| Recently,increasing evidences show a strong positive relation of obesity with hyperlipidemia,hypertension,cardiovascular disease,and Type 2 diabetes,etc.Excessive energy intake is the primary factor leading to obesity worldwide.The dietary survey report of Chinese residents shows that the oil intake per capita is 43 g/d,which is much higher than the25-30 g/d recommended by the dietary guidelines of residents,and the fat supply ratio is as high as 35.5%.The novel functional health oils could improve lipid metabolism and avoid excessive accumulation of body fat and have became the primary concern for improving the health status of Chinese residents and solving obesity and its complications.In this study,two functional oils of diacylglycerol(DAG)and LML(Long-Middle-Long)-type structured lipids with health promoting properties of reducing body fat accumulation and improving lipid metabolism were chosen as research objects.Aiming at the shortcomings of the existing preparation processes of DAG and LML-type structured lipids,and the research status of two kinds of structured lipids lacking of application characteristics,the processes of preparing DAG and LML structured lipids by whole enzymatic method were systematically investigated.Meanwhile,the application characteristics of two functional oils were systematically examined.To date,glycerolysis and esterification are typically employed for the preparation of DAG.For the esterification method,fatty acids should be firstly prepared and then the prepared fatty acids and glycerol are used as substrates for the preparation of DAG by lipase with strict sn-1,3 specificity catalyzed esterification.The process is complicated and the purity of the final product is usually less than 85%owing to the formation of triacylglycerol(TAG)during esterification.Therefore,in view of the complexity of the current processes and the low purity of the final DAG product,this study developed a whole enzymatic method for simultaneous preparation of high-purity and medium-purity DAG.First,DAG was prepared through lipase DF 15 with strict sn-1,3 specificity catalyzed hydrolysis under the conditions of a water content of 20 wt%,an enzyme loading of 0.25 wt%,a reaction temperature of 40~oC and a reaction time of 3 h,and a hydrolysate with 35.52%DAG was obtained.After purification by molecular distillation,the final product containd 56.6%DAG.Subsequently,the collected distillate was used as substrate for further preparation of DAG using a novel partial glyceride lipase of AOL-V269D that excaveted by our research group under the conditions of a molar ratio of MAG to FFA of 5:1,a reaction temperature of 40 ~oC,an enzyme loading of 0.1 wt%and a reaction time of 12 h.A esterification mixture containing 56.69%DAG was obtaibed.The above mixture was then purified by molecular distillation getting a final product with 98.11%DAG.The whole enzymatic process proposed in this study is safe and environmental friendliness.It could not only obtain high-purity(98.11%)and medium-purity(56.6%)DAG products,but also elevated the comprehensive utilization rate of raw materials.Currently,medium and long chain triacylglycerol(MLCT)is usually prepared by enzymatic esterification and acidolysis,which are complicated and the product is usually a mixture of various types of structured lipids.Additionally,the current research on MLCT is mainly focused on the preparation of MLM-type structured lipids and the evaluation of its functional properties.However,less information is available on the preparation,physicochemical properties and physiological functions of LML-type structured lipids.In this study,immobilized lipase TTL with strict sn-1,3 specificity was employed for the preparation of LML-type structured lipids by transesterification.First,lipase TTL was immobilized and the optimal conditions for lipase TTL immobilization were determined.Subsequently,the immobilized lipase TTL was used to prepare LML-type structured lipids by transesterification of tricaprylin and ethyl linoleate.A product containing 53.19 mol%double long-chain TAG(DL-TAG)was obtained under the conditions of a reaction temperature of 60 ~oC,an enzyme loading of 6 wt%(relative to the total weight of the substrates),and a substrate molar ratio of tricaprylin to ethyl linoleate of 1:6.The product was then purified by molecular distillation and the final product contained 85.24 mol%DL-TAG.Finally,fatty acid at sn-2 position of the final product was analyzed and the results showed that the final product contained at least82.7 mol%of LML-type structured lipids.At present,few reports are available on studying the changes of the physicochemical properties of DAG and LML-type structured lipids during high temperature utilization.In this study,the physicochemical properties of the prepared DAG and LML type structured lipids and their properties in high temperature frying were studied.Results showed that the two structured lipids have similar fatty acid composition,viscosity,density,acid and peroxide value with the raw soybean oil.Compared with soybean oil,DAG and LML-type structured lipid have a 20-30 ~oC reduction in onset weight loss temperature and weight loss peak temperature in thermogravimetric analysis.DSC analysis showed that when the sample contained DAG,the melting rate of the sample phase transition curve would decrease,and when the DAG content in the sample reached 90%,the phase transformation process would be shortened to some extent.Additionally,FTIR analysis of the three oils showed that the absorption peaks of LML-type structured lipids and soybean oil were basically the same,while DAG showed more difference with soybean oil.Finally,the functional properties of DAG and LML-type structured lipids were evaluated through animal studies.After a single intragastric administration,it was found that DAG and LML-type structured lipids significantly reduced the postprandial lipid levels in experimental mice by comparison with control group(intragastric administration of soybean oil),however,the differences between the blood glucose and cholesterol were not significant.Subsequently,through intervention experiments with long-term feeding of high-fat and high-cholesterol feeds with different doses of DAG,it was found that the fasting blood lipids,blood glucose,serum total cholesterol and LDL levels of the mice in the H-DAG group were significantly decreased,whereas L-DAG group reduced but not significant.By analyzing the TAG and cholesterol levels in the liver of mice,long-term intake of DAG could significantly reduce the TAG and cholesterol levels in the liver of mice compared with long-term intake of soybean oil.Further analysis of liver pathology of experimental mice revealed that long-term intake of DAG could alleviate fatty liver disease caused by high-fat,high-cholesterol feeding in experimental animals. |
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