EPA-DHA fish oil ethyl esterification method

Abstract

The invention discloses an EPA-DHA fish oil ethyl esterification method. Fish oil, absolute ethyl alcohol and a metal modified zirconium sulfate catalyst are mixed, light-phase ethyl ester is separated out and ethyl alcohol is recycled after stirring and standing layering, and then pure ethyl ester type fish oil is obtained through washing. The method is suitable for most fish oil with high acid value. Compared with the prior art, by accurately controlling the reaction conditions and adopting the efficient metal modified zirconium sulfate solid catalyst, the relative content of the EPA-DHA ethyl ester type fish oil can be remarkably increased to a certain extent, and the nutritional value of the product is enhanced; secondly, through effective separation and recovery steps, the purity and quality of the product are ensured, and the generation of byproducts is reduced; in addition, due to the recycling of the catalyst, the production cost is reduced, the environmental pollution is reduced, and the sustainability of the whole process is improved.

Description

Technical Field

[0001] The invention relates to the technical field of edible fish oil processing, in particular to a method for ethyl esterification of EPA-DHA fish oil. Background Art

[0002] Fish oil is composed of triglycerides, phospholipids, free fatty acids, vitamins and other ingredients. Triglycerides are the main component of fish oil and usually account for the majority of fish oil ingredients. In high-quality fish oil, the content of triglyceride-type Omega-3 fatty acids (mainly EPA and DHA) is relatively high. These triglycerides are composed of a glycerol molecule and three fatty acid molecules, the fatty acids of which may include saturated fatty acids, monounsaturated fatty acids and polyunsaturated fatty acids, and polyunsaturated fatty acids such as EPA and DHA are the key components for fish oil to exert many health benefits. Fish oil also contains a certain amount of free fatty acids, and its content varies depending on the quality of the fish oil, the processing process and other factors. The presence of free fatty acids will affect the acid value of fish oil. Excessive free fatty acids may cause the fish oil to become rancid and reduce the quality of the fish oil.

[0003] Generally speaking, triglycerides account for about 50%-90% of fish oil. However, this ratio varies depending on factors such as the source of the fish oil (different types of fish), extraction and processing methods.

[0004] CN102994236A discloses a method for preparing fatty acid ethyl esters with an Omega-3 content greater than 90%. By molecular distillation, transesterification, urea inclusion technology and activated clay adsorption, fatty acid ethyl esters with an Omega-3 content greater than 90% are extracted from fish oil. This method not only fills the domestic production technology gap, but also ensures that the product meets the European and American pharmacopoeia standards, is suitable for the fields of nutritional supplements, medicines, foods and additives, and has significant economic value. It is recorded that pretreatment is required before the fish oil is transesterified, and the acid value of the fish oil is controlled to be below 0.5mgKOH / g by molecular distillation means, which is not suitable for large-scale industrial production.

[0005] CN115109654A discloses a production process for ethyl esterification of edible fish oil. By using a mixture of phosphoric acid and glycerol instead of sulfuric acid as a catalyst, the difficulty in utilizing high-acid-value fish oil and the hidden danger of food safety are solved, while expanding the application of fish oil resources and reducing production costs. The method simplifies the operating steps, improves the efficiency of ethyl esterification, and the ethanol solvent can be recycled and utilized, which is easy to promote. However, the process is complicated and the processing difficulty is high. Summary of the invention

[0006] In order to solve the above technical problems, the present invention provides a method for ethyl esterification of EPA-DHA fish oil, wherein the fish oil is mixed with anhydrous ethanol and a metal-modified zirconium sulfate catalyst, and after stirring and standing for stratification, the light phase ethyl ester is separated and the ethanol is recovered, and then the pure ethyl ester fish oil is obtained by water washing. In this process, the catalyst can be filtered, recovered and reused. And it is suitable for most fish oils with high acid value.

