Abstract:
The present invention relates to a process for preparing a triglyceride, comprising reacting glycerol with a polyunsaturated fatty acid having at least 20 carbon atoms and at least three double bonds or a C 1-4  alkyl ester thereof, for a reaction time in the range of 24-48 hours at a temperature between 40° and 80° C. in the presence of a mixture of lipase A and lipase B obtained from Candida antarctica which is immobilized, to form (i) the triglyceride and (ii) water or a C 1-4  alcohol, while removing the water or the C 1-4  alcohol during the reaction, and recovering the triglyceride.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of application Ser. No. 07/927,405, filed Sep. 25, 1992, now abandoned, which is a continuation of PCT/DK91/00,100 filed Apr. 16, 1991, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates to a process for the preparation of a triglyceride wherein all three fatty acids are C 20+   polyunsaturated fatty acids (with at least three double bonds), by esterification of glycerol with free polyunsaturated fatty acid or its C 1-4  lower alkyl ester in the presence of a lipase. The invention also relates to a triglyceride composition with a high content of polyunsaturated fatty acid. 
     BACKGROUND ART 
     It is known that triglycerides of poly-unsaturated fatty acid (PUFA), such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), have beneficial medical effects, and within the last decade much attention has been directed to methods of producing triglyceride compositions with a high content of these acids, and particularly a high content of triglycerides with three such acids in the molecule. 
     PUFA in the form of free fatty acid or lower alkyl (e.g. methyl or ethyl) ester is available in high purity and have been used to prepare triglycerides with high PUFA content. 
     Thus, JP-A 61-43143 (Nisshin Flour Mill et al.) and EP 300,844 (R. F. Azar et al.) describe chemical interesterification of lower alkyl PUFA ester with triacetin or tributyrin. Sodium methylate was used as catalyst, and vacuum was used to remove lower alkyl acetate or butyrate formed in the reaction. The former describes production of triglyceride with 90% PUFA content. 
     JP-A 61-246146 (Nissui Seiyaku) describes halogenation of PUFA free acid, followed by reaction of PUFA acyl chloride with glycerine. The first step was carried out with oxalyl chloride at 65°-90° C. for 4 hours, and the second step under reflux for several hours in chloroform in the presence of quinoline or pyridine. 
     The above-mentioned processes use highly reactive chemicals that require special precautions in handling, these reactive chemicals react with part of the labile PUFA acyl groups, and the resulting reaction mixture in each case requires complex purification. 
     JP-A 62-91188 (Nisshin Oil) describes lipase-catalyzed production of PUFA glycerides from glycerol and PUFA free acid or ethyl ester, using positionally specific lipase in native form or immobilized on a weakly basic anion exchange resin. It is stated that addition of water is necessary. After reaction and removal of unreacted fatty acid, the glyceride mixture contained at most 86% triglyceride together with at least 14% diglyceride+monoglyceride. The product contained at most 85% polyunsaturated fatty acids together with at least 15% of other fatty acids. 
