Method for preparing special grease for infant formula milk powder by enzyme method

Abstract

The invention relates to the technical field of enzymatic preparation of grease, in particular to a method for enzymatic preparation of special grease for infant formula milk powder, which comprises the following steps: constructing a pH response type interface biocatalysis system, and synthesizing and enriching functional ester; the preparation method comprises the following steps: preparing an sn-2 palmitic acid diglyceride intermediate based on alcoholysis reaction, and reacting the sn-2 palmitic acid diglyceride intermediate with mixed fatty acid to synthesize the composite structure lipid; and preparing the target mixed grease from the functional ester, the composite structure grease and the fat-soluble vitamin by adopting a low-temperature high-speed shearing and homogenizing technology. According to the method for preparing the grease special for the infant formula milk powder through the enzyme method, pH response type magnetic nano immobilized lipase is prepared, the enzyme activity is controllable, and the grease is recycled and reused through the dual mechanisms of magnetic separation and pH induced aggregation; a fat-soluble vitamin-structural fat-essential fatty acid composite synergistic system is constructed, the functions of energy supply, mineral absorption and micronutrient supplementation are achieved, and a target grease product is purified by adopting a multi-stage molecular distillation step.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of enzymatic preparation of oil and fat, in particular to a method for enzymatically preparing special oil and fat for infant formula milk powder. BACKGROUND

[0002] Enzymatic preparation of special oil and fat for infant formula milk powder is a green manufacturing technology that utilizes the biological catalytic action of lipase to rearrange natural oil and fat molecules under mild conditions, thereby constructing special oil and fat that highly simulates breast milk fat in composition and structure. Unlike traditional chemical methods, which cannot control the position of fatty acids on the triglyceride molecule and can only obtain a mixture of various structural lipids, enzymatic preparation of special oil and fat enables precise synthesis, thus having a very broad application prospect and being a key driving force for upgrading the infant formula milk powder industry and even the entire special food industry.

[0003] According to the search, the invention patent with the Chinese patent publication number CN114181982B discloses a method for enzymatically preparing special oil and fat for infant formula milk powder. The method in the invention patent enzymatically processes natural oil and fat rich in sn-2 palmitic acid to prepare sn-2 palmitic acid monoglyceride, and then directly synthesizes UPU with the prepared sn-2 palmitic acid monoglyceride and an oleic acid-based / linoleic acid-based acyl donor. The sn-2 palmitic acid monoglyceride is purified by solvent crystallization, and the water activity is controlled to reduce the transfer of sn-2 palmitic acid acyl groups, thereby ensuring the obtaining of high-content sn-2 palmitic acid monoglyceride, which is beneficial to improving the yield of UPU. However, the enzymatic preparation method uses free sn-1,3 specific lipase to catalyze the reaction, and only centrifugal filtration is performed after the reaction, which cannot realize recycling and reuse. In addition, the obtained oil and fat product has a single function and lacks essential micronutrients, and the product is purified by solvent crystallization, resulting in low purity. Based on the existing technical deficiencies, a method for enzymatically preparing special oil and fat for infant formula milk powder is proposed. SUMMARY

[0004] (I) Technical problems to be solved In view of the deficiencies of the prior art, the present application provides a method for enzymatically preparing special oil and fat for infant formula milk powder, which has the advantages of enzyme catalysis recycling and reuse and multi-stage molecular distillation efficient purification, and solves the problems of low purity of nutritional components and oil and fat product in the prior art.

[0005] (II) Technical solutions To achieve the above-mentioned enzyme catalytic recycling and multi-stage molecular distillation efficient purification purposes, the application provides the following technical solutions: a method for preparing special oil for infant formula milk powder by enzyme method, comprising the following steps: S1. The palm stearin rich in palmitic acid is used as raw oil to realize degumming, deacidification and deodorization refining treatment, and a pH-responsive interface biocatalysis system is constructed to synthesize and enrich functional esters; S2. The sn-2 palmitic acid glycerol diester intermediate is prepared based on alcoholysis reaction, and the sn-2 palmitic acid glycerol diester intermediate is subjected to acidolysis esterification reaction with mixed fatty acids to accurately synthesize composite structure fat; S3. The low-temperature high-speed shearing homogenization technology is used for the functional esters, the composite structure fat and the fat-soluble vitamins prepared and synthesized to ensure uniform dispersion of components, and finally low-temperature soft tower deodorization and antioxidant treatment are performed.