[0007] The specific technical solutions are as follows:

[0008] A method for ethyl esterification of EPA-DHA fish oil comprises the following steps:

[0009] 1kg fish oil, 1-3kg anhydrous ethanol, and 40-100g metal-modified zirconium sulfate solid catalyst are mixed evenly and stirred at a temperature of 50-90°C for 2-6h; the mixed solution is allowed to stand for 1-4h to separate the layers, and the upper layer of liquid is separated, which is the light phase ethyl ester; the lower layer of liquid is separated and filtered, and the filtrate is the heavy phase glycerin, and the filter residue is the catalyst; the upper layer of liquid (light phase ethyl ester) is evaporated under reduced pressure to recover anhydrous ethanol; after removing the ethanol, distilled water is added for washing, and the oil layer is separated to obtain ethyl ester fish oil. The filter residue is the catalyst, which can be recovered after drying and reused repeatedly.

[0010] The reduced pressure evaporation is reduced pressure evaporation at 0.08-0.09 MPa and 40-60°C.

[0011] Preferably, the metal-modified zirconium sulfate solid catalyst is any one of a zinc-modified zirconium sulfate solid catalyst, a cerium-modified zirconium sulfate solid catalyst, a cobalt-modified zirconium sulfate solid catalyst, a tin-modified zirconium sulfate solid catalyst, and a zinc / tin-modified zirconium sulfate solid catalyst.

[0012] Preferably, the preparation method of the zinc-modified zirconium sulfate solid catalyst is: dissolving 0.1 mol of zirconium oxychloride octahydrate and 0.005-0.02 mol of zinc nitrate hexahydrate in 100-500 mL of deionized water; adding 10-30 wt% concentrated ammonia water dropwise under stirring until the pH value of the solution is 10; standing at room temperature for 6-18 hours; filtering, taking the precipitate and repeatedly washing it with deionized water until the washing liquid does not contain chloride ions; drying and crushing the obtained precipitate; then soaking it in a 0.1-1 mol / L sulfuric acid aqueous solution for 2-8 hours; after filtering, calcining the filter cake at 400-650° C. for 1-6 hours to obtain a zinc-modified zirconium sulfate catalyst.

[0013] Preferably, the preparation method of the cerium-modified zirconium sulfate solid catalyst is: dissolving 0.1 mol of zirconium oxychloride octahydrate and 0.005-0.02 mol of cerium (III) nitrate hexahydrate in 100-500 mL of deionized water; adding 10-30 wt% concentrated ammonia water dropwise under stirring until the pH value of the solution is 10; standing at room temperature for 6-18 hours; filtering, taking the precipitate and repeatedly washing it with deionized water until the washing liquid does not contain chloride ions; drying and crushing the obtained precipitate; then soaking it in a 0.1-1 mol / L sulfuric acid aqueous solution for 2-8 hours; after filtering, calcining the filter cake at 400-650° C. for 1-6 hours to obtain a cerium-modified zirconium sulfate catalyst.

[0014] Preferably, the preparation method of the cobalt-modified zirconium sulfate solid catalyst is: dissolving 0.1 mol of zirconium oxychloride octahydrate and 0.005-0.02 mol of cobalt nitrate hexahydrate in 100-500 mL of deionized water; adding 10-30 wt% concentrated ammonia water dropwise under stirring until the pH value of the solution is 10; standing at room temperature for 6-18 hours; filtering, taking the precipitate and repeatedly washing it with deionized water until the washing liquid does not contain chloride ions; drying and crushing the obtained precipitate; then soaking it in a 0.1-1 mol / L sulfuric acid aqueous solution for 2-8 hours; after filtering, calcining the filter cake at 400-650° C. for 1-6 hours to obtain a cobalt-modified zirconium sulfate catalyst.

[0015] Preferably, the preparation method of the tin-modified zirconium sulfate solid catalyst is: dissolving 0.1 mol of zirconium oxychloride octahydrate and 0.005-0.02 mol of tin chloride in 100-500 mL of deionized water; adding 10-30 wt% concentrated ammonia water dropwise under stirring until the pH value of the solution is 10; standing at room temperature for 6-18 hours; filtering, taking the precipitate and repeatedly washing the precipitate with deionized water until the washing liquid does not contain chloride ions; drying and crushing the obtained precipitate; then soaking in a 0.1-1 mol / L sulfuric acid aqueous solution for 2-8 hours; after filtering, calcining the filter cake at 400-650° C. for 1-6 hours to obtain a tin-modified zirconium sulfate catalyst.