     It is an object of the invention to provide a simple process, avoiding the use of aggressive chemicals, to produce triglyceride with a low content of mono- and diglycerides, having a high content of PUFA, especially a high content of triglycerides with three PUFA in the molecule. It is also an object to provide triglyceride compositions with high PUFA content. 
     STATEMENT OF THE INVENTION 
     We have found that the yield of triglyceride can be increased and the amount of mono- and diglyceride decreased by removing water or lower alcohol formed during the reaction, by using positionally non-specific lipase, or by using a lipase immobilized by adsorption on a particulate, macroporous adsorbent resin of the acrylic type. 
     Accordingly, the invention provides a process for the preparation of a triglyceride wherein all three fatty acids are C 20+   polyunsaturated fatty acids (with at least three double bonds), by esterification of glycerol with free polyunsaturated fatty acid or its C 1-4  lower alkyl ester in the presence of a lipase. In the first aspect of the invention, the process is characterized by removing water or lower alcohol by evaporation during the reaction. In another aspect, the process is characterized in that the lipase is positionally non-specific. In a third aspect, the process is characterized in that the lipase is immobilized by adsorption on a particulate, macroporous adsorbent resin of the acrylic type. 
     The invention also provides a triglyceride composition, characterized by at least 95% by weight (preferably at least 98%) of the fatty acids in the triglyceride molecules being polyunsaturated C 18-22  acid. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Polyunsaturated fatty acid 
     The process of the invention is applicable to C 20+   PUFA with 3 or more double bonds such as eicosapentaenoic acid (EPA, C 20:5 ) and docosahexaenoic acid (DHA, C 22:6 ). 
     Thus, the process of the invention can be used to prepare triglyceride compositions with high content of PUFA by using a reactant mixture wherein the free acid or lower alkyl ester contains at least 90%, preferably at least 95% and most preferably at least 98%, of PUFA. In this way it is possible to prepare triglyceride with PUFA in all three positions in good yield. In this connection, it has surprisingly been found that pure EPA or DHA is incorporated particularly fast. 
     Preferably, the PUFA reactant is free fatty acid, methyl or ethyl ester; These are easily available, and the resulting water, methanol or ethanol is very volatile and is easily removed. 
     The PUFA free fatty acid or ester may be prepared by known methods, and some are commercially available in high purity, e.g. EPA and DHA as free acid and ethyl ester in 99% purity from Idemitsu Petro Chemical Co., Ltd., Japan. 
     Lipase 
     The lipase should be sufficiently thermostable for the temperature and reaction time in question, e.g. 24 hours at 40°-80° C. It is preferred to use immobilized lipase. 
     One aspect of the invention uses a positionally non-specific lipase. Examples of non-specific lipases are those derived from strains of Candida, especially C. antarctica iipase (WO 88/02775, incorporated herein by reference), and lipase from C. rugosa (also known as C. cylindracea). It is particularly preferred to use a lipase preparation containing both lipase A and lipase B of C. antarctica described in said reference. 
     Three strains of C. antarctica described in WO 88/02775 have been deposited at Deutsche Sammlung von Mikroroganismen (DSM) under the terms of the Budapest Treaty as follows: 
     