[0006] Preferably, in step S1, the solid palm stearin obtained by low-temperature fractionation of palm oil is used as raw oil, the composite phospholipase preparation system is used to remove phospholipids, the active clay-silica-activated carbon composite adsorption system is used for adsorption and decolorization of the raw oil, most of the free fatty acids are removed in advance by molecular distillation method, and then the remaining free fatty acids and odor molecules are removed in the structured packing tower to realize physical refining treatment of the palm stearin; The purity of retinol (all-trans vitamin A alcohol) is greater than 95%; the purity of palmitic acid after molecular distillation is greater than 98%; the free fatty acid impurities are reduced to reduce the inhibition of the enzyme catalyst; the Candida antarctica lipase B (CALB) is modified and fixed on the pH-responsive group of the magnetic nanometer microspheres to prepare a pH-responsive immobilized lipase as a catalyst; t-butyl alcohol is used as a green hydrophobic solvent; and a 3Å type molecular sieve is used to construct a pH-responsive interface biocatalysis system; The specific steps for preparing functional esters by esterification synthesis reaction are as follows: 1) In a jacketed glass reaction kettle, retinol and palmitic acid are added as reaction substrates in a molar ratio of 1:(1.2~1.5), t-butyl alcohol is added in a solvent to reaction substrate volume ratio of 3:1, the temperature is raised to 50~60℃ by circulating water bath heating, nitrogen is continuously introduced for protection during stirring to ensure that the substrates are fully dissolved and prevent oxidation of retinol; 2) The pH value of the reaction system is adjusted to 6.5~7.5 by adding 0.1M dilute NaOH solution or dilute HCl solution in the reaction kettle system, the pH-responsive immobilized lipase is added in an amount of 3%~5% of the total substrate, and the molecular sieve is activated at high temperature; 3) stirring at 200-300 rpm for 8-12 h at 50-60 °C to ensure uniform suspension of the catalyst and prevent breakage due to shear force, sampling every 2 h, and monitoring retinol palmitate formation and retinol residual amount by high performance liquid chromatography (HPLC).

[0007] Preferably, after the ester synthesis reaction is completed, the pH of the reaction solution is adjusted to less than 5.0 to change the charge on the surface of the immobilized enzyme carrier from a highly dispersed state to an aggregated and precipitated state; under the dual action of magnetic separation and pH-induced aggregation, the catalyst is separated and recovered from the reaction solution, washed with anhydrous solvent, and then dried; The reaction solution after separation of the catalyst is distilled under reduced pressure at 50 °C and a vacuum degree of -0.095 MPa, most of the solvent is recovered, and then warm water at 50 °C is added to the remaining concentrated solution for washing to remove residual palmitic acid and catalyst. After standing and separating, the organic phase is collected to obtain the crude functional ester product. The steps for further purification and enrichment of the product include: 1) controlling the feed temperature of the short-path molecular distillation device to be 80 °C, the distillation temperature to be 120 °C, the condensation temperature to be 5 °C, and the feed rate to be 1.0 mL / min, and removing the light components of residual solvent, water, and a small amount of free palmitic acid by first-stage molecular distillation under a system pressure of less than 0.01 kPa; 2) performing second-stage molecular distillation treatment on the remaining heavy components in the device, controlling the feed temperature to be 100 °C, the distillation temperature to be 180-200 °C, the condensation temperature to be 30 °C, and the feed rate to be 0.5 mL / min, and recovering unreacted retinol in the condenser after distillate I and obtaining high-purity retinol palmitate in the main collector after distillate II as the target functional lipid; 3) detecting the purity of the final product by HPLC-UV and analyzing the molecular structure by LC-MS, dissolving the obtained high-purity retinol palmitate in a small amount of neutral edible vegetable oil to prepare a stock solution, and storing it in a nitrogen-filled sealed container away from light and in a freezer.