[0016] Preferably, the preparation method of the zinc / tin modified zirconium sulfate solid catalyst is: dissolving 0.1 mol of zirconium oxychloride octahydrate, 0.003-0.01 mol of tin chloride, and 0.003-0.01 mol of zinc nitrate hexahydrate in 100-500 mL of deionized water; adding 10-30 wt% concentrated ammonia water dropwise under stirring until the pH value of the solution is 10; standing at room temperature for 6-18 hours; filtering, taking the precipitate and repeatedly washing the precipitate with deionized water until the washing liquid does not contain chloride ions; drying and crushing the obtained precipitate; then soaking it in a 0.1-1 mol / L sulfuric acid aqueous solution for 2-8 hours; after filtering, calcining the filter cake at 400-650° C. for 1-6 hours to obtain a tin-modified zirconium sulfate catalyst.

[0017] Beneficial effects of the present invention:

[0018] A method for ethyl esterification of EPA-DHA fish oil. The fish oil is mixed with anhydrous ethanol and a metal-modified zirconium sulfate catalyst, stirred, and allowed to stand for stratification, and the light phase ethyl ester is separated and the ethanol is recovered, and then the pure ethyl ester fish oil is obtained by washing with water; it is suitable for most fish oils with high acid values. Compared with the prior art, by precisely controlling the reaction conditions and using an efficient metal-modified zirconium sulfate solid catalyst, the relative content of EPA-DHA ethyl ester fish oil can be significantly increased to a certain extent, and the nutritional value of the product can be enhanced; secondly, the method ensures the purity and quality of the product through effective separation and recovery steps, and reduces the generation of by-products; in addition, the recycling and reuse of the catalyst not only reduces the production cost, but also reduces environmental pollution, and improves the sustainability of the entire process. DETAILED DESCRIPTION

[0019] The test of the acid value of fish oil in the embodiment was carried out with reference to the method specified in the national food safety standard GB 5009.229-2016 Determination of acid value in food.

[0020] The ethyl esterification conversion rate of the fish oil in the embodiment is calculated by detecting the change in triglyceride content, and the formula is as follows: (1-triglyceride content in the fish oil raw material / triglyceride content in the fish oil raw material)×100%.

[0021] The triglyceride content test was carried out with reference to the national standard GBT44614-2024 Grain and Oil Inspection Determination of triglyceride composition in vegetable oils by high performance liquid chromatography.

[0022] Example 1

[0023] A method for ethyl esterification of EPA-DHA fish oil comprises the following steps:

[0024] 1 kg of fish oil, 2 kg of anhydrous ethanol, and 50 g of zinc-modified zirconium sulfate solid catalyst were added to a reaction tank and stirred at 60°C for 5 hours to carry out ethyl esterification reaction; the initial acid value of the fish oil was 10.3 mg KOH / g;

[0025] The mixed solution was allowed to stand for 2 hours to separate into layers, and the upper layer of liquid was separated, which was the light phase of ethyl ester; the lower layer of liquid was separated and filtered, and the filtrate was the heavy phase of glycerin, and the filter residue was the catalyst;

[0026] The upper liquid (light phase ethyl ester) was evaporated under reduced pressure at 0.09 MPa and 45° C. to recover anhydrous ethanol; after removing the ethanol, 1 kg of distilled water was added for washing, and the oil layer was separated to obtain ethyl ester fish oil.

[0027] After testing, the acid value of ethyl ester fish oil was 0.7 mg KOH / g and the ethyl esterification rate was 97.9%.

