nonpolar and polar components from the heated oil performed on silica gel prior to MS analysis significantly enhanced the detection of oxidized components. The spectra contained signals that were assigned to triacylglycerols (TAG), diacylglycerols (DAG), triacylglycerol oxidative dimers, oxidized TAG, and TAG fragments arising from the homolytic β-scission of linoleyl, peroxy, and alkoxy radicals. Enrichment of the polar compounds prevented mass spectrometric ion suppression, thus allowing the detection of minor species originating from thermal oxidation. In addition, this allowed the monitoring of polar compounds in vegetable oils undergoing mild thermal treatment. As such, chromatographic separation coupled with MALDI-TOF MS analysis provided a rapid, sensitive, and specific tool to assess the thermal oxidation of vegetable oils. KEYWORDS: Sunflower oil; virgin olive oil; thermo-oxidation; polar and nonpolar fractions; MALDI-TOF MS INTRODUCTION Thermal stress strongly oxidizes unsaturated vegetable oils, which has been determined to cause losses in nutrients and sensory attributes (1 ). The most important reaction in over- heated/fried oils is the radical-mediated autoxidation of unsatu- rated/polyunsaturated triacylglycerols (TAG) that primarily leads to the formation of conjugated fatty acid hydroperoxides (Figure 1). As hydroperoxides, the primary products of lipid oxidation, are unstable at frying temperatures, peroxy and alkoxy radicals are formed from the major unsaturated alkyl chains, such as linolenoyl (18:3 n-3), linoleoyl (18:2 n-6), and oleoyl (18:1 n-9). These radical compounds readily decompose to form a wide variety of secondary oxidation products (Figure 1b). Some addi- tional decomposition TAG reactions (e.g., hydrolysis, polymer- ization, cyclization, and cracking) could occur to form β-scission products, cyclic fatty acids, or monomeric and oligomeric oxidized TAG (1 ). Both nonvolatile and volatile compounds are formed during the frying process at temperatures in the 170-200 °C range (2 ). For the most part, volatile fried-flavors and off-flavors escape from the frying medium. The nonvolatile decomposition products, including nonpolar nonoxygenated and polar oxygenated fractions, gradually accumulate in the oil, are absorbed by the fried foods, and are finally ingested. The level of oxidized products present and the kinetics of oxidation depend on the chemical structure of the TAG and on a series of other factors including the fat blend, heating temperature, frying time, food dryness, oxygen accessibility, and presence of transition metal ions and antioxidants. In addition to the off-flavor (3 ), the oxidized compounds of fried oils may contribute to adverse health effects (4 ). Among the chemical and physical indices, total polar compounds (TPC) is considered to be one of the most objective indicators for the evaluation of the deterioration of oils and fats during deep-frying (5, 6). TPC of oils increase with frying time, and generally ∼25% by weight is considered to be the safe upper limit (7 ). Although accurate, the standard method based on silica gel column chromatography is relatively expensive and time- consuming (8 ). To characterize the oxidized and polymerized TAG, the complex mixture resulting from fried oils has been fractionated by solid phase extraction (SPE) on silica gel followed by high-performance size exclusion chromatography (HPSEC) (6, 9, 10). Alternative strategies, such as near-infrared spectroscopy (NIR), dielectric capacitance measurement (11 ), pressurized differential scanning calorimetry (3 ) combined with NMR, or ultrasonic-based technologies (12 ), have also been utilized. Although a rapid test to determine the level of TPC has been developed (13 ), a universally accepted test procedure for monitoring TAG oxidation is still lacking. Furthermore, there is a need to validate straightforward and high-throughput procedures capable of providing precise information about the chemical nature of the compounds formed during frying. *Authors to whom correspondence should be addressed [(F.A.) telephone +39 081 2539355, fax +39 0817762580, e-mail addeo@ unina.it; (G.P.) telephone +39 0825 299510, fax +39 0825 781585, e-mail picariello@isa.cnr.it].