[0008] Preferably, the magnetic nanometer immobilized lipase is used to catalyze the alcoholysis reaction of palm stearin and glycerol, specifically hydrolyze the sn-1,3 ester bond to generate sn-2 palmitic acid glycerol diester intermediates. Refined palm stearin is used as the acyl donor, high-purity glycerol is used as the nucleophile, and the magnetic nanometer immobilized lipase is used as the catalyst. Phosphate buffer is added simultaneously with activated 3 Å molecular sieves as the dehydrating agent. The sn-2 palmitic acid glycerol diester intermediate preparation steps include: 1) The reaction substrate of palm stearin and glycerol is prepared in a molar ratio of 1:(3-4). The palm stearin is added to a jacketed stainless steel reaction kettle, and heated to 65-70°C to completely melt. The glycerol and 10%-15% of the total mass of the reaction system of phosphoric acid buffer are added under low-speed stirring at 100 rpm; 2) The high-shear dispersion emulsifier is started, and emulsified at a speed of 10,000-12,000 rpm for 3-5 min to form a uniform water-in-oil microemulsion, which is used to increase the interface area of the oil-water two phases to improve the enzyme catalytic efficiency; 3) The temperature of the reaction system is controlled at 60°C, and the speed is reduced to 300-400 rpm. The magnetic nano-immobilized lipase is added at 5%-8% of the mass of the palm stearin, and the high-temperature activated 3Å molecular sieve is added at 15%-20% of the total mass of the reaction system. The reaction is maintained at a constant pressure for 6-8 h; 4) The reaction process is monitored by high-performance liquid chromatography every 1-1.5 h by using a micro-sampler, diluting and filtering with n-hexane and isopropyl alcohol at a volume ratio of 1:1. When the content of sn-2 monoglyceride reaches the peak value, the reaction is terminated.

[0009] Preferably, after the alcoholysis reaction is completed, the contents of the reaction kettle are cooled to 50°C to maintain fluidity, and the reaction slurry is slowly pumped into a high-strength magnetic separator through the bottom outlet of the reaction kettle. Under the action of the magnetic field, the magnetic immobilized enzyme is captured in the separator, and the reaction liquid is separated and flows out. The recovered catalyst is washed 2-3 times with anhydrous acetone to wash away the adsorbed oil and glycerol on the surface, and is stored at low temperature under nitrogen protection; The three-phase mixture of oil, glycerol and water flowing out from the magnetic separator is transferred to a high-speed tubular centrifuge, and centrifuged at a speed of 10,000 rpm for 10-15 min. After separation, the upper oil phase is obtained, which contains sn-2 MP, unreacted triglyceride and diglyceride; the middle water phase is the phosphoric acid buffer; and the bottom layer is the glycerol phase. The upper oil phase is collected, and the steps for purifying the sn-2 MP intermediate include: 1) The feeding temperature of the short-path molecular distillation device is controlled at 90°C, the distillation temperature is 140°C, the condensation temperature is 5°C, the feeding speed is 1.5 mL / min, and the first-stage molecular distillation is used to remove residual water, free glycerol and most of the free fatty acids under a system pressure of less than 0.005 kPa; 2) The remaining heavy components are subjected to secondary molecular distillation treatment in the device, the feeding temperature is controlled at 110°C, the distillation temperature is 180-200°C, the condensation temperature is 40°C, the feeding speed is 1.0 mL / min, and the distillate I containing the target product sn-2 MP is collected after the system pressure is less than 0.0005 kPa, and the unreacted glycerol is collected after the distillate I; 3) Dissolve the diglyceride of fraction I in acetone with a feed liquid ratio of 1:3, cool and slowly stir, the solubility of target product sn-2 MP tends to acetone, at the same time, the diglyceride gradually crystallizes out, and high-purity sn-2 MP is obtained after filtration; 4) Qualitative and quantitative analysis of the final product is performed using a chiral column HPLC, to ensure that the percentage of sn-2 palmitic acid in the total palmitic acid is greater than 85% and the total sn-2 MP content is less than 90%, and the high-purity sn-2 MP intermediate is stored at low temperature under nitrogen protection and away from light.

[0010] Preferably, in step S2, the high-purity sn-2 MP intermediate is used as a reaction substrate together with mixed fatty acids including oleic acid, linoleic acid and ARA algal oil, and a magnetic nano-immobilized lipase is used as a catalyst, wherein the complex structured fat synthesis step includes: 1) Put the sn-2 MP intermediate into the reactor, heat to 70°C to completely melt, and add mixed fatty acids with a molar ratio of 1:(2.2~2.5) to the sn-2 MP under nitrogen protection, stir at 200 rpm and start the high-shear disperser for a short time, stir at 5000 rpm for 1-2 min, and ensure that the liquid fatty acid and sn-2 MP are fully homogenized; 2) Control the temperature at 65°C, and add 6%~10% of the total reaction mass of magnetic immobilized lipase and 20% of the total reaction mass of activated molecular sieve, and perform acidolysis reaction at a stirring speed of 250~350 rpm for 4~8 h; 3) Continue to pass nitrogen protection, use online near-infrared NIR spectrometer combined with partial least squares model to monitor the intensity change of OPO and OPL characteristic peaks in the reaction system in real time, take samples every 1 h, and use HPLC-APCI-MS offline analysis; 4) When the NIR model prediction or HPLC analysis shows that the ratio of OPO to OPL in the product reaches the set target range 1:(1.1~1.3) and the total triglyceride content no longer increases significantly, it is determined that the acidolysis reaction reaches the end point; After the acidolysis reaction is completed, the reactor is cooled to 50°C, the reaction slurry is pumped into a high-strength magnetic separator, the magnetic nano-immobilized lipase is separated and captured, the remaining oil mixture is discharged, the recovered catalyst is washed with n-hexane for 2~3 times, the adsorbate is removed, and vacuum drying is performed.

