Variation Rule Of Polar Compound Composition Of Oil During Heating And Frying Process And Its Application

Total polar compound(TPC)refers to the substances that showed more polarity than normal triglycerides in oils,mainly including the oxidized triglyceride oligomer(ox TGO),oxidized triglyceride dimer(ox TGD),oxidized triglyceride monomer(ox TGM),diacylglycerol(DG),and free fatty acid(FFA).TPC value is often used to measure the degree of oil deterioration,and many countries have set it as the main indicator to judge whether frying oil should be discarded.However,due to the differences in relative contents of polar compounds and their safety risks,the TPC value may not reflect the true freshness and safety of oil.Therefore,because of the fitting difficulty with the existing empirical equation and its unclear chemical meaning of parameters,the thesis established and evaluated the theoretical kinetic equation for the oxidation of oil.On this basis,changes of polar compound composition during high-temperature heating and frying were studied,the feasibility of polar compouds,especially ox TGM,as a new indicator was evaluated,and the influence of water and polar compounds on rapid results of TPC was analyzed.Main contents are as follows:1.Based on the hypothesis of the free radical chain reaction during the oxidation of oil at high temperature,the kinetic equation was found as Y=Y_steady/[exp(-k*t)(Y_steady/Y₀-1)+1].The equation was validated by 93 sets of experimental data,and the adj-R² value is 0.9585?0.9967,and the RMSE value is 2.426?28.66,indicating the good performance of the kinetic equation.With methods of data training and equation transformation,k_f,Y_steady',and Y₀'were found to represent the reaction rate constant,the maximum and minimum value of oxidation index,respectively.It can also explain the meaning of parameters in former empirical equation as Y=Y₀'+(Y_steady'-Y₀')*[1/(1+(Y_steady'/Y₀'-1)*exp(-k_f*t))-Y₀'/Y_steady']*[1/(1+exp(k_de*t)/(Y_steady'/Y₀'-1))+Y₀'/Y_steady'].2.According to the kinetic equation obtained above,with further deduction,the relationship between peroxide value(PV)and ox TGM is In(ox TGM)=0.7782*In(PV)-1.2823(R²=0.9383).Combined with the comparison results of the characteristic kinetic time of induction and intersection timepoints,during the high-temperature oxidation process,in terms of the accumulation rate of oxidation products,the characteristic time of ox TGM is greater than that of peroxide value,and the characteristic time of(ox TGO+ox TGD)is greater than that of the acid value,indicating that the ox TGM and(ox TGO+ox TGD)may be more suitable to evaluate the oxidation degree of oil heated at high temperature.3.Changes in TPC and its composition of oil heated at six temperatures from 90°C to170°C were monitored.Results showed that values of TPC,ox TGO,ox TGD,and ox TGM all increased with the heating time.The TPC and(ox TGO+ox TGD),TPC and ox TGM were significantly and linearly correlated(P<0.05,R²=0.9409 and 0.9492),which showed their simultaneous increase along with the oxidation.Therefore,all these three indices can be used as indicators to represent the oxidation degree during the high-temperature heating process.Considering that ox TGM showed the strongest potential toxicity,it may be meaningful to set the index of ox TGM to evaluate the degradation of heated oil.4.Changes of TPC and its composition of oil fried at high temperatures were monitored.Results showed that values of TPC,ox TGO,and ox TGD all increased with the frying time.It was found that the TPC and(ox TGO+ox TGD)were significantly and linearly correlated(P<0.05,R²=0.9703),while TPC cannot be significantly correlated with the ox TGM(P>0.05).The content of ox TGM fluctuated and showed irregular rules,which is different from the results of heated oil.The reason may be that the water,protein,fat,and trace components contained in the fried food participated in the oxidation,which affects the continuous generation of ox TGM.Both TPC and ox TGM were needed to evaluate the deterioration of frying oil,which showed that the ox TGM is more meaningful as an indicator to judge the deterioration of high-temperature fried oil.5.TPC results of the rapid test and traditional method by the GB 5009.202 standard were compared.The influence of moisture and polar compound composition on the deviation of rapid test was evaluated as well.Results showed that the TPC values by the rapid test were significantly correlated with the results of standard method(Y=0.7625*X+3.681,R²=0.8734,P<0.05).But when the TPC value is greater than 20,results were underestimated by the rapid test,which were 2 lower than the right value.The reason can be that the signal conversion formula built in the rapid detection instrument ignores the actual situation that the dielectric constant of each polar compound is different.It is also found that the moisture content in the oil affected the rapid test results.When the water content of oil is greater than 0.45 g/L,the impact is significant(P<0.05).The higher moisture content resulted in the higher rapid test results.The changes in polar compound composition also significantly affect the rapid TPC results(P<0.05,R²=0.768),which may be caused by the difference in the dielectric constant of each polar compound.In summary,the thesis established the theoretical kinetic equation for the oxidation of oil based on the free radical reaction,studied the high-temperature oxidation kinetics of oil,monitored the change of polar compound composition of heated and fried oil,and evaluated the effect of moisture and polar compound composition on the rapid detection.Results showed the feasibility and necessity of using ox TGM to characterize the high-temperature oxidation of oil and to judge whether frying oil should be discarded.