Preparation Of OPO Structured Lipid And Characterization Of Its Oil-in-Water Emulsion

1,3-Dioleyl-2-palmitoylglycerol?OPO?,an important ingredient in human milk fat,has attracted experts'attention because of its special structure.In recent years,enzymatic esterification is mainly adopted by the synthesis of human milk fat substitute which is due to its mild reaction,less by-products and recycling.During enzymatic esterification,acyl migration is unavoidable and it can only be minimized by controlling the reaction factors.It is well-known that oils and fats are non-polar components,and their water-insoluble properties are not conducive to their bioavailability in the human body.Formula milk is ingested by an infant in the form of an oil-in-water emulsion and an excellent delivery system that combines encapsulation,protection,and targeted delivery of lipid components.The protein in the formula milk is the main oil-water interface component to maintain the stability of the emulsion,and other small molecules and minerals also play a role in stabilizing the emulsion and regulating lipid digestion.Breast milk contains 0.8-0.9%protein whose the composition ratio of whey protein and casein is different during different lactation phases.That is,the ratio of whey protein:casein is approximately 6:4 in mature human milk?early lactation,9:1;late lactation,5:5?.However,it is rarely reported how different protein ratios and other components such as minerals affect the physicochemical properties of the emulsion system and the impact on the release of the OPO structured lipid.Therefore,this paper focuses on the preparation of OPO structured lipid and the effect of the proportions of different proteins and metal ions(Ca²⁺,K⁺)on the the physicochemical properties of OPO emulsions and the release of fatty acids during in vitro digestion to explore the interactions of fat-protein-mineral components in breast milk,which provide theoretical basis for optimizing infant formula.First,OPO structured lipid was prepared by enzymatic reaction,namely,1,3position specific lipase-catalyzed acidolysis of tripalmitin?PPP?with fatty acids from camellia oil.The effect of acyl migration on OPO structured lipid was carried out by investigating the factors of two immobilized lipases?Lipozyme RM IM,Novozyme435?,solvents?hexane,t-butanol,acetone?,water activity?0.11,0.53,0.97?,reaction time?0,2,4,6,8 h?.The results showed that water activity is 0.53,the degree of acyl migration is minimal;the hydrophobic organic solvent is more conducive to the acyl migration during the esterification of oils and fats;at 6 h,obtained OPO structured lipid is desirable products.In other words,under optimized conditions for reaction of6h,water activity of 0.53,acetone system,the desirable products was obtained.In short,the percentage of sn-2 P and sn-1,3 O which is released by Lipozyme RM IM and Novozyme 435-catalyzed OPO structured lipid is 94.97%?62.00%and 76.55%?39.67%,respectively.Secondly,the OPO human milk fat substitute was prepared into an oil-in-water emulsion.The effects of different protein ratios?whey protein:casein,10:0,9:1,6:4,5:5,0:10?and ion types(Ca²⁺,K⁺)on the physicochemical properties of OPO emulsions were studied by means of analysis of particle size distribution,zeta potential,particle size index,creaming index,rheological properties,and oxidative stability.The physicochemical stability of the three groups of emulsions under the same protein ratio is as followed:Ca²⁺<K⁺<blank which was mainly caused by the difference of ionic valence.Divalent ions can affect the electrostatic interactions between lipid droplets and the interactive effects of ion surface adsorption,while monovalent ions can only promoted electrostatic screening at reducing electrostatic repulsion between lipid droplets.In addition,the effect of different protein ratios on the physical stability of the emulsion is not significant as a whole under the same ionic type conditions.In term of the chemical stability,emulsions made from casein alone as emulsifiers have relative oxidation values that are greater than those of other different emulsifiers.This may be due to differences in the purity of raw protein and solubility in the buffer.Finally,in vitro simulated gastrointestinal digestion system was used to explore the effects of different protein ratios and minerals and their interactions on the digestion of OPO human milk fat substitute.The figures showed that in the early stage of gastric digestion,the lipid digestion rate of the 0:10 was greater than 10:0 and6:4;with the decrease of the whey protein:casein content,the FFA release rate of 0:10was the lowest,followed by 6:4,but the FFA release rate of 10:0 and 6:4 was almost the same at the concentration of 20 mmol/L CaCl₂.In the early stage of intestinal digestion,the digestive rate was as follows:10:0<6:4<0:10,and then the digestive rate was generally similar for the three groups of different emulsifier ratio.At the concentration of 20 mmol/L CaCl₂,the final digestive degrees of the three emulsions was as follows:10:0?55.69%?>6:4?47.82%?>0:10?45.12%?,with no significant change.Meanwhile,with the same emulsifier ratio?10:0,6:4,0:10?and different CaCl₂ concentrations?0,5,10,20 mmol/L?,the bioavailability of the final product increased with the increase of CaCl₂ concentration.Therefore,the effect of free fatty acids and monoglyceride components and different calcium salts on the lipid digestion of OPO emulsions was investigated with the protein ratio of 6:4 and 20mmol/L CaCl₂.The results show that the OPO emulsion was digested whose free fatty acid and monoglyceride were mainly composed of palmitic acid and oleic acid,next to stearic acid,linoleic acid and myristic acid.In addition,the effect of different calcium salts on lipid digestion was consistent with the expected results during the intestinal digestion phase which was attributed to the solubility of different calcium salts in the aqueous phase.That is,the bioavailability is as follows:calcium citrate>calcium chloride>calcium lactate>calcium carbonate.