Formation Mechanism Of Malondialdehyde,4-hydroxy-2-hexenal,and 4-hydroxy-2-nonenal In Vegetable Oils During Thermal Processing

Since increasing attention has been paid to nutrition and health,the consumption of vegetable oils,especially those rich in polyunsaturated fatty acid(PUFA),is also increasing in China.As known,PUFA is highly susceptible to lipid oxidation,resulting in the formation of oxidative products.Some of them,which are also the accompanying contaminant of lipid oxidation,are toxic to our health.Malondialdehyde(MDA),4-hydroxy-2-hexenal(HHE),and 4-hydroxy-2-nonenal(HNE)are three typical accompanying contaminants,which are closely related to the peroxidation of PUFA,and has been found with high contents in various vegetable oils and oil-based foods.Increasing attentions have been paid to these three toxic reactive aldehydes,and the Threshold of Toxicological Concern(TTC)level had been set by the European Food Safety Authority(EFSA)scientific committee in terms of the daily intake of MDA,HHE,and HNE.In order to inhibit the formation of MDA,HHE,and HNE in vegetable oils,and guarantee the safety and nutrition of the PUFA-rich vegetable oils in our country,the present study established the simultaneous determination method for MDA,HHE,and HNE;the formation and distribution of these three toxic aldehydes were explored.Factors affected the formation of MDA,HHE,and HNE were also illustrated.Besides,the distribution and migration mechanism of MDA,HHE,and HNE in different fried foods were investigated.And finally,model oil systems were constructed to investigate the formation mechanism of MDA,HHE,and HNE.The main results were listed as follows:(1)The simultaneous determination of MDA,HHE,and HNE in vegetable oils by RP-HPLCBased on the RP-HPLC combined with PDA detector,the method for the simultaneous analysis of MDA,HHE,and HNE in vegetable oils were established.The results showed that the linear ranges were 0.02-10.00μg/mL for MDA,0.02-4.00μg/mL for HHE,and 0.03-40μg/mL for HNE with the satisfactory correlation coefficient of>0.999 for all detected aldehydes.The limit of detection(LOD)and limit of quantification(LOQ)of MDA,HHE,and HNE were～0.021and 0.020μg/mL,～0.009 and 0.020μg/mL,and～0.014 and 0.030μg/mL,respectively.Their recoveries were 99.64-102.18%,102.34-104.61%,and 98.87-103.04%for rapeseed oil and 96.38-98.05%,96.19-101.34%,and 96.86-99.04%for French fries,separately.Under the selected conditions,the developed methods was successfully applied to the simultaneous determination of MDA,HHE,and HNE in different tested vegetable oils.The results indicated that this method could be employed for the quality assessment of vegetable oils.(2)The distribution of MDA,HHE,and HNE in vegetable oils during thermal processing and the formation mechanisms of themThe formation of MDA,HHE,and HNE in eight commonly consumed vegetable oils in our country was investigated at 180°C.The formation mechanism of them were further explored by different fatty acid methyl esters(FAMEs).HHE was only detectable in soybean(SBO),rapeseed(RO),and linseed oils(LO).MDA and HNE was measured in all tested oils.High MDA content was found in SBO,RO,sunflower oil(SO),and LO,while HNE was observed mainly in corn oil(CO),SO,and SBO.Oil-dependent formation of MDA/HHE/HNE were remarkably observed.Furthermore,different fatty acid methyl esters(FAMEs)in tricaprylin,as model oil systems,were constructed to demonstrate their characteristic contribution to MDA/HHE/HNE formation.Both methyl linolenate(MLN)and methyl linoleate(ML)could generate MDA,while HHE and HNE originated from the oxidative degradation of MLN and ML,respectively.Particularly,low content of HHE was also detected in ML.The formation of MDA/HHE/HNE was concentration-dependent with their corresponding precursors.Low concentrations of MLN(<5.0%)and ML(<1.0%)produced no detectable MDA/HHE/HNE.Furthermore,MLN/ML could induce both HHE/HNE formation and acted as pro-oxidation at higher concentrations.Unexpectedly,methyl stearate(MS)and methyl oleate(MO)slightly promoted HHE/HNE formation,which might be attributed to free radical transfer mechanisms during thermal oxidation.