Migration And Conversion Mechanism Of Phytosterols During The Refining Of Corn Oil

Phytosterols possess various biological properties,especially the cholesterol-lowering activity.However,the content of phytosterols is gradually lost during the corn oil neutralization,bleaching and deodorization.At present,the moderate refining of vegetable oils was proposed to prevent the excessive nutrients loss and to retain the inherent nutrients in China.Therefore,this study systematically researched the migration and conversion of phytosterols in the neutralization,bleaching and deodorization of corn oil with molecular insights for retaining phytosterols.Critical factors were investigated to ascertain mechanism for the loss of phytosterol by the physical migration and the chemical conversion in the corn oil neutralization,bleaching and deodorization,and provide the theoretical basis and technical support for the appropriate refining.The main results were summarized as follows:（1）Migration and conversion of phytosterols during the neutralization of corn oilThe loss of phytosterols during the alkali neutralization process of corn oil was mainly attributed to the migration of free phytosterols（FPs）into soapstock,and the migrated content of FPs decreased with increasing Na OH concentrations.The content of FPs increased with increasing water content in soapstock,which could be attributed to the hydrogen bonding between water and FPs.At the same time,slight chemical conversions between FPs and phytosterol esters（PEs）occurred partly in the liquid-liquid heterogeneous system during the neutralization of corn oil,including hydrolysis,transesterification and esterification.The hydrolysis of PEs would occur when the 26°BéNa OH solution was added;the FP interesterification with triglycerides or esterification with free fatty acids（FFAs）would occur when the 16°BéNa OH solution was added or the Na OH solution was insufficient.The kind of fatty acid in oil was also an important factor affecting the loss of FPs.The loss of FPs caused by sodium palmitate was greater than that caused by sodium oleate and sodium linoleate.Further analysis of the structure of soapstock showed that sodium fatty acid formed a lamellar structure with FPs.When the Na OH concentration gradually increased,the area of polar head group in soapstock decreased,resulting in the reduction of FPs loss.（2）Effect of adsorbents on the adsorption loss of phytosterols during the bleaching of corn oilBy comparing the bleaching of corn oil with six categories of adsorbents,the FP loss was the dominant phytosterol loss.Among them,the activated carbon C and activated clays（ZL/SXI/SXII）were the main adsorbents causing the loss of phytosterols,and the loss of FPs was as follows:the activated clay SXI（22.48±1.67%）,the activated clay SXII（18.57±1.24%）,the activated carbon C（17.70±0.79%）and the activated clay ZL（10.10±1.34%）.In the heptane system,the adsorption capacity of FPs by the activated carbon C was about 10times that of activated clays（ZL/SXI/SXII）.The equilibrium data of the activated carbon C and activated clays（ZL/SXI/SXII）fitted best for Freundlich adsorption,indicating that these might be multilayer heterogeneous adsorption of FPs.The adsorption of FPs by the activated carbon C belonged to the physical adsorption,and the adsorption of FPs by activated clays（ZL/SXI/SXII）belonged to physico-chemical synergistic adsorption.According to the correlation analysis between the model fitting parameter K_Fand the physical properties of adsorbents,it was speculated that the specific surface area and the microporous specific surface area were the key factors leading to the adsorption loss of FPs.（3）Chemical conversions of FPs during the bleaching of corn oilThe content of FPs in corn oil gradually decreased with the prolongation of the bleaching time,and the content of FPs reached a constant level at 95℃for 15 min.The concentration of campesterol,stigmasterol andβ-sitosterol decreased to 85.33±2.85%,73.44±2.68%and 93.05±1.50%,respectively.The degradation of FPs in the heptane system was according with the first-order kinetics,and the degradation kinetic parameters at95℃were ranked as stigmasterol(0.033 min^-1)>campesterol(0.031 min^-1)>β-sitosterol(0.026 min^-1),which was consistent with the degradation order of FPs in the bleaching of corn oil.The degradation rate of different types of FPs varied from unsaturated bonds and branched chain lengths.In the bleaching temperature（75-95℃）,the presence of oxygen and the activated clay ZL promoted the degradation of FPs,increased oxidation products and added to the formation of sterenes.Sterenes were formed under the catalysis of the activated clay ZL,and the calculated apparent active energy（E_a）of stigma-3,5-diene was higher than that of campesta-3,5-diene and stigma-3,5,22-triene,indicating higher difficulty in the formation of stigma-3,5-diene.The chemical loss of FPs in the bleaching of the activated clay ZL could be attributed to two reasons:one was that FPs was catalyzed by the activated clay ZL to dehydrate to form sterenes;the other was that FPs was catalyzed by the activated clay ZL to form polar oxides,such as 7-hydroxy and 7-ketone derivatives,and were adsorbed by the activated clay ZL simultaneously.（4）Chemical conversions between FPs and PEs and their migration during the bleaching of corn oilAcyl donors in corn oil,especially FFAs,promoted the conversion of FPs to PEs at deodorization temperatures.In comparison,saturated palmitic acid had a higher capability of esterifying FPs to PEs than unsaturated oleic acid and linoleic acid.The formation of stigmasteryl ester had a competitive advantage over that of sitosteryl ester by quantum chemistry simulation.Based on the deodorization parameters,temperature played a role in the formation of PEs with a time-dependent manner,and the formation of by-product sterenes was intensified by the extension of the heating time.For the different deodorization experiments,the content of PEs in the steam deodorized oil was higher than that in the nitrogen deodorized oil.In addition,the formation of steroids was also detected in corn deodorized oil,which was partially distilled to the distillate.（5）Formation and analysis of FP thermal degradation products during the deodorization of corn oilStigmasterol,an example of FPs,underwent degradation in corn oil matrix at deodorization temperatures.5-Ethyl-6-methyl-3-heptene-2-one was a marker of such degradation,3-hydroxy-steroid,3-ketostigmasterol,3,7-diketostigmasterol and oligomer were formed at deodorization temperatures.By quantum chemical simulations,the contents of 3-hydoxy-steroid and 5-ethyl-6-methyl-3-heptene-2-one were higher than those of 3-ketostigmasterol at the same deodorized condition.The presence of 3-ketostigmasterol and3,7-diketostigmasterol might help elucidate the linkage of oligomers via C-3 and C-7.