[0011] Preferably, the discharged oil mixture is subjected to pre-deacidification treatment by short-path molecular distillation, the feeding temperature is 80°C, the distillation temperature is 160°C, the condensation temperature is 10°C, and the unreacted excess mixed fatty acid is removed under a system pressure of less than 0.002 kPa, and the remaining fatty acid is recovered and purified; The steps for purifying the target product complex structured fat include: 1) With the feed temperature of 100℃, the distillation temperature of 190 ~210℃, the condensation temperature of 45℃, the feed speed of 1.0mL / min, the trace pigments, oxides and reaction byproducts are removed by using a two-stage short path molecular distillation device under the system pressure less than 0.0005k Pa, and the high-purity composite structure fat is collected on the condenser; 2) The sn-2 position fatty acid molecular composition is analyzed by using 13C-NMR to confirm the positioning success rate of palmitic acid and ARA, the specific content and proportion of OPO, OPL and other MLCT in the final product are quantitatively analyzed by using HPLC-APCI-MS, and after nitrogen filling, the product is stored in the dark and refrigerated to be used for the infant formula milk powder product deployment.

[0012] (Three) Beneficial effects Compared with the prior art, the present application provides an enzyme method for preparing special oil for infant formula milk powder, which has the following beneficial effects: 1. The enzyme method for preparing special oil for infant formula milk powder, by preparing pH-responsive magnetic nanometer immobilized lipase, realizes controllable enzyme activity, recovers the enzyme by using magnetic separation and pH-induced aggregation double mechanism, and after washing and drying with anhydrous acetone, the enzyme can be reused, thereby reducing the enzyme cost.

[0013] 2. The enzyme method for preparing special oil for infant formula milk powder, by synchronously synthesizing retinol palmitate functional ester, preparing vitamin A active carrier by pH-responsive catalysis, solving the problem of uneven dispersion of fat-soluble vitamins, constructing fat-soluble vitamin-structure fat-essential fatty acid composite synergistic system, retinol palmitate promoting ARA transport and absorption, realizing the trinity function of energy supply-mineral absorption-micro-nutrient supplementation.

[0014] 3. The enzyme method for preparing special oil for infant formula milk powder, the sn-2 MP intermediate is purified by using two-stage short path molecular distillation, the water and free glycerol are removed by primary distillation, the glycerol diester is separated by secondary distillation, combined with acetone recrystallization, and the online near-infrared spectrum combined with partial least squares method model is introduced to monitor the characteristic peak intensity of the composite structure fat OPO / OPL in real time, and the chiral column HPLC qualitative and quantitative analysis and HPLC-APCI-MS offline analysis are matched to ensure the purity of the target product. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 It is a pH response catalytic system flow chart of the special oil for infant formula milk powder of the present application; Figure 2 It is a sn-2 MP preparation flow chart of the special oil for infant formula milk powder of the present application; Figure 3The flow chart for synthesizing the complex structured lipids in the oil special for the infant formula milk powder of the present application is shown in the following; Figure 4 The flow chart for product blending and post-treatment of the oil special for the infant formula milk powder of the present application is shown in the following. DETAILED DESCRIPTION

[0016] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the embodiments of the present application and the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all the other embodiments obtained by those skilled in the art without creative work fall within the scope of the present application.

[0017] Embodiment one In the present embodiment, before the alcoholysis reaction of the step S1 magnetically immobilized lipase, the palm stearin raw material oil needs to be sequentially treated by degumming, bleaching, deacidification and deodorization. The specific steps include: 1. Enzymatic degumming 1) The palm stearin is heated to 50-55℃, and an appropriate amount of water and citric acid are added to adjust the pH to 5.0-5.5, so as to ensure that the activity is optimal after adding the enzyme under the condition. 2) A complex enzyme system of phospholipase A1 and phospholipase C is prepared with a mass ratio of (4-5):1. 3) The complex enzyme system is added to the palm stearin, and stirred at 200-300 rpm for 4-6 h. The phospholipase A1 hydrolyzes the sn-1 position of hydrated and non-hydrated phospholipids, and the phospholipase C cuts off the polar head of the phospholipid to generate diglycerides and remain in the oil.