         ______________________________________Deposit No.       Deposit date______________________________________DSM 3855          September 29, 1986DSM 3908          December 8, 1986DSM 3909          December 8, 1986.______________________________________ 
    
     Positionally specific (1,3-specific) lipase may be used in some embodiments of the invention. Examples are lipase derived from Humicola, especially H. lanuginosa (WO 89/06278) and recombinant Humicola lipase (EP 305,216) and Mucor lipase (EP 140,542). 
     One aspect of the invention uses iipase immobilized by adsorption on a particulate, macroporous adsorbent (i.e. non-ionic) resin of the acrylic type according to WO 89/02916. 
     An example of a lipase preparation that may be used is SP 382 from Novo Nordisk A/S (mixture of lipases A and B from C. antarctica, immobilized according to WO 89/02916). 
     Reaction conditions 
     A suitable amount of lipase is generally in the range 0.5-10 BIU/g (typically 1-5 BIU/g) of reactant mixture (BIU=Batch Interesterification Unit, see WO 89/06278) by use of immobilized lipase, or 50-500 LU/g of oil (LU=Upase Unit, see WO 88/02775) by use of native (non-immobilized) lipase. 
     It is preferable to use the two reactants at about the stoichiometric ratio or with a moderate excess (e.g. 0-50%, especially 0-20%) of the PUFA acid or ester. It is preferable to let the reaction continue until at least 90% (particularly more than 95, especially more than 98%) of the glycerol has been converted into triglyceride. 
     It is generally not necessary to use a pH buffer or an organic solvent in the process. 
     A temperature of 40°-80° C., especially 60°-80° C., is generally suitable for the reaction and the evaporation. The reaction time will generally be from 24-48 hours. 
     Removal of volatile alcohol or water 
     The removal by evaporation of volatile alcohol or water may be done continuously from a stirred tank. Reactants may be added batch wise, semi-batch wise or continuously. If the lipase is immobilized it can be separated off after the reaction and reused. 
     Alternatively, the reaction may occur in two or more steps, and evaporation can be done between the steps. Each process step can be made in a stirred tank, or immobilized iipase can be used continuously in a fixed bed. 
     The evaporation is most conveniently done under vacuum, e.g. below 200 Pa and especially below 20 Pa. 
     EXAMPLES 
     Example 1 
     Preparation of triglycerides containing approx. 85% EPA/DHA 
     Immobilized lipase derived from Candida antarctica (SP-382 from Novo Nordisk A/S; activity approx. 30 BIU/g; 1.53 g; moisture-free) was added to a mixture of glycerol (99% from Sigma; 1.37 g, 14.9 mmol) and 87% PUFA (free acid) concentrate (55% EPA and 32% DHA; M.wt. 311.0 g/mol; 14.1 g, 45.3 mmol). The mixture was gently stirred on a magnetic stirrer hot-plate at 65° C. under continuous vacuum of 0.1 mm Hg. The volatile water or lower alcohol, when using lower alkyl ester concentrates, produced during the progress of the reaction was continuously condensed into a liquid nitrogen cooled trap, which could be separated and weighed regularly during the process by disconnecting the reaction by replacing the vacuum with dry nitrogen or argon atmosphere. After 30 hours the reaction was discontinued, hexane added and the enzyme separated off by filtration. The hexane was removed in vacuo on a rotary evaporator. Titration was applied to determine the free fatty acid content of the crude reaction product (3% FFA content, corresponding to 97% incorporation, which is equivalent to 91% triglyceride content). The titration results were confirmed by latroscan studies, which indicated 91% triglyceride content, after the product had been freed from free fatty acids by washing the organic phase a few times with 0.25M sodium hydroxide in 1:1 water/ethanol solution. 100% pure triglycerides were afforded by preparatory High Performance Liquid Chromatography (HPLC) eluting with 10% ether in hexane solvent, which was confirmed by latroscan studies. Capillary Gas Liquid Chromatography (GLC) analysis showed fatty acid composition identical to the original PUFA concentrate. 
     The following equation was used to calculate the % incorporation of fatty acids into glycerol from the weight measurements: 
     
         %incorporation=1.71·[Wt.sub.water /Wt.sub.glycerol ]·100% 
    
     This was deduced from the following equation: ##EQU1## 
     The following results were obtained: 
     
         ______________________________________Time           % incorporation*hours          min.*    max.*______________________________________1              50       562              67       726              90       9624              103**   10830              103**   108______________________________________ *The incorporation was determined by mass measurements of the entrapped product. The minimum incorporation was based on the assumption that all the moisture had escaped from the immobilized lipase. **Due to inaccuracy in the mass measurements the minimum incorporation passes the 100% incorporation level. 
    
     Example 2 
     Preparation of triglycerides containing approx. 60-70% EPA/DHA 
     The procedure of Example 1 was followed in details by using immobilized lipase SP-382 (1.10 g; 10% moisture content), which was added to a mixture of glycerol (1.00 g, 10.9 mmol) and 66% PUFA concentrate (38% EPA and 28% DHA; M.wt. 309.5 g/mol; 10.2 g, 33.0 mmol) or 59% PUFA concentrate (29% EPA and 30% DHA; M.wt. 306.4 g/mol; 10.1 g; 33.0 mmol). 
     The following results were found: 
     
         ______________________________________    % incorporation*Time,      59% PUFA            66% PUFAhours      min.    max.        min. max.______________________________________1          53      72          34   532          67      85          61   803          77      96          73   926          89      108         79   9724          106**  125         90   10928          106**  125         90   109______________________________________ *The incorporation was determined by mass measurements of the entrapped product. The minimum incorporation was based on the assumption that all the moisture had escaped from the immobilized lipase. **Due to inaccuracy in the mass measurements the minimum incorporation passes the 100% incorporation level. 
    
