Abstract:
There is disclosed a method for the preparation of ester compound. It is distinguished by comprising reaction of an alcohol with an organic acid in the vapor state and at a temperature of 25° to 55° C., in the presence of lipase. Unlike production in aqueous solutions, the ester compounds prepared according to the method have no difficulty in separating from the reaction system because they are in the vapor state. Accordingly, the method is suitable for mass production.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates, in general, to a method for the preparation of ester compound and, more particularly, use of enzyme to prepare ester compounds from vapor state substrates. 
     2. Description of the Prior Art 
     Ester compounds as used herein include ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, ethyl propionate, ethyl butyrate, which all are used as natural flavor. 
     Lipase, an esterase, catalyzes the production of ethyl acetate (Langrand et al., Biotechnol, Lett., 12, 581, 1990), as illustrated by the following reaction formula: 
     
         ethanol+acetic acid→ethyl acetate+water 
    
     To speak generally, this reaction is expressed as follows: 
     
         alcohol+carboxylic acid→ester compound+water 
    
     It is possible to synthesize ester compounds in chemically synthetic processes. However, as natural compounds are increasingly demanded, there have been undertaken extensive researches and studies for utilizing ferments of microorganism or enzymes, to produce ester compounds in a large quantity (Armstrong et al., Biotechnol. Bioeng. 26, 1038, 1984; Williams et al., Ann. New York Acad. Sci., 542, 406, 1988; Murray and Duff, Appl. Microbiol. Biotechnol. 33, 202, 1990; Fukuda et al., Curr. Genet., 20, 49, 1991; Fukuda et al., J. Ferm. Bioeng. 75, 288, 1993). The reactions suggested in supra articles are carried out in aqueous solution. It is reported that the reactivities of the enzymes are confirmed. However, those processes are problematic in that the solubility of ester compound produced in water is extremely low. 
     Enzymatic reaction systems (Langrand et al., Biotechnol. Lett. 12, 581, 1990; Welsh and Williams, Enzyme Microbiol. Technol., 12, 743, 1990; Carta et al., Biotechnol. Bioeng., 37, 1004, 1991; Manjon et al., Biotechnol. Lett., 13, 339, 1991) show mass transfer limitation in solvents other than water and it is difficult to isolate the products from the solvents (Dordick, Enzyme Microbiol. Technol., 11, 194, 1989; Kery et al., J. Chem. Tech. Biotechnol., 48, 201, 1990). 
     As explained above, it is recognized that solubility of ester compounds in aqueous solutions is low and recovery of them from other solvents (reaction media) is not easy. Taking advantage of vaporization of the substrates (reactants) and the ester compounds (products), the present inventor made an attempt to catalyze the reaction in the vapor state. 
     There had been an attempt to react ethanol vapor with acetic acid vapor in the presence of lipase. However, it is reported that, when ethanol vapor is treated with acetic acid vapor at 30° C. in the presence of lipase from Candida rugosa, ethyl acetate is not produced (Ross and Schneider, Enzyme Microbiol. Technol.. 13, 370, 1991). It is believed that this is attributed to a fact that water content in the lipase is high. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to overcome the above problems encountered in prior arts and to provide a method for the preparation of natural ester compound from alcohols and organic acids in the vapor state, capable of taking advantage of lipase. 
     It is another object of the present invention to provide a method for the preparation of natural ester compound, applicable to industrialization. 
     In accordance with the present invention, the above objects could be accomplished by a provision of a method for the preparation of ester compounds, comprising reaction of an alcohol with an organic acid in the vapor state in the presence of lipase. 
     These and other objects and advantages of the present invention will be more apparent as following description proceeds. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Lipases used in the present invention include those derived from pancreas of pig, enzymes obtained from microorganisms at powder or liquid phase, immobilized lipases, and cells or immobilized cells containing lipase and preferably have a water content ranging from about 1 to about 25% by weight. 
     A more detail is shown as given in the following Table I. 
     