[0018] 2. Adsorption bleaching 1) A small amount of food-grade silica gel and activated carbon is added to the activated clay to establish a composite adsorption system of activated clay-silica gel-activated carbon, so as to realize the adsorption of bleaching and phospholipid residues. 2) Based on the mass of the palm stearin, 0.3%-1.0% activated clay, 0.1%-0.3% food-grade silica gel and 0.05%-0.2% activated carbon are added, and the total addition amount of the composite adsorption system is controlled within 1.0%. 3) The palm stearin is bleached at 95-105℃ and under a vacuum environment of -0.085 MPa to reduce thermal oxidation.

[0019] 3. Distillation deacidification and deodorization The bleached raw material oil is introduced into a short-path molecular distillation device, and most of the free fatty acids, part of the odor substances and volatile oxides are removed in advance under the condition of 160-190℃ and a vacuum degree of less than 1 Pa by utilizing the difference in the average free path of free fatty acids FFA and triglycerides.

[0020] Example Two In this embodiment, a low-temperature ultra-high-pressure nanohomogenizer is used to break the material based on the mechanical effects of high-frequency shear force, impact and cavitation effect generated by the rotor and stator, which is not conducive to the use of ultra-high pressure to promote the material to pass through the slit for nanometerization to protect the heat-sensitive ingredients. The specific operation steps include: 1) Take 30-40 parts of functional oil as the oil phase; take 40-50 parts of lactose, 10-20 parts of whey protein and 0.5-4 parts of skim milk powder as the water phase, and add water to prepare a wall material solution with a solid content of 20-30%; and take an appropriate amount of compound emulsifier and emulsification auxiliary agent as the emulsifier; 2) Slowly add the oil phase to the water phase wall material solution under stirring, stir at 50-60°C for 15-20 min, and preliminarily mix and emulsify; 3) Connect a cooling circulation system to maintain the low-temperature state of the material, start the homogenizer at a low speed first to break it up preliminarily, then switch to high-speed shearing at 15,000-25,000 rpm, and process intermittently for 10-60 s; 4) The homogenized emulsion is transported to the top of the drying tower at a flow rate of 30-50 L / h by a high-pressure feed pump, converted into micron-sized fine droplets by a centrifugal atomizing disc, and the target compound oil is wrapped inside by the powder wall material formed by the co-current contact of hot air with the atomized droplets in the tower, with the inlet air temperature being 180-200°C and the outlet air temperature being 80-95°C; 5) The powder enters the vibration fluidized bed and is supplied with dehumidified air at 30-50°C for secondary drying and cooling. After drying, the powder is collected from the bottom of the drying tower and the outlet of the fluidized bed, sieved by a vibrating screen, and then subjected to microencapsulation treatment by nitrogen sealing.

[0021] Example Three In this embodiment, a low-temperature soft tower deodorization process is used to achieve mild deodorization of the product, including: 1) The deacidified oil enters the deodorization tower, a small amount of steam is directly introduced at 210-230°C, the vacuum degree is controlled at 0.1-0.5 kPa, the structured packing provides a large specific surface area to ensure maximum stripping efficiency, and low-temperature short-time operation is realized; 2) After stripping for 20-40 min, the remaining free fatty acids and volatile odor molecules are removed, and the heat-sensitive ARA and vitamin substances are maximally retained; 3) A three-stage steam jet vacuum system is deployed in the middle of the deodorization tower, and the absolute pressure is maintained at less than 3 mbar. The preheating of the raw material oil by the hot oil after deodorization is used to build an integrated energy-saving heat exchange circulation network; The addition of natural antioxidants, including mixed tocopherols or ascorbyl palmitate, during the cooling stage of the refined oil, and immediate nitrogen flushing, ensures the oxidative stability of the oil product during storage.

