Analysis of target volatile compounds related to fishy off-flavor in heated rapeseed oil: A comparative study of different headspace techniques

Off-flavors, characterized by a fishy odor, are emitted during the heating of rapeseed oil in a fryer. In order to study the headspace composition of degraded rapeseed oil, and more specifically the compounds responsible for the fishy odor, four headspace techniques were evaluated: Static headspace (SHS), headspace trap (HS Trap), headspace solid phase microextraction (HS-SPME), and headspace sorptive extraction (HSSE) coupled to gas chromatography-mass spectrometry (GC-MS). The selected model compounds to develop the headspace methods were 1-penten-3-one, 1-octen-3-one, and E,Z-2,6-nonadienal as known fishy odor markers. Extraction conditions for the four techniques were optimized and evaluated in terms of repeatability and sensitivity. They were then applied to the analysis of real samples of degraded rapeseed oil. SHS was not suitable for the analysis of odorant volatile compounds because of LOD values lower than those required for a quantitative analysis of the target compounds. Concerning HS-SPME and HSSE, a discrimination against the more volatile compounds was observed. Finally, HS Trap-GC-MS was considered the most appropriate technique for the analysis of the headspace of rapeseed oil in this study with LOD values less than 2 mu g/L for the three target compounds. More than 60 compounds were identified by HS Trap-GC-MS and two compounds responsible for the fishy odor could be detected. The evolution of the relative areas of these compounds was reported for fresh oil and rapeseed oil degraded by 1, 5, and 10 heating cycles. Practical applications: The characterization of the volatile compounds responsible for the fishy odor is a key industrial issue. Moreover, the developed method with HS-Trap could be used to quantify several volatile compounds characteristic of the degradation of rapeseed oil, and to evaluate the possible effect of antioxidants on oils.