     Example 3 
     Preparation of triglycerides containing 99% EPA 
     Immobilized lipase SP 382 (0.50 g; moisture-free) was added to a mixture of glycerol (99% from Sigma; 0.44 g, 4.78 mmol) and 99% EPA as free fatty acids (M.wt. 302.5 g/mol; 4.40 g, 14.54 mmol). The mixture was gently stirred on a magnetic stirrer hot-plate at 65° C. under continuous vacuum of 0.5-0.1 mmHg. The volatile water produced during the progress of the reaction was continuously condensed into a liquid nitrogen cooled trap, which could be separated and weighed regularly during the process by disconnecting the reaction by replacing the vacuum with dry nitrogen or argon atmosphere. After 30 hours the reaction was discontinued, hexane added and the enzyme separated off by filtration. The organic solvent was removed in vacuo on a rotary evaporator to afford the crude product as a slightly yellowish oil (4.18 g, 93%). Weight measurements indicated 105-108% incorporation, but NMR spectroscopy indicated 98% incorporation, which had increased to 99% after 48 hours. Titration was applied to determine the free acid content of the crude reaction product (less than 1% FFA content, corresponding to at least 99% incorporation, which is equivalent to 97% triglyceride content). The crude product was directly introduced into HPLC eluting with 10% ether in hexane solvent to afford 100% pure triglycerides (3.51 g, 84% recovery from HPLC, but overall yield 78%) which was confirmed by iatroscan studies. 
     [250 MHz- 1  H NMR (CDCl 3 ): δ5.41-5.26 (m, 31 H, =C--H and --CH 2  --CH--CH 2  --), 4.30 (dd,J=11.90 Hz, J=4.34 Hz, 2 H, --CH 2  --CH--CH 2  --), 4.14 (dd,J=11.90 Hz, J=5.93 Hz, 2 H, --CH 2  --CH--CH 2  --), 2.90-2.78 (m, 24 H,=C--CH 2  --C=), 2.33(t,J=7.34 Hz, 2H, OOC--CH 2  --), 2.32 (t,J=7.36 Hz, 4 H, OOC--CH 2  --), 2.15-2.01 (m, 12 H, --CH 2  --CH 2  --C=), 1.75-1.61 (m, 6H, =CH--CH 2  --CH 3 ), and 0.97 ppm (t,J=7.52 Hz, 9 H, --CH 3 ).  13  C NMR (CDCl 3 ): δ172.9(s), 172.6(s), 132.0(d), 128,9(d), 128.7(d), 128.5(d), 128.2(d), 128.1(d), 128.1(d), 128.0(d), 127.8(d), 127.0(d), 68.9(d), 62.1(t), 33.5(t), 33.3(t), 26.4(t), 25.6(t), 25.6(t), 25.5(t), 24.7(t), 24.6(t), 20.5(t) and 14.2 ppm(q), IR (neat liquid): v max  3020 (vs,C=C--H), 2970 (s, CH 3 ), 2935 (s, CH 2 ), 2875 (s, CH 3 ), 2850 (w, CH 2 ), 1745 (vs, C=O) and 1645 cm -1  (ms, C=C). m/e (El): 945 (M + , 100%); found 944.68784 C 63  H 92  O 6  requires 944.68939 amu]. 
     The following results were obtained: 
     
         ______________________________________Time     % incorporationhours    min.*         max.*   NMR______________________________________1        56            62      602        72            78      744        91            97      896        92            98      9112       95            101     9624       99            105     9730       99            105     9848       99            105     99______________________________________ *The incorporation was determined by mass measurements of the entrapped product. The minimum incorporation was based on the assumption that all the moisture had escaped from the immobilized lipase. 
    