[0022] The beneficial effects of the present application are: the enzyme method for preparing special oil for infant formula milk powder, using phospholipase A1 and phospholipase C complex system, realizing enzymatic degumming of palm stearin raw material oil, directional hydrolysis and hydration of sn-1 ester bond and polar head of non-hydrated phospholipids, generating glycerol diester retained in the oil phase, reducing the inhibition of phospholipids on subsequent enzyme catalysis; constructing active white clay-silica-activated carbon composite adsorption system to complete the decolorization of raw material oil under vacuum conditions, synchronously adsorbing residual phospholipids and pigments, avoiding high temperature oxidation; through short path molecular distillation pretreatment, using the difference in average free path of free fatty acids and triglycerides, pre-removing free fatty acids and volatile odor substances, improving the purity of the raw material; The pH-responsive magnetic nano-immobilized lipase is prepared, the enzyme activity is controllable, the enzyme is recovered by magnetic separation and pH-induced aggregation dual mechanism, and after being washed and dried by anhydrous acetone, the enzyme can be repeatedly used, the enzyme cost is reduced, the toxic chloroform or dichloromethane is replaced by green hydrophobic solvent tert-butyl alcohol, 3Å molecular sieve is used for dehydration, a food-grade reaction system is constructed, and food safety standards are met; The functional ester retinol palmitate is synthesized synchronously, the vitamin A active carrier is prepared by pH-responsive catalysis, the problem of uneven dispersion of fat-soluble vitamins is solved, a composite structure lipid system is constructed, a synergistic system of fat-soluble vitamins-structure lipids-essential fatty acids is formed, retinol palmitate promotes the transport and absorption of ARA, and the trinity function of energy supply-mineral absorption-micro-nutrient supplementation is realized. The sn-2 MP intermediate is purified by two-stage short path molecular distillation, water and free glycerol are removed by first-stage distillation, and diglyceride is separated by second-stage distillation, combined with acetone recrystallization, and online near-infrared spectroscopy combined with partial least squares model is introduced to monitor the characteristic peak intensity of the composite structure lipid OPO / OPL in real time, and qualitative and quantitative analysis is performed by combining chiral column HPLC and HPLC-APCI-MS offline analysis, so as to ensure the purity of the target product.

[0023] Although embodiments of the present application have been shown and described, it is to be understood that various modifications, substitutions, replacements and changes can be made to these embodiments without departing from the principles and spirit of the present application, and the scope of the present application is defined by the appended claims and their equivalents.

Claims

1. A method for enzymatically preparing an oil for infant formula, characterized in that, Comprise the following steps: S1. With palm stearin rich in palmitic acid as raw oil, realize degumming, deacidification and deodorization refining treatment, construct pH responsive interface biocatalysis system to synthesize and enrich functional ester; S2. Based on alcoholysis reaction to prepare sn-2 palmitic acid glycerol diester intermediates, sn-2 palmitic acid glycerol diester intermediates and mixed fatty acids are subjected to acidolysis esterification reaction to accurately synthesize composite structure fat; S3. The low-temperature high-speed shearing homogenization technology is used for the functional ester, the composite structure fat and the fat-soluble vitamin prepared and synthesized to ensure uniform dispersion of the components, and finally low-temperature soft tower deodorization and antioxidant treatment is carried out.

2. The method for preparing oil for infant formula by enzymatic method according to claim 1, characterized in that, In step S1, the solid palm stearin obtained by low-temperature fractionation of palm oil is used as raw oil, and a composite phospholipase preparation system is used to remove phospholipids. Then, an active clay-silica-activated carbon composite adsorption system is used for adsorption and decolorization of the raw oil. Most of the free fatty acids are removed by molecular distillation method in advance, and then the remaining free fatty acids and odor molecules are removed in a structured packing tower to realize physical refining of palm stearin; The purity of retinol (all-trans vitamin A alcohol) is greater than 95%; the purity of palmitic acid is greater than 98% and it is molecularly distilled to reduce the inhibition of free fatty acid impurities on the enzyme catalyst; The Candida antarctica lipase B (CALB) is modified and fixed on the pH responsive group of the magnetic nanometer microspheres to prepare a pH responsive immobilized lipase as a catalyst; tert-butyl alcohol is used as a green hydrophobic solvent, and a 3Å type molecular sieve is used to construct a pH responsive interface biocatalysis system; The specific steps for preparing functional ester by esterification synthesis reaction are as follows: 1) In a jacketed glass reaction kettle, add retinol and palmitic acid as reaction substrates in a molar ratio of 1:(1.2~1.5), add tert-butyl alcohol in a solvent to reaction substrate volume to mass ratio of 3:1, heat to 50~60℃ by circulating water bath, continuously pass nitrogen gas to protect the substrate, and ensure that the substrate is fully dissolved and prevent oxidation of retinol; 2) Adjust the pH value of the reaction system to 6.5~7.5 by adding 0.1M dilute NaOH solution or dilute HCl solution in the reaction kettle system, add 3%~5% of the total substrate mass of pH responsive immobilized lipase, and add 10%~20% of the total mass of the reaction system of molecular sieve activated at high temperature; 3) Under the condition of 50~60℃, uniformly stir at 200~300rpm for 8~12h to ensure that the catalyst is uniformly suspended and not broken by shear force, take samples every 2h, and monitor the generation of retinyl palmitate and the residual amount of retinol by high performance liquid chromatography (HPLC).

