The Evolution And Structural Characteristic Of Polar Compounds In Soybean Oil During Deep-frying

As a basic food processing means,frying is widely used in food factory production and home cooking.Because of its unique taste and tempting flavor,the fried foods are very popular with consumers.However,under the interaction of atmospheric oxygen,water in food,and high temperature during frying,triglycerides are oxidized,polymerized,hydrolyzed and pyrolyzed,,resulting in the formation of total polar compounds(TPC).The obtained TPC can be further divided into oxidized triglyceride oligomer(TGO),oxidized triglyceride dimers(TGD),oxidized triglyceride monomer(ox-TGM),diacylglycerol(DAG)and free fatty acids and sterols(FFA & Sterols)via the use of high-performance of size-exclusion chromatography,which makes it possible to simultaneously evaluate the oxidation and hydrolysis of oils.However,the research is mainly focused on the content of TPC in the frying oil without further analysis of its composition.Moreover,a large number of the researches were focused on the frying oil.But the frying system contains two parts,including the oil and the food,which is lacking of much attention.Therefore,in order to ensure the security of fried food,it is necessary to carry out the research of TPC and its components in fried food,providing theoretical guidance for the preparation of healthy fried food.In this study,changes in the content and the composition of TPC in heating system,frying system of French fries and chicken wings of soybean oil was systematically determined,and the related mechanism was also discussed.The effects of the types of oils on the composition of TPC were explored with the comparison the changes in the content and composition of TPC in the camellia seed oil,olive oil,soybean oil and palm oil during heating,providing the theoretical basis for the selection of frying oil.Simultaneously,the structure characteristics of TPC were further analyzed via gas chromatography-mass spectrometry,infrared spectroscopy and differential scanning calorimeter and the understanding of TPC was further extended.(1)In this study,changes in the content and the composition of TPC in heating system,frying system of French fries and chicken wings of soybean oil was systematically determined via medium-pressure preparative chromatography and high performance of size-exclusion chromatography.Results indicated that the content of TPC,TGO and TGD in all samples of the control,the remaining oil after frying potatoes,the extracted lipid from fried potatoes,the remaining oil after frying chicken wings,and the extracted lipid from fried chicken wings increased linearly with the numbers of cycle,and a significant higher increasing rate of TGD content was found in all samples than of TGO.Except the sample of the extracted lipid from fried chicken wings,the content of ox-TGM in samples increased linearly with the numbers of cycles.In the consecutively frying experiment for 7 days with the the replenishment with fresh oil,It was found that the increasing of TPC content in samples of control,the remaining soybean oil after frying potatoes,the extracted lipid from fried potatoes,and the remaining soybean oil after frying chicken wings was more fitted to polymeric curve,but a linear increasing was still found in the sample of the extracted lipid from fried chicken wings.The same linearly increasing trend of TGO content was found in all samples,while a polymeric curve of TGD and ox-TGM content was found in the sample of control,the remaining soybean oil after frying potatoes and the extracted lipid from fried potatoes,and a linear increase was also found in the sample of the extracted lipid from fried chicken wings,but no obvious trend was found in the sample of the remaining soybean oil after frying chicken wings.The lifetime of frying oil can be evaluated according to the limit of TPC content(27%)in the frying oil in China.The use time of soybean oil in the system of heating,frying potatoes,and frying chicken wings was respectively 34,36,and 40 without replenishment,and was respectively 43,53,58 with replenishment.Under this condition,the total content of TGD and TGO,namely TGP,in the sample of control,the remaining soybean oil after frying potatoes,and the remaining soybean oil after frying chicken wings was higher than the limit value of TGP set by the international standards.If according to this standards,the use time of soybean oil in the system of heating,frying potatoes,and frying chicken wings was respectively 24,26,and 28.(2)The results showed that a significant(p<0.05)and positive correlation relationship was found between them and their corresponding frying oil,which was also found a a significant(p<0.05)and positive correlation relationship with the content of oil and moisture loss in potatoes and chicken sings.Results showed that the moisture loss of fried potatoes and chicken wings can promote their accumulation in the content of oil,TPC and its polar components during frying,indicating that the content of oil and TPC and its polar components can be decreased via lower the moisture loss during frying.The accumulation amount of TPC,TGO,TGD,ox-TGM could be linearly expressed by the model with their contents in the corresponding oil.(3)Frying conditions were as an external condition affecting the variation of TPC in oils,and could be effectively controlled by good manufacturing practices to reduce the increase of TPC in frying oil.However,as the internal factors influencing the change of TPC content,the selection of the type of oils plays a very important role in frying oil.In this study,four kinds of oil,including olive oil of the west,camellia seed oil which is praised as “oriental olive oil”,soybean oil which was commonly used in north China,and the frequently used frying oil of palm oil were chose.The effects of the types of oils on the composition of TPC were explored with the comparison the changes in the content and composition of TPC,especially of TGO,TGD,ox-TGM,to provide the theoretical basis for the selection of frying oil.Results showed that as regards for Camilla seed oil and olive oil,the increasing rate of ox-TGM was higher than that of TGD,while for palm oil and soybean oil,the increasing rate of ox-TGM was much lower than of TGD.It can be seen that the difference in the type of oil can influence the change of TPC,especially of ox-TGM,TGD and TGO during heating.(4)According to the existing theoretical research gap on the structural characteristics of TPC,the gas chromatography-mass spectrometry,infrared spectroscopy and differential scanning calorimeter were applied to analyze the fatty acid composition,chemical bond or group,thermal stability of the obtained TPC.The structural characteristics of TPC were explored with the comparison with that of oil and NPC.The formation mechanism and the trend of TPC were further analyzed,providing the theoretical basis for the establishment of rapid monitoring technology for evaluating TPC.Results showed that as regard for soybean oil and Camilla seed oil,the total content of fatty acids of non-polar compounds reduced with heating time,but the total content of fatty acids of TPC increased with heating time.Both the absorption peak area of TPC from soybean oil and camellia seed oil changed significantly during heating,and varied with that of the oil.From the results of DSC,it was found that the stability of non-polar compounds and TPC from soybean oil was less than that from camellia seed oil.