Study On Synthesis Of β-sitosterol Fatty Acid Esters By Chemical Method

Phytosterols are important functional components which can reduce cholesterol in serum.However,the high melting points and poor oil or water solubility of phytosterols limit their application in food,medicine and other industries.β-sitosterol is one of the most abundant and widely distributed phytosterols and has unique physiological functions such as prostatic prevention and anti-inflammatory,which has important research value.Esterification ofβ-sitosterol with long-chain fatty acids to synthesizeβ-sitosterol fatty acid esters is an effective method to improve the oil solubility and bioavailability ofβ-sitosterol.β-sitosterol fatty acid esters not only retain the advantages ofβ-sitosterol,but also have the property of fatty acids.Therefore,theβ-sitosterol fatty acid esters synthesized with different fatty acids are different.At present,the synthesis methods of phytosterol fatty acid esters mainly include enzymatic method and chemical method.The enzymatic synthesis is expensive and difficult to realize industrial production in the short term.The traditional chemical method has the problems of high reaction temperature and side reactions.Therefore,the purpose of this study is to find a chemical catalyst that can realize low temperature catalysis and establish an efficient synthesis method ofβ-sitosterol fatty acid esters.The esterification ofβ-sitosterol with saturated fatty acid（lauric acid）was studied using dodecylbenzene sulfonic acid DBSA as catalyst.The results showed that DBSA could catalyze the synthesis of sterol esters at a lower reaction temperature（40℃）.Through single factor and response surface methodology,the optimized reaction conditions were as follows:reaction solvent was n-hexane,reaction temperature was 69℃,reaction time was 9 h,the molar ratio of the substrate（β-sitosterol:lauric acid）was 1:1.5,catalyst amount was 14%.Under these optimized conditions,the conversion ofβ-sitosterol was 97.0%.The crude product ofβ-sitosterol laurate obtained was separated and purified,and the yield ofβ-sitosterol laurate was 95.6%.The esterification ofβ-sitosterol with monounsaturated fatty acid（oleic acid）was studied using DBSA as catalyst under solvent free condition.Through single factor and response surface methodology,the optimized reaction conditions were as follows:reaction temperature was 71℃,reaction time was 11 h,the molar ratio of the substrate was 1:1.3,catalyst amount was 9.8%.Under these optimized conditions,the conversion ofβ-sitosterol was 91.4%.The crude product ofβ-sitosterol oleate obtained after the reaction was isolated and purified,and then identified by FT-IR.The esterification ofβ-sitosterol with polyunsaturated fatty acid（linolenic acid）was studied using DBSA as catalyst.Through single factor and response surface methodology,the optimized reaction conditions were as follows:reaction temperature was 76℃,reaction time was 12 h,the molar ratio of the substrate was 1:1.7,catalyst amount was 16%.Under these optimized conditions,the conversion ofβ-sitosterol was 95.6%.β-sitosterol linolenic ester was identification by FT-IR,and it was proved thatβ-sitosterol linolenic ester was successfully synthesized under mild conditions.The reaction kinetics ofβ-sitosterol esters synthesized with three kinds of acyl donors（fatty acid,oil and fatty acid methyl ester）using DBSA as catalyst were studied.The results showed that the esterification reaction ofβ-sitosterol with fatty acid and the transesterification reaction with triglyceride and methyl fatty acid were the second order irreversible reactions.Reaction activation energies of DBSA catalyzed the esterification ofβ-sitosterol with fatty acids from high-oleic acid sunflower seed oil,the transesterification ofβ-sitosterol with high-oleic acid sunflower seed oil,and the transesterification ofβ-sitosterol with methyl esters of high-oleic acid sunflower seed oil were 75.09 k J mol^-1,114.17 k J mol^-1,and 93.23 k J mol^-1,respectively.Combined with the kinetic data and the effect of water,the catalytic reaction mechanism of DBSA was discussed.The results showed that DBSA catalyzed esterification reaction can form a reverse micelle system,which can realize water isolation,avoid the side reaction hydrolysis reaction,and promote the esterification reaction equilibrium and forward movement.