3. The method for preparing oil for infant formula by enzymatic method according to claim 2, characterized in that, After the esterification synthesis reaction is completed, the pH value of the reaction liquid is adjusted to less than 5.0 to change the charge on the surface of the immobilized enzyme carrier from a highly dispersed state to an aggregated and precipitated state; under the dual action of magnetic separation and pH-induced aggregation, the catalyst is separated and recovered from the reaction liquid, and then washed with anhydrous solvent and dried. The reaction liquid after separation of the catalyst is distilled under reduced pressure at 50℃ and a vacuum degree of-0.095MPa, most of the solvent is recovered, and then the remaining concentrated liquid is washed with warm water at 50℃ to remove residual palmitic acid and catalyst. After standing and separating, the organic phase is collected to obtain the functional ester crude product. The steps for further purification and enrichment of the product include: 1) Control the feed temperature of the short path molecular distillation device at 80℃, the distillation temperature at 120℃, the condensation temperature at 5℃, and the feed speed at 1.0mL / min. Remove the light components of residual solvent, moisture and a small amount of free palmitic acid by first-stage molecular distillation under a system pressure of less than 0.01kPa; 2) Perform second-stage molecular distillation treatment on the remaining heavy components in the device, control the feed temperature at 100℃, the distillation temperature at 180~200℃, the condensation temperature at 30℃, and the feed speed at 0.5mL / min. Recover the unreacted retinol after fraction I in the condenser and obtain high-purity retinyl palmitate as the target functional lipid after fraction II in the main collector; 3) Detect the purity of the final product by HPLC-UV and analyze the molecular structure by LC-MS. Dissolve the obtained high-purity retinyl palmitate in a small amount of neutral edible vegetable oil to prepare a stock solution, seal it with nitrogen, and store it in the dark and refrigerated.

4. The method for preparing oil for infant formula by enzymatic method according to claim 1, characterized in that, The magnetic nano-immobilized lipase catalyzes the alcoholysis reaction of palm stearin and glycerol, specifically hydrolyzes the sn-1,3 ester bond, and generates an sn-2 palmitic acid glycerol diester intermediate. The refined palm stearin is used as the acyl donor, high-purity glycerol is used as the nucleophile, and the magnetic nano-immobilized lipase is used as the catalyst. The sn-2 palmitic acid glycerol diester intermediate preparation step includes: 1) Prepare the reaction substrate of palm stearin and glycerol with a molar ratio of 1:(3~4). Add palm stearin to a jacketed stainless steel reaction kettle, heat to 65~70℃ to completely melt, and add glycerol and 10%~15% of the total mass of the reaction system of phosphoric acid buffer under low-speed stirring at 100 rpm; 2) Start the high-shear dispersion emulsifier and emulsify for 3~5min at a speed of 10000~12000rpm to form a uniform water-in-oil microemulsion, which increases the interface area of the oil-water two phases to improve the enzyme catalytic efficiency; 3) Control the reaction system temperature at 60℃ and reduce the speed to 300~400rpm. Add 5%~8% of the mass of palm stearin to the magnetic nano-immobilized lipase, and add 15%~20% of the total mass of the reaction system to the high-temperature activated 3Å molecular sieve. Maintain airtight and normal pressure and react for 6~8h; 4) Take samples every 1~1.5h with a micro-sampler, dilute with n-hexane and isopropyl alcohol at a volume ratio of 1:1, filter, and monitor the reaction progress by high-performance liquid chromatography. Terminate the reaction when the sn-2 monoglyceride content reaches the peak value.