[0028] The preparation method of the zinc-modified zirconium sulfate solid catalyst is as follows: dissolving 32.22g of zirconium oxychloride octahydrate and 2.97g of zinc nitrate hexahydrate in 250mL of deionized water; adding 24wt% concentrated ammonia water dropwise under stirring until the pH value of the solution is 10; standing at room temperature for 12 hours to promote the formation of precipitation; filtering with 400-mesh filter paper, taking the precipitate and repeatedly washing it with deionized water until the washing liquid does not contain chloride ions; drying the obtained precipitate at 105°C for 24 hours and then crushing it to 200 mesh; then soaking it in a 0.5mol / L sulfuric acid aqueous solution at a mass ratio of 1:10 for 6 hours; filtering with 400-mesh filter paper, calcining the filter cake at 600°C for 3 hours to obtain the zinc-modified zirconium sulfate catalyst.

[0029] When the acid value in the fish oil raw material is too high, alkaline substances such as sodium ethoxide are generally not used as catalysts, but acids are used as catalysts. Concentrated sulfuric acid is commonly used at present, which can provide H and absorb water produced by the ethylation reaction. However, edible fish oil is food grade, and sulfuric acid has not yet entered the food additive catalog of oil and fat technology, and is prone to residue, and its safety is unknown. The metal-modified zirconium sulfate solid catalyst of the present invention is easy to separate, has no residue, and has high safety.

[0030] Comparative Example 1

[0031] A method for ethyl esterification of EPA-DHA fish oil comprises the following steps:

[0032] 1 kg of fish oil, 2 kg of anhydrous ethanol, and 8 g of sodium ethoxide were added to a reaction tank and stirred at 60°C for 5 hours to perform an ethyl esterification reaction; the initial acid value of the fish oil was 10.3 mg KOH / g;

[0033] The mixed solution was allowed to stand for 2 hours to separate the layers. The upper layer of liquid was the light phase of ethyl ester, and the lower layer of liquid was the heavy phase of glycerol.

[0034] The upper liquid (light phase ethyl ester) was evaporated under reduced pressure at 0.09 MPa and 45° C. to recover anhydrous ethanol; after removing the ethanol, 1 kg of distilled water was added for washing, and the oil layer was separated to obtain ethyl ester fish oil.

[0035] After testing, the acid value of ethyl ester fish oil was 4.6 mg KOH / g and the ethyl esterification rate was 83.1%.

[0036] Normally, the acid value in fish oil raw materials is high, and the catalyst sodium ethoxide will undergo saponification reaction with free fatty acids, affecting the esterification process, and easily causing difficulty in separating fish oil ethyl esters and soaps, which greatly reduces the reaction yield. Using sodium ethoxide as a catalyst requires too high quality of fish oil raw materials. Increasing the use of the catalyst sodium ethoxide will bring a series of problems. For example, sodium ethoxide easily absorbs moisture and carbon dioxide in the air and deteriorates, interfering with the ethyl esterification reaction and affecting the final catalytic effect. The strong alkalinity of sodium ethoxide may cause side reactions such as isomerization of unsaturated fatty acids in fish oil.

[0037] Comparative Example 1 uses sodium ethoxide as a catalyst. The strong alkalinity of sodium ethoxide will cause saponification reaction with free fatty acids, affecting the normal progress of the esterification reaction. This saponification reaction consumes part of the raw materials used for ethyl esterification, and at the same time generates impurities such as fatty acid salts, making the ethyl esterification reaction incomplete. The metal element (zinc, cobalt, cerium, tin) modified zirconium sulfate solid catalyst in Examples 1-5 can effectively promote the ethyl esterification reaction, reduce the residual free fatty acids, and thus make the acid value of ethyl ester type fish oil lower.

[0038] Example 2

[0039] A method for ethyl esterification of EPA-DHA fish oil comprises the following steps:

[0040] 1 kg of fish oil, 2 kg of anhydrous ethanol, and 50 g of cerium-modified zirconium sulfate solid catalyst were added to a reaction tank and stirred at 60°C for 5 hours to carry out ethyl esterification reaction; the initial acid value of the fish oil was 10.3 mg KOH / g;

[0041] The mixed solution was allowed to stand for 2 hours to separate into layers, and the upper layer of liquid was separated, which was the light phase of ethyl ester; the lower layer of liquid was separated and filtered, and the filtrate was the heavy phase of glycerin, and the filter residue was the catalyst;

[0042] The upper liquid (light phase ethyl ester) was evaporated under reduced pressure at 0.09 MPa and 45° C. to recover anhydrous ethanol; after removing the ethanol, 1 kg of distilled water was added for washing, and the oil layer was separated to obtain ethyl ester fish oil.

