Study On Polar Compounds Yielded By Lipids Under High Temperature Conditions

Due to unique organoleptic and sensorial properties of fried food, the lipids are usually heated at a higher temperature for frying foods in food processing or household kitchen. Under the condition of high temperature, lipids thermal oxidation produce complex mixtures of volatile, and non-volatile compounds, which are known to react with nitrogenous materials in biological systems including amino acids, proteins, bases of phospholipids, and DNA to induce a number of chronic and acute disease.To analyze polar compounds formed at high temperature, soybean oil was heated at 180℃ for 1 h,3 h,5 h and 7 h, respectively. Aliphatic aldehydes were detected by reverse-phase HPLC. The remaining aliphatic aldehydes content in frying oils was increased from 1.2205 mmol/kg to 1.6724 mmol/kg and subsequently reduced to 1.4661 mmol/kg at 7 h. Polar compounds were separated by silica-gel chromatography column, different polar fractions were stepwise eluted by n-hexane/diethyl ether system (87:13, 75:25,50:50,0:100, v/v), respectively. With the extension of heating time, the content of triglyerides eluted by n-hexane/diethyl ether (87:13 v/v) was decreased from 82.9% to 74.2%. The content of carbonyl compounds in polar fractions that eluted by n-hexane/diethyl ether (50:50 v/v) was 31-49% of total carbonyl compounds content. The special structure of different polar fractions was identified by Fourier Transform Spectrometer, and the results shown that the polar compounds eluted by n-hexane/diethyl ether (75/25, v/v) included trans structure altered triglycerides, polar fractions eluted by n-hexane/diethyl ether (0/100, v/v) included benzene ring which may induce severe toxicity and carcinogenic.Reversed-phase high-performance liquid chromatography with UV-detector and ELSD was applied to analyze the distribution of polar fractions. The compounds obtained from soybean oil heated for 7 h eluted by n-hexane/diethyl ether (87/13, v/v) may include a lot of triglycerides and some tocopherol and a bit of aliphatic carbonyl compounds. The compounds obtained from soybean oil heated for 7 h eluted by n-hexane/diethyl ether (75/25, v/v) may include tocopherol and aldehydes or ketones on the triglycerides. The compounds obtained from soybean oil heated for 7 h eluted by n-hexane/diethyl ether (50/50, v/v) may include tocopherol and diglyceride and aldehydes or ketones on the diglyceride. The polar compounds eluted by n-hexane/diethyl ether (0/100, v/v) may include free fatty acids and tocopherol and monoglyceride and aldehydes or ketones on the monoglyceride.