(3)The application of HHE and HNE as the characteristic indicator to evaluate the oxidation of vegetable oilsBased on the Schaal over test,the formation of MDA,HHE,and HNE in palm oil(PO),corn oil(CO),rapeseed oil(RO),camellia oil(CLO),and linseed oil(LO)was investigated,and the applicable of HHE/HNE as new indicator by heat map to evaluate the oxidation in vegetable oils was also explored.The results showed that HHE was only observed in linseed oil(LO)and rapeseed oil(RO),while MDA and HNE were found in all the tested oils.Meanwhile,the potentials of MDA-,HHE-,and HNE-formation in different oils were closely related to their initial polyunsaturated fatty acid(PUFA)content.The formation of HHE and HNE in vegetable oils was significantly suppressed by intrinsic tocopherols with little oil type-dependent effect on the inhibition rates(10.49?16.59%).Correlation analyses showed that MDA was inappropriate to evaluate the oxidation of oils with low PUFA content such as palm oil and camellia oil,for the low coefficients,while HHE and HNE showed good correlations with other oxidation indices in all the tested oils.Finally,regression analysis showed the content of HHE could predict the oxidation ofω-3 type oil,while of HNE as a good indicator to estimate oxidative deterioration ofω-6 andω-9 type oils.(4)Understanding the difference of MDA,HHE,and HNE formation in different vegetable oils:a kinetic and thermodynamic studyThe contents of MDA,HHE,and HNE in five different vegetable oils during thermal process(100-200°C)were investigated and the results modeled.The results showed that HHE was determined in rapeseed oil and linseed oil,other than in palm oil,corn oil,and camellia oil,while MDA and HNE was determined in all tested oils and the contents varied significantly depending on the oil type.For all MDA/HHE/HNE-detected vegetable oils,the increase in both MDA,HHE and HNE contents during thermal process was well described by the pseudo-first-order kinetic model.The equilibrium contents of MDA/HHE/HNE increased and the duration until equilibrium decreased as a function of thermal time.The calculated reaction rate contents followed the Arrhenius law.The rate of MDA/HHE/HNE formation was remarkably matrix-dependent,which was also in accordance with the calculated kinetic and thermodynamic parameters,activation energies(E_a)and enthalpies(?H~?).Accordingly,the above results can contribute to a prediction of MDA/HHE/HNE formation in different types of vegetable oils during thermal process,especially deep-frying system under air generally requiring temperature and time of higher than 100°C and 6 h,respectively.(5)The formation and distribution of MDA,HHE,and HNE during frying and the migration mechanisms towards different foodsThe formation and distribution of MDA,HHE,and HNE during frying was explored.Besides,the migration mechanism of them in different fried foods was also investigated.The results showed that during the whole frying period,high amounts of MDA,HHE,and HNE were observed in both frying oils and fried foods,indicating the easy incorporation of these three toxic aldehydes into fried food.Regardless of the oil type,the distribution of MDA,HHE,and HNE in different fried foods were not the same.In French fries(FF)HNE content was higher,whereas lower in fried chicken breast meat(FCBM).In contrast,MDA content in FF was lower,while higher in FCBM.Regardless of the oils types,distribution of MDA,HHE,and HNE was only related to the food types.Result of model oil system showed that content of HNE was higher in FF for the higher hydrophobic property than MDA/HHE which would be preferred bounded into the hydrophobic helical structures,whereas lower content of HNE was observed in FCBM for its higher reactivity towards nucleophilic group,namely protein in FCBM.The reactivity towards proteins were as follow:HNE>HHE>MDA.Finally,the probable migration mechanism of MDA,HHE and HNE in different food matrixes was proposed for the first time.