[0043] After testing, the acid value of ethyl ester fish oil was 1.5 mg KOH / g and the ethyl esterification rate was 97.8%.

[0044] The preparation method of the cerium-modified zirconium sulfate solid catalyst is as follows: 32.2g of zirconium oxychloride octahydrate and 4.34g of cerium (III) nitrate hexahydrate are dissolved in 250mL of deionized water; under stirring, 24wt% concentrated ammonia water is added dropwise until the pH value of the solution is 10; the solution is allowed to stand at room temperature for 12 hours to promote the formation of a precipitate; the solution is filtered through a 400-mesh filter paper, and the precipitate is repeatedly washed with deionized water until the washing liquid does not contain chloride ions; the obtained precipitate is dried at 105°C for 24 hours and then crushed to 200 mesh; the precipitate is then immersed in a 0.5mol / L sulfuric acid aqueous solution at a mass ratio of 1:10 for 6 hours; the filter cake is filtered through a 400-mesh filter paper and calcined at 600°C for 3 hours to obtain a cerium-modified zirconium sulfate catalyst.

[0045] Example 3

[0046] A method for ethyl esterification of EPA-DHA fish oil comprises the following steps:

[0047] 1 kg of fish oil, 2 kg of anhydrous ethanol, and 50 g of cobalt-modified zirconium sulfate solid catalyst were added to a reaction tank and stirred at 60°C for 5 hours to carry out ethyl esterification reaction; the initial acid value of the fish oil was 10.3 mg KOH / g;

[0048] The mixed solution was allowed to stand for 2 hours to separate into layers, and the upper layer of liquid was separated, which was the light phase of ethyl ester; the lower layer of liquid was separated and filtered, and the filtrate was the heavy phase of glycerin, and the filter residue was the catalyst;

[0049] The upper liquid (light phase ethyl ester) was evaporated under reduced pressure at 0.09 MPa and 45° C. to recover anhydrous ethanol; after removing the ethanol, 1 kg of distilled water was added for washing, and the oil layer was separated to obtain ethyl ester fish oil.

[0050] After testing, the acid value of ethyl ester fish oil was 1.4 mg KOH / g and the ethyl esterification rate was 97.8%.

[0051] The preparation method of the cobalt-modified zirconium sulfate solid catalyst is as follows: 32.2g of zirconium oxychloride octahydrate and 2.91g of cobalt nitrate hexahydrate are dissolved in 250mL of deionized water; under stirring, 24wt% concentrated ammonia water is added dropwise until the pH value of the solution is 10; the solution is allowed to stand at room temperature for 12 hours to promote the formation of a precipitate; the solution is filtered through a 400-mesh filter paper, and the precipitate is repeatedly washed with deionized water until the washing liquid contains no chloride ions; the obtained precipitate is dried at 105°C for 24 hours and then crushed to 200 meshes; the precipitate is then immersed in a 0.5mol / L sulfuric acid aqueous solution at a mass ratio of 1:10 for 6 hours; the filter cake is filtered through a 400-mesh filter paper and calcined at 600°C for 3 hours to obtain a cobalt-modified zirconium sulfate catalyst.