     Example 4 
     Preparation of triglycerides containing 99% DHA 
     Immobilized lipase SP 382 (0.50 g; moisture-free) was added to a mixture of glycerol (99% from Sigma; 0.41 g, 4.45 mmol) and 99% DHA as free fatty acids (M.wt. 328.5 g/mol; 4.43 g, 13.48 mmol). The mixture was gently stirred on a magnetic stirrer hot-plate at 65° C. under continuous vacuum of 0.5-0.1 mmHg. The volatile water produced during the progress of the reaction was continuously condensed into a liquid nitrogen cooled trap, which could be separated and weighed regularly during the process by disconnecting the reaction by replacing the vacuum with dry nitrogen or argon atmosphere. After 30 hours the reaction was discontinued, hexane added and the enzyme separated off by filtration. The organic solvent was removed in vacuo on a rotary evaporator to afford the crude product as a slightly yellowish oil (4.36 g, 95%). Weight measurements indicated 100-106% incorporation which remained constant from 24 to 72 hours, whereas NMR spectroscopy indicated 97% incorporation after 24 hours which had increased to 100% after 72 hours. The crude product was directly introduced into HPLC eluting with 10% ether in hexane solvent to afford pure triglycerides (3.46 g, 80% recovery from HPLC, but overall yield 76%) which was confirmed by iatroscan studies. 
     [250 MHz  1  H NMR (CDCl 3 ): δ5.44-5.25 (m, 37 H, =C--H and --CH 2  --CH--CH 2  --), 4.30 (dd,J=11.90 Hz, J=4.36 Hz, 2 H, --CH 2  --CH--CH 2  --), 4.15 (dd,J=11.90 Hz, J=5.89 Hz, 2 H, --CH 2  --CH--CH 2  --), 2.90-2.79 (m, 30 H, =C--CH 2  --C=), 2.39-2.38 (m, A 2  B 2 , 12 H, =CH--CH 2  --CH 2  --COOH), 2.13-2.02 (m, 6 H, =CH--CH 2  --CH 3 ), and 0.97 ppm (t,J=7.53 Hz, 9 H, --CH 3 ).  13  C NMR (CDCl 3 ): δ172.5(s), 172.1(s), 132.0(d), 129.5(d), 128.5(d), 128.3(d), 128.3(d), 128.2(d), 128.2(d), 128.0(d), 127.9(d), 127.8(d), 127.6(d), 127.0(d), 69.0(d), 62.2(t), 34.0(t), 33.8(t), 25.6(t), 25.6(t), 25.6(t), 25.6(t), 25.5(t), 22.6(t), 20.5(t) and 14.2 ppm(q), IR (neat liquid): v max  3020 (vs,C=C--H), 2970 (S, CH 3 ), 2930 (s, CH 2 ), 2870 (s, CH 3 ), 2850 (w, CH 2 ), 1750 (vs, C=O) and 1650 cm -1  (ms, C=C). m/e (El): 1023 (M + , 100%); found 1022.7340 C 69  H 98  O 6  requires 1022.7363 amu]. 
     The following results were obtained: 
     
         ______________________________________Time     % incorporationhours    min.*         max.*   NMR______________________________________1        18            24      242        43            49      454        59            65      676        71            77      7712       91            97      9124       100           106     9730       100           106     9848       100           106     9972       100           106     100______________________________________ *The incorporation was determined by mass measurements of the entrapped product. The minimum incorporation was based on the assumption that all the moisture had escaped from the immobilized lipase. 
    
     Example 5 
     Preparation of triglycerides with reuse of lipase 
     Immobilized lipase SP-382 (9.31 g; moisture-free) was added to a mixture of glycerol (9.00 g; 97.7 mmol) and free fatty acids from cod liver oil (9% EPA and 9% DHA; M.wt. 285.0 g/mol; 84.1 g; 295 mmol). The mixture was gently stirred at 65° C. under a continuous vacuum of 0.1 mm Hg. The volatile water produced during the progress of the reaction was condensed into a liquid nitrogen cooled trap, which was weighed regularly during the progress of the reaction. After 48 hours the reaction was discontinued and the lipase directly separated off without an organic solvent by filtration under dry nitrogen by the aid of a pressure equalized funnel equipped with a sintered glass filter plate and inlets/outlets to the nitrogen and the vacuum lines to aid the filtration, which was controlled by teflon key stopcocks. The oil was collected for further analysis. 
     The immobilized lipase was reintroduced into the reaction vessel. This was repeated 5 times, reusing the same immobilized lipase. 
     The following results were obtained: 
     
         ______________________________________Run           % incorporation*No.           min.     max.______________________________________1             93       962             93       973             93       974             93       975             94       98______________________________________ *As determined by mass measurements of the entrapped product after 48 hours.