Study on the Formation and Adsorptive Elimination Mechanisms of Glycidyl Esters in Vegetable Oils during Refining
Author:Cheng Wei Wei
Supervisor:liu guo qin
Chemical contaminants formed in edible vegetable oil during thermal processing pose the potential threat to human health.In recent years,with the increasingly published toxicology data and wide occurance of these subtances in edible oils,it is currently greatly urgent to conduct the research on their formation and elimination during oil processing.Glycidyl fatty acid estters(GEs)has recently been found as a novel potential carcinogen in refined vegeatble oils.The hydrolysate of GEs,that is,glycidol,was categorized as“probably carcinogenic to humans”in group 2A by International Agency for Research on Cancer(IARC).Presently,the European Commission(EC)has set maximum levels for GEs in edible oil and infant fumula.Accordingly,to effectively inhibit the formation of GEs and ensure the safe production of vegetable oil,this study used vegetable oils(soybean oil,camellia oil,and palm oil)with high consumption as research matrix to investigate the key factors of affecting GEs formation during vegetatable oil preparation and refining.Model oil and chemical model system were formulated to acertain the reaction mechanisms of GEs formation by means of modern instrument analysis technology and kinetic model fitting,and finally develop the highly effective method for GEs elimination in vegetable oil.The Main results were shown as follows:(1)Effects of oil preparation method and refining steps on GEs formationThe variation of GEs contents at different refining stages(drgumming,deacidation,bleaching,and deodorization)for tested vegetable oils(Soybean oil,camellia oil,and palm oil)prepared by different methods(cold pressing,hot pressing,and solvent extraction)was investigated.Results showed that there was significant difference for GEs contents in vergeatable oils obtained by different preparation methods and refining steps.For the same vegetable oil with different preparation methods,GEs contents were ranked as hot-pressed oil>cold-pressed oil>solvent-extracted oil;For different vegetable oils with the same preparation method at the same refining step,GEs contents were ranked as palm oil>camellia oil>soybean oil;For the same vegetable oil with the same preparation method at differet refining stages,GEs contents were ranked as deodorized oil>bleached oil>deacidized oil>degummed oil>crude oil,in which GEs contents at the refining stages before deodorization varied slightly,and thus oil deodorization was the main step that generated great amounts of GEs.Deodorization temperature and time were the important factors for GEs formation.When deodorization temperature exceeded 200°C,GEs content in tested oils presented a trend of increasing first and decreasing then and the time point corresponding to the top GEs levels was in the range of1-2 h,suggesting that the formation and degradation of GEs occurred synchronously under high temperature.(2)Precusors and free radical-mediated mechanisms of GEs formationBased on the separation of trace components and then excluding one by one,combined with proton nuclear magnetic resonance and fourier transform infrared spectroscopy technology,it was well proved that the precursors of GEs formation were diacylglyceride(DAG)and monoacylglyceride(MAG)and the formation ability of MAG was higher than that of DAG in this work.However,MAG content is generally very low in vegetable oil,and thus their contribution to GEs content in vegetable oil was ranked as DAG>MAG.The intermediate of GEs formation from DAG and MAG had cyclic acyloxonium-containing structure,and GEs could not be formed directly from triacylglyceride(TAG).Model oils containing the same amounts of pure 1,2-dipalmitoyl-glycerol and hexadecane chemical model systems were formulated to remove the effect of precursors on GEs formation in different model oils.From the change of GEs contents during heating at 200°C,it was observed that different GEs formation in model oils was not related to the formed polar fractions during heating but was related to oil type.Ferric sulfate,tert-butylhydroquinone(TBHQ),and oxygen contact could significantly affect the formation of GEs in model oil,in which ferric sulfate and oxygen contact play a promotion role while TBHQ plays an inhibition role on GEs formation,which was in accrordance with the results in chemical modyl