[0052] Example 4

[0053] A method for ethyl esterification of EPA-DHA fish oil comprises the following steps:

[0054] 1 kg of fish oil, 2 kg of anhydrous ethanol, and 50 g of tin-modified zirconium sulfate solid catalyst were added to a reaction tank and stirred at 60°C for 5 hours to carry out ethyl esterification reaction; the initial acid value of the fish oil was 10.3 mg KOH / g;

[0055] The mixed solution was allowed to stand for 2 hours to separate into layers, and the upper layer of liquid was separated, which was the light phase of ethyl ester; the lower layer of liquid was separated and filtered, and the filtrate was the heavy phase of glycerin, and the filter residue was the catalyst;

[0056] The upper liquid (light phase ethyl ester) was evaporated under reduced pressure at 0.09 MPa and 45° C. to recover anhydrous ethanol; after removing the ethanol, 1 kg of distilled water was added for washing, and the oil layer was separated to obtain ethyl ester fish oil.

[0057] After testing, the acid value of ethyl ester fish oil was 0.4 mg KOH / g and the ethyl esterification rate was 98.5%.

[0058] The preparation method of the tin-modified zirconium sulfate solid catalyst is as follows: dissolving 32.2g of zirconium oxychloride octahydrate and 1.90g of tin chloride in 250mL of deionized water; adding 24wt% concentrated ammonia water dropwise under stirring until the pH value of the solution is 10; standing at room temperature for 12 hours to promote the formation of precipitation; filtering with 400-mesh filter paper, taking the precipitate and repeatedly washing it with deionized water until the washing liquid does not contain chloride ions; drying the obtained precipitate at 105°C for 24 hours and then crushing it to 200 mesh; then soaking it in a 0.5mol / L sulfuric acid aqueous solution at a mass ratio of 1:10 for 6 hours; filtering with 400-mesh filter paper and calcining the filter cake at 600°C for 3 hours to obtain the tin-modified zirconium sulfate catalyst.

[0059] Tin-modified zirconium sulfate solid catalyst may have stronger affinity or catalytic activity for free fatty acids. During the reaction, tin may more effectively promote the esterification reaction between free fatty acids and ethanol, so that free fatty acids can be more fully converted into ethyl esters, thereby lowering the acid value.

[0060] Comparative Example 2

[0061] A method for ethyl esterification of EPA-DHA fish oil comprises the following steps:

[0062] 1 kg of fish oil, 2 kg of anhydrous ethanol, and 50 g of zirconium sulfate solid catalyst were added to a reaction tank and stirred at 60°C for 5 hours to carry out ethyl esterification reaction; the initial acid value of the fish oil was 10.3 mg KOH / g;

[0063] The mixed solution was allowed to stand for 2 hours to separate into layers, and the upper layer of liquid was separated, which was the light phase of ethyl ester; the lower layer of liquid was separated and filtered, and the filtrate was the heavy phase of glycerin, and the filter residue was the catalyst;

[0064] The upper liquid (light phase ethyl ester) was evaporated under reduced pressure at 0.09 MPa and 45° C. to recover anhydrous ethanol; after removing the ethanol, 1 kg of distilled water was added for washing, and the oil layer was separated to obtain ethyl ester fish oil.

[0065] After testing, the acid value of ethyl ester fish oil was 3.0 mg KOH / g and the ethyl esterification rate was 93.4%.

[0066] The preparation method of the zirconium sulfate solid catalyst is as follows: dissolving 32.2g of zirconium oxychloride octahydrate in 250mL of deionized water; adding 24wt% concentrated ammonia water dropwise under stirring until the pH value of the solution is 10; standing at room temperature for 12 hours to promote the formation of precipitation; filtering with 400-mesh filter paper, taking the precipitation and repeatedly washing it with deionized water until the washing liquid does not contain chloride ions; drying the obtained precipitation at 105°C for 24 hours and then crushing it to 200 mesh; then soaking it in a 0.5mol / L sulfuric acid aqueous solution at a mass ratio of 1:10 for 6 hours; filtering with 400-mesh filter paper and calcining the filter cake at 600°C for 3 hours to obtain the zirconium sulfate catalyst.

[0067] The catalytic activity of the zirconium sulfate solid catalyst is inferior to that of the modified catalyst in Examples 1 to 5. The metal-modified zirconium sulfate solid catalyst in Examples 1 to 5 improves the reaction activity and selectivity by the synergistic effect of metals, so that more free fatty acids in fish oil are converted into ethyl esters, thereby reducing the acid value.

