Are the occurrence of 4-hydrox-2-trans-nonenal and 4-hydrox-2-hexenal a safety concern in vegetable oils?

4-hydroxy-2-trans-nonenal (HNE) is a secondary lipid peroxidation product derived from the oxidation of n-6 polyunsaturated fatty acids, namely linoleic acid, and 4-hydroxy-2-hexenal (HHE) is formed by the oxidation of n-3 polyunsaturated fatty acids, namely docosahexaenoic and eicosapentaenoic fatty acids. Compounds such as HNE and HHE are formed as a consequence of fatty acids deterioration in the presence of oxygen, and when vegetable oils are exposed to high temperatures. This type of compounds are of particular interest, since they are quickly absorbed and are highly reactive substances, that can modify proteins, nucleic acids and other important biomolecules in vivo. HNE is also a mutagenic compound and is related with several diseases, such as atherosclerosis, low density lipoprotein oxidation, stroke, Parkinson’s and Alzheimer’s diseases, among others. The data found in the literature regarding HNE and HHE content in vegetable oils is scarce. Therefore, in the last years, great attention has been devoted to the analysis of these compounds in foods and vegetable oils. Most of the analytical methods reported in the literature involve several extraction steps, which sometimes can induce important losses, leading to an underestimation of the actual amount present in the sample. For the analytical determination, the following methodologies were reported: high performance liquid chromatography with ultraviolet detection (HPLC-UV) or mass spectrometry detection (HPLC-MS/MS), and gas chromatography with mass spectrometry detection (GC-MS). Regarding the occurrence of these compounds in vegetable oils, in general, HNE is present in higher amounts than HHE. With respect to the different vegetable oils (soybean, corn, sesame and olive oils), it is not possible to define a trend, since there are considerable differences among the same type of oil (e.g. for different soybean oils, the following values were reported for HNE: 0.006; 0.499; 2.45; 3.73 mg/kg). On the other hand, one of the main concerns pointed out on the studies carried out up to now is the migration of these compounds from the oil into the fried food. Recent findings report similar quantities of these compounds in the oil (524 ± 61 mg/kg) and in the oil extracted from the fried food (596 ± 119 mg/kg). With respect to heat treatment applied to vegetable oils, such as frying, some results reported in the literature were performed under unrealistic frying temperatures (>200 ºC). Future trends in this area of research should include studies that suppress the lack of data regarding the formation of these toxic compounds in the presence of food and using real frying conditions, considering the different types of fryer, duration and temperature, and also the different types of food matrices and vegetable oils.This work was funded by National Institute of Health Dr. Ricardo Jorge, I.P., under the project “PTranSALT (2012DAN828)” and by the project “Operação NORTE-01-0145-FEDER-000011 - denominada Qualidade e Segurança Alimentar - uma abordagem (nano)tecnológica” and the project UID/QUI/50006/2013 - POCI/01/0145/FEDER/007265 with financial support from FCT/MEC through national funds and co-financed by FEDER. Tânia Gonçalves Albuquerque is grateful for the PhD fellowship (SFRH/BD/99718/2014) funded by the FCT, FSE and MEC.N/