systems.These findings suggested that GEs formation in model oil might be related to oil oxidation.By anlysing the difference of GEs contents in ferric sulfate-,TBHQ-,and V_E-DPG model systems,it was ascertained that ferric sulfate,TBHQ,and V_E could affect directly GEs formation,and the inhibition ability of TBHQ was higher than that of V_E.FTIR results further indicated that ferric sulfate actually promoted the formation of cyclic acyloxonium intermediate(CAI),but contrarily for V_E.By using 5,5-Dimethyl-1-pyrroline N-oxide(DMPO)as radical trap,the structural form of CAI was characterized indirectly by quadrupole time-of-flight tandem mass spectrometry(Q-TOF MS/MS).The results revealed that CAI generated during GEs formation was in the form of free radical.Therefore,a free radical mediated mechanism for the formation of GEs from DAG at high temperature was proposed in the present study.The reaction process involved in the formation of CAI′and CAI″might be initiated by the removal of either fatty acid radical(R·)at the sn-2 position or hydroxyl radical at the sn-3 position combining with the thermally resulting radical(such as ROO·)that most likely originated from oil peroxidation,or the alternatively direct fracture of covalent bond under high-temperature conditions.The ester carbonyl group at the sn-1 position might be immediately attacked by the sn-2 carbon-centered radical to form CAI′free radical.Also,the ester carbonyl group at the sn-2 position might also be attacked by the sn-3 carbon-centered radical to form CAI″free radical.Subsequently,unstable CAI(CAI′or CAI″)free radical could perform a further intramolecular rearrangement to form GEs in which the free radical might participate in the termination stage of oil autoxidation.As a consequence,it was well understood that ferric sulfate promoted the formation of GEs,which was also inhibited by TBHQ and V_E,not only by indirectly regulating lipid free radical-oxidation,but also by directly affecting CAI formation.(3)Kinetic models of GEs formationBy making ideal assumptions about GEs reaction generation conditions,the chemical reaction rate equation was established and solved based on boundary conditions.The kinetics of GEs levels in soybean oil,camellia oil,and palm oil during heating at 200-240℃in 2 h were fitted pseudo first-order reaction model.The obtained k1 and k2 levels at different temperatures followed Arrhenius equation and the caculated apparent active energy(E_a)of GEs formation was ranked as camellia oil(35.64 kJ/mol)>palm oil(34.58 kJ/mol)>soybean oil(29.58 kJ/mol).However,the Ea levels of GEs degradation for different tested oils varied slightly(12.87-14.13 kJ/mol),suggesting that GEs degradation was not related to oil type.The pseudo first-order formation reaction rate constant of GEs levels in ferric sulfate-and TBHQ-model palm oil system during heating at 200℃had significant difference compared to that in model palm oil but no difference for GEs degradation reaction rate constant,suggesting that pro-oxidant and antioxidant could affect GEs formation but degradation.GEs formation reaction rate from 1-monopalmitoyl-glycerol(MPG)was higher that that from DPG in hexadecane chemical model system.(4)Adsorption elimination mechanisms of GEsAcid-washed oil palm wood-based activated carbon(awOPAC)was used for the adsorptive removal of GEs in palm oil,and GEs contents were 0.17 mg/kg in adsorbed palm oil.The corresponding removal rate was 95.49%,which was significantly higher than other tested adsorbents(P<0.05)in this work.The GEs-awOPAC adsorption test was conducted in palmitoyl glycidyl esters(PGE)-hexadecane system,which was evaluated by adsorption thermodynamic,equilibrium isotherm,and kinetic model.Thermodynamic parameters(ΔH,ΔS,andΔG)indicated that the sorption process was feasible,spontaneous and exothermic in nature.The equilibrium data fitted best for Langmuir adsorption isotherm and the maximum capacity of awOPAC was found to be 36.23 mg/g.The result of kinetic study shows that the adsorption of PGE could be described by the pseudo-second-order equation.These observations indicated that the adsorption process was presumably physisorption,controlled by both film diffusion and intraparticle diffusion.The desorption test indicated the removal of GEs from palm oil was attributed to not only the adsorption of GEs on acid-washed OPAC,but also the degradation of GEs adsorbed at activated sites with acidic character.