[0068] Example 5

[0069] A method for ethyl esterification of EPA-DHA fish oil comprises the following steps:

[0070] 1 kg of fish oil, 2 kg of anhydrous ethanol, and 50 g of zinc / tin modified zirconium sulfate solid catalyst were added to a reaction tank and stirred at 60°C for 5 hours to carry out ethyl esterification reaction; the initial acid value of the fish oil was 10.3 mg KOH / g;

[0071] The mixed solution was allowed to stand for 2 hours to separate into layers, and the upper layer of liquid was separated, which was the light phase of ethyl ester; the lower layer of liquid was separated and filtered, and the filtrate was the heavy phase of glycerin, and the filter residue was the catalyst;

[0072] The upper liquid (light phase ethyl ester) was evaporated under reduced pressure at 0.09 MPa and 45° C. to recover anhydrous ethanol; after removing the ethanol, 1 kg of distilled water was added for washing, and the oil layer was separated to obtain ethyl ester fish oil.

[0073] After testing, the acid value of ethyl ester fish oil was 0.1 mg KOH / g and the ethyl esterification rate was 99.5%.

[0074] The preparation method of the zinc / tin modified zirconium sulfate solid catalyst is as follows: 32.2g of zirconium oxychloride octahydrate, 0.95g of tin chloride, and 1.49g of zinc nitrate hexahydrate are dissolved in 250mL of deionized water; 24wt% concentrated ammonia water is added dropwise under stirring until the pH value of the solution is 10; the mixture is allowed to stand at room temperature for 12 hours to promote the formation of a precipitate; the mixture is filtered through a 400-mesh filter paper, and the precipitate is repeatedly washed with deionized water until the washing liquid contains no chloride ions; the obtained precipitate is dried at 105°C for 24 hours and then crushed to 200 meshes; the precipitate is then immersed in a 0.5mol / L sulfuric acid aqueous solution at a mass ratio of 1:10 for 6 hours; the filter cake is filtered through a 400-mesh filter paper and calcined at 600°C for 3 hours to obtain a zinc / tin modified zirconium sulfate catalyst.

[0075] Test Example 1

[0076] According to the national food safety standard GB 5009.227-2023, the determination of peroxide value in food, the ethyl ester fish oil obtained in the embodiment and the comparative example was tested for peroxide value after being sealed and stored at room temperature for 50 days. The test results were obtained by taking the average value of the test results of three samples for each sample.

[0077] Table 1

[0078] Peroxide value (meq / kg) Example 1 2.2 Comparative Example 1 6.4 Example 2 2.9 Example 3 3.0 Example 4 1.5 Comparative Example 2 4.8 Example 5 0.8

[0079] As can be seen from the above table, the peroxide value of the ethyl ester fish oil of Examples 1-4 after being stored for a period of time is significantly lower than that of Comparative Examples 1-2. The reason is that the metal element-modified zirconium sulfate solid catalyst has more active sites than sodium ethoxide, achieves a more efficient catalytic effect, reduces easily oxidized components such as unsaturated fatty acids, and thus achieves a lower peroxide value after long-term storage.

[0080] In comparison, the effect of using tin-modified zirconium sulfate solid catalyst as a catalyst in Example 4 is better. This is because the higher electronegativity and electronic structure of the tin element may make the tin-modified zirconium sulfate solid catalyst more effective in the ethyl esterification reaction, reducing easily oxidized components such as unsaturated fatty acids, thereby achieving a low peroxide value for ethyl ester fish oil after long-term storage.

[0081] Further comparison revealed that the synergistic effect of zinc and tin in the zinc / tin bimetallic modified zirconium sulfate solid catalyst may improve the activity and selectivity of the catalyst. This bimetallic modification may produce more effective active sites during the catalytic process, thereby improving the efficiency of the ethylation reaction and the stability of the product, resulting in the peroxide value of the ethyl ester fish oil in Example 5 after long-term storage being significantly lower than that in Examples 1-4.

