Determination of vegetable oil volatile compounds during frying by dynamic headspace/GCMS analysis

1. Determination of vegetable oil volatile compounds during frying by dynamic headspace/GCMS analysis Centro Ricerche per l’Industria Olearia Università degli Studi di Napoli Federico II Dipartimento di Scienza degli Alimenti Parisini C.*, Savarese M.*, De Marco E.*, Falco S. * Sacchi R. *‡ * CRIOL, Centro Ricerche per l’Industria Olearia, Industria Olearia Biagio Mataluni, Zona Industriale, 82016 , Montesarchio (BN) ‡ University di Naples “Federico II”, Department of Food Science, via Università 100, 80055, Portici (NA) email: criol@mataluni.com; sacchi@unina.it INTRODUCTION OBJECTIVES • Aim of the present work was: • to study the changes in vegetable oil volatile profile during discontinuous deep-fat frying • to compare two vegetable oil mixtures as regards resistance to a prolonged frying process. Deep-fat frying is largely used both in industrial and home-made food processing. During deep frying, food is completely surrounded by the frying medium at high temperatures (180-200°C) in presence of oxygen and moisture. Under these conditions a gradual and not reversible degradation of the frying fat takes place. These chemical and physical changes are mainly due to the oxidative reactions of polyunsaturated fatty acids leading to the formation of odourless hydroperoxides and to their further decomposition to off-flavour volatile compounds (Frankel, 1998). This process depends on the oil fatty acid composition and on the type of hydroperoxides formed in the initial stages of the oxidation process (Kanavouras et al., 2006). The development of desirable and undesirable flavours has a great impact on the fried food acceptability by consumers and helps establishing when oil has to be discarded. Table 1. Headspace composition (mg/kg of Isobutyl Acetate) of two vegetable oil mixtures at time 0, as determined by DHS Purge and Trap GCMS analysis. MATERIALS and METHODS Samples: Two vegetable oil mixtures Mix 1: sunflower/soybean/palm oil Mix 2: sunflower/palm oil were provided by Industria Olearia Biagio Mataluni s.r.l. (Montesarchio, Benevento, Italy). Frying trials: An electrical deep fat fryer (Tefal, Milan, Italy), operating at a temperature of 180 °C, was used for frying frozen pre-fried potato chips (Arena, Milan, Italy) purchased in a local market. The trial was conducted with the two selected oil mixtures. The fryer was filled with 1 l of oil (initial volume) and two frying cycles of 4 h were carried out with 1 h interval between each other. During each cycle, 8 chips samples (100 g) were fried. In order to determine the effect of frying on oil volatile profile, oil samples (50 ml) were collected after 0, 4 and 8 hours and stored at -20 °C until analysis. Figure 1. Total Ion Current (TIC) resulting from DHS/GCMS analysis of a vegetable oil mixture before frying (fresh oil) and after 8 hours frying. Peaks identificationis reported in Table 2. Table 2. Volatile compounds identified in a vegetable oil mixture after 8 hours of frying by DHS/GCMS analysis. Dynamic headspace/GCMS analysis of oil volatile fraction: The dynamic headspace (DHS) analysis was carried out using the Purge and Trap method (Teledyne Velocity XPT Purge and Trap sample concentrator, Tekmar-Dohrmann, Ohio, USA). An oil sample (3 ml) was placed in a 5 ml fritless glasswere and purged for 30 minutes applying a helium flow (200 ml/min flow rate). Stripped volatiles were, then, adsorbed on a Trap C (Tenax, silica gel, activated charcoal) (Supelco, Bellefonte, USA) kept at room temperature. Volatiles desorption was carried out by heating the trap for 10 min at 260°C. A helium flow at 200 ml/min was applied to desorbe molecules and to convey them into the gas chromatograph (GC). For GC analysis a QP2010 gas chromatograph tandem mass spectrometer (Shimadzu, Milan, Italy) was used. Separation of volatile compounds was performed on a Supelcowax-10 column (60 m x 0,32 mm I.D., 0, 50 mm film thickness) (Supelco, Bellefonte, USA). Fresh oil mixtures Oil mixtures after 8 h frying • Conditions used for GC analysis were: • initial temperature, 40°C for 4 minutes, increased to 240°C at a rate of 3,5 ml/min, held for 3 minutes. • Injector temperature 190°C. • The operating MS conditions were: • interface temperature 250°C, • ion source temperature 200°C, • scan range 40 - 400 m/z, • scan speed 0,4 scan/s. Figure 2. Evolution of the main volatile compounds responsible for deep-fat frying flavours and off-flavours in two vegetable oil mixtures during frying. Quantification was carried out using Isobutyl Acetate as Internal Standard (Sigma-Aldrich, St.Louis, USA) (Morales and Aparicio, 1993). Peak identification was obtained comparing mass spectra with those of the NIST library. All samples were analysed in duplicate. RESULTS and DISCUSSION • 82 volatile compounds were identified in the oil mixtures after 8 hours of frying (Table 2). • The chromatographic profiles resulting from the DHS/GCMS analysis of vegetable oil samples before frying (fresh oil) and after 8 hours of frying are reported in Figure 1. The increase in the identified volatile compounds as a consequence of frying is evident. • The volatile compounds responsible for deep-frying flavour and off-flavour are primarily alkenals and alkadienals, such as (E)-2-octenal, (E or Z)-2-heptenal, 2,4-octadienals, 2,4-nonadienals and 2,4-decadienals, though present in low concentrations. The amount af these compounds, formed directly from the decomposition of linoleic hydroperoxides (Warner, 2001), increase in both oil mixtures after 8 hours of frying (Figure 2) but at a higher rate in Mix 1. • The two vegetable oil mixtures, already differing in the volatile fraction before frying (Table1), show a different resistance to a prolonged frying process. In particular Mix 2 seems to be more stable to thermal oxidation than Mix 1 (Figure 2). This result can be explained by the difference in fatty acid composition between the two mixtures: Mix 1 is characterized by a higher content of polyunsaturated fatty acids (data not shown). ACKNOWLEDGEMENTS This work was supported by the Italian Ministry of University and Research (MIUR) and by Industria Olearia Biagio Mataluni (project “Controllo Qualità ed Innovazione Tecnologica nell’Industria Olearia”, DM 593 del 8/08/2000, Prot. MIUR 1866 del 18/02/2002). REFERENCES Frankel, E.N. 1998; The Oily Press, Dundee, Scotland. 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