5. The method for preparing oil for infant formula by enzymatic method according to claim 1, characterized in that, After the alcoholysis reaction is completed, the reactor contents are cooled to 50℃ to maintain fluidity, and the reactor contents are slowly pumped into a high-strength magnetic separator through the bottom outlet of the reactor. Under the action of the magnetic field, the magnetic immobilized enzyme is captured in the separator, and the reaction liquid is separated and then flows out. The recovered catalyst is washed 2-3 times with anhydrous acetone to remove the adsorbed oil and glycerol on the surface, and is stored at low temperature under nitrogen protection; The three-phase mixture of oil, glycerol and water flowing out from the magnetic separator is transferred to a high-speed tubular centrifuge, and centrifuged at a speed of 10,000 rpm for 10-15 min. After separation, the upper oil phase is obtained, which contains sn-2 MP, unreacted triglycerides and diglycerides; the middle water phase is a phosphoric acid buffer; and the bottom layer is a glycerol phase; The upper oil phase is collected, and the steps for purifying the sn-2 MP intermediate include: 1) The feed temperature of the short-path molecular distillation device is controlled at 90℃, the distillation temperature is 140℃, the condensation temperature is 5℃, the feed speed is 1.5 mL / min, and the first-stage molecular distillation is used to remove residual water, free glycerol and most of the free fatty acids under a system pressure of less than 0.005 kPa; 2) The remaining heavy components are subjected to secondary molecular distillation treatment in the device, the feed temperature is controlled at 110℃, the distillation temperature is 180-200℃, the condensation temperature is 40℃, the feed speed is 1.0 mL / min, and the fraction I is collected after the system pressure is less than 0.0005 kPa to obtain the diglyceride containing the target product sn-2 MP, and the unreacted glycerol is collected after the fraction I; 3) The diglyceride of the fraction I is dissolved in acetone at a material-liquid ratio of 1:3, cooled and slowly stirred, the solubility of the target product sn-2 MP tends to be in acetone, and the diglyceride is gradually precipitated in the form of crystals, and high-purity sn-2 MP is obtained after filtration; 4) The final product is qualitatively and quantitatively analyzed by using a chiral column HPLC to ensure that the percentage of sn-2 palmitic acid in the total palmitic acid is greater than 85% and the total sn-2 MP content is less than 90%, and the high-purity sn-2 MP intermediate is stored at low temperature under nitrogen protection and in the dark.

6. The method for preparing oil for infant formula by enzymatic method according to claim 1, characterized in that, In step S2, the high-purity sn-2 MP intermediate and mixed fatty acids are used as reaction substrates, the mixed fatty acids include oleic acid, linoleic acid and ARA algal oil, and the magnetic nano-immobilized lipase is used as a catalyst. The complex structured lipid synthesis step includes: 1) The sn-2 MP intermediate is put into the reactor, heated to 70℃ to completely melt, and the mixed fatty acids with a molar ratio of 1:(2.2-2.5) to the sn-2 MP are added under nitrogen protection, stirred at 200 rpm and started for a short time to disperse the high shear disperser, and stirred at 5,000 rpm for 1-2 min to ensure that the liquid fatty acid and the sn-2 MP are fully homogenized; 2) The temperature is controlled at 65℃, the magnetic immobilized lipase with a total reaction material quality of 6%-10% and the activated molecular sieve with a total reaction material quality of 20% are put in, the acidolysis reaction is carried out at a stirring speed of 250-350 rpm, and the reaction is continued for 4-8 h; 3) Continuous nitrogen protection, using online near-infrared NIR spectrometer combined with partial least squares model, real-time monitoring of the intensity of OPO and OPL characteristic peak in the reaction system, every 1 h sampling, using HPLC-APCI-MS offline analysis; 4) When the NIR model predicts or HPLC analysis shows that the ratio of OPO to OPL in the product reaches the set target range 1:(1.1~1.3), and the total glycerol content no longer increases significantly, the acidolysis reaction is determined to be completed; After the acidolysis reaction is completed, the reactor is cooled to 50℃, the reaction slurry is pumped into a high-strength magnetic separator, the magnetic nano-immobilized lipase is separated and captured, and the remaining oil mixture is discharged. The recovered catalyst is washed with n-hexane 2~3 times to remove the adsorbate and vacuum dried.

7. The method for preparing oil for infant formula by enzymatic method according to claim 6, characterized in that, The discharged oil mixture is treated by short path molecular distillation to remove unreacted excess mixed fatty acids at a feed temperature of 80℃, a distillation temperature of 160℃, and a condensation temperature of 10℃, and a system pressure of less than 0.002 kPa. The remaining fatty acids are recovered and purified; The steps for purifying the target product complex structured fat include: 1) Using a two-stage short path molecular distillation device to remove trace amounts of pigments, oxides and reaction byproducts at a feed temperature of 100℃, a distillation temperature of 190~210℃, a condensation temperature of 45℃, a feed rate of 1.0 mL / min, and a system pressure of less than 0.0005 kPa. High-purity complex structured fat is collected on the condenser; 2) Using 13C-NMR to analyze the sn-2 fatty acid molecular composition, confirming the success rate of palmitic acid and ARA positioning, using HPLC-APCI-MS to quantitatively analyze the specific content and ratio of OPO, OPL and other MLCT in the final product, and storing in a nitrogen-filled, light-protected refrigerator after freezing for use in infant formula product formulation.

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