[0082] The preferred specific embodiments of the present invention are described in detail above. It should be understood that a person skilled in the art can make many modifications and changes based on the concept of the present invention without creative work. Therefore, any technical solution that can be obtained by a person skilled in the art through logical analysis, reasoning or limited experiments based on the concept of the present invention on the basis of the prior art should be within the scope of protection determined by the claims.

Claims

1. A method for ethyl esterification of EPA-DHA fish oil, characterized in that: The steps include: 1kg of fish oil, 1-3kg of anhydrous ethanol and 40-100g of metal-modified zirconium sulfate solid catalyst are mixed evenly, and stirred at a temperature of 50-90°C for 2-6h; the mixed solution is allowed to stand for 1-4h to separate the layers, and the upper layer of liquid is separated, which is the light phase ethyl ester; the lower layer of liquid is separated and filtered, and the filtrate is the heavy phase glycerin, and the filter residue is the catalyst; the upper layer of liquid is evaporated under reduced pressure to recover the anhydrous ethanol; after removing the ethanol, distilled water is added for washing, and the oil layer is separated to obtain ethyl ester fish oil.

2. The method for ethyl esterification of EPA-DHA fish oil according to claim 1, wherein: The reduced pressure evaporation is reduced pressure evaporation at 0.08-0.09 MPa and 40-60°C.

3. The method for ethyl esterification of EPA-DHA fish oil according to claim 1, characterized in that: The metal-modified zirconium sulfate solid catalyst is any one of a zinc-modified zirconium sulfate solid catalyst, a cerium-modified zirconium sulfate solid catalyst, a cobalt-modified zirconium sulfate solid catalyst, a tin-modified zirconium sulfate solid catalyst, and a zinc / tin-modified zirconium sulfate solid catalyst.

4. The method for ethyl esterification of EPA-DHA fish oil according to claim 1, characterized in that: The preparation method of the metal-modified zirconium sulfate solid catalyst is as follows: dissolving 0.1 mol of zirconium oxychloride octahydrate and 0.005-0.02 mol of metal salt in 100-500 mL of deionized water; adding 10-30 wt% concentrated ammonia water dropwise under stirring until the pH value of the solution is 10; standing at room temperature for 6-18 hours; filtering, taking a precipitate and repeatedly washing the precipitate with deionized water until the washing liquid does not contain chloride ions; drying and crushing the obtained precipitate; then soaking it in a 0.1-1 mol / L sulfuric acid aqueous solution for 2-8 hours; after filtering, calcining the filter cake at 400-650° C. for 1-6 hours to obtain the metal-modified zirconium sulfate catalyst.

5. The method for ethyl esterification of EPA-DHA fish oil according to claim 4, characterized in that: The metal salt is any one of zinc nitrate hexahydrate, cerium (III) nitrate hexahydrate, cobalt nitrate hexahydrate, and tin chloride.

6. The method for ethyl esterification of EPA-DHA fish oil according to claim 1, characterized in that: The metal-modified zirconium sulfate solid catalyst is a zinc / tin-modified zirconium sulfate solid catalyst.

7. The method for ethyl esterification of EPA-DHA fish oil according to claim 6, characterized in that: The preparation method of the zinc / tin modified zirconium sulfate solid catalyst is as follows: dissolving 0.1 mol of zirconium oxychloride octahydrate, 0.003-0.01 mol of tin chloride and 0.003-0.01 mol of zinc nitrate hexahydrate in 100-500 mL of deionized water; adding 10-30 wt% concentrated ammonia water dropwise under stirring until the pH value of the solution is 10; standing at room temperature for 6-18 hours; filtering, taking a precipitate and repeatedly washing the precipitate with deionized water until the washing liquid does not contain chloride ions; drying and crushing the obtained precipitate; and then soaking it in a 0.1-1 mol / L sulfuric acid aqueous solution for 2-8 hours; after filtering, calcining the filter cake at 400-650° C. for 1-6 hours to obtain the zinc / tin modified zirconium sulfate catalyst.