                       TABLE I______________________________________                 Concentration ofKind of Lipase        Ethyl acetate (μM)______________________________________Lipase GL (sold by Amano Co., Ltd.)                 0.331Lipase AY (sold by Amano Co., Ltd.)                 22.543Lipase CES (sold by Amano Co., Ltd.)                 31.703Lipase CE (sold by Amano Co., Ltd.)                 1.675Lipase PS (sold by Amano Co., Ltd.)                 5.182Lipase 7023C (sold by R ohm Co., Ltd.)                 3.997Lipase 22-12E (sold by R ohm Co., Ltd.)                 3.416Lipase (sold by Novo Co., Ltd.)                 28.716Lypozyme IM (sold by Novo Co., Ltd.)                 0.666Lipase MY             40.010Lipase D* (sold by Amano Co., Ltd.)                 0.219Lipase EAP 15         0.304Lipase R (sold by Amano Co., Ltd.)                 1.772Lipase Jozo (sold by Toyo Co., Ltd.)                 17.457______________________________________ 
    
     The expression &#34;ester compounds&#34; as used in the present specification mean ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, ethyl propionate, and ethyl butyrate. 
     In case of applying the method according to the present invention to a batch format, the reaction can be performed by, for example, charging lipase in a 15 ml vial with a silicon rubber septum and injecting ethanol vapor and acetic acid vapor into the vial by means of a gas-tight syringe (commercially available from Hamilton Co. Ltd.) at room temperature or more. At the moment, the temperature of outside of the vial may be ordinary temperature and preferably maintained in a range of 25° to 55° C. 4 to 10 hours is enough for the reaction. 
     It was observed that water, a product of the reaction, acts as a suppressor for the reaction. Accordingly, the present inventor found that the reaction can proceed in a fed-batch or continuous manner with the proviso that the concentration of water is so maintained as not to suppress the reaction. 
     Rapid cooling of the reactor into 5° C. or less liquidizes ethyl acetate with the aim of recovering the product. 
     Produced ester compounds were quantitatively measured by means of gas chromatography such as that sold by Hewlett-Packard Co. Ltd., under the trademark designation &#34;Model 5890&#34;. In this connection, a capillary column (inner diameter 0.53 mm, film thickness 1.0 mm, length 30 m) packed with Carbowax (trademark)-coated silica was heated at 60° C. for 5 min. and up to 160° C. in a rate of 20° C. per min, and the temperature raise was stopped at this temperature for 5 min. Nitrogen gas with a flowing rate of 5 ml per min. was employed as a carrier. FID was maintained at 200° C. Ethanol, acetic acid and ethyl acetate were detected at 3.9, 11.5 and 3.4 min., respectively. Under the above experimental conditions, ester compounds were produced and thus, the present invention was realized. 
     The preferred embodiment of the present invention will now be further described with reference to the following specific example. 
    
    
     EXAMPLE 1 
     Into 15 ml vial with a silicon rubber septum, 10 mg of lipase derived from pancreas of pig, commercially available from Sigma Co., Ltd. was charged. Subsequently, 5 ml of ethanol (commercially available from Merck Co. Ltd.)-saturated vapor and 5 ml of acetic acid (commercially available from Merck Co., Ltd.)-saturated vapor were injected by means of respective syringes for gas into the vial. Thereafter, 100 μl of the resulting gas was taken from the vial at predetermined intervals and subjected to gas chromatography with the aim of quantitative analysis. 
     The analysis results are given as shown in the following Table II. 
     
                       TABLE II______________________________________Amount of Produced Ethyl acetate by TimesTime       Ethyl acetate (μM)______________________________________1.5        2.43          4.14.5        4.86          5.28          5.49.5        5.111         5.1______________________________________ 
    
     As apparent from the above Table II, maximum production amount appears after 8 hours. 
     EXAMPLES 2 THROUGH 7 
     10 mg of lipases derived from pancreas of pig were treated in such a manner that their water contents might be 0.96, 2.96, 9.96, 18.11, 24.96 and 30%. Thereafter, the same reaction as in Example 1 was repeated. The water content was controlled according to a method suggested in Biotechnol. Bioeng., 37, 1004, 1989, to Barzana et al., in which the vial containing dry lipase was exposed to air in a vessel containing saturated saline. 
     Under the same conditions as in Example 1, alcohol vapor and acetic acid vapor were reacted with each other for 8 hours, followed by analysis. Ethyl acetate was produced in a concentration of 1.4, 5.4, 4.7, 3.5, 2.8 and 0.01 μM. When the water content was 2.96%, ethyl acetate was produced in the largest amount. In addition, it was found that over 25% of water content restrained the reaction. 
     EXAMPLES 8 THROUGH 11 
     Ethyl acetate was produced in the same manner as in Example 1, except that the reaction temperature was changed into 25°, 35°, 45° and 55° C. Initial reaction rates at the temperatures were 0.10, 0.14, 0.15 and 0.17 μM/hr/enzyme mg. From these Examples, it was apparent that the reaction could proceed at 55° C. 
     EXAMPLES 12 THROUGH 25 
     Into 15 ml vial with a silicon rubber septum, 10 mg of lipases with a water content of 4%, derived from microorganisms, as given in Table III, was charged. Subsequently, 5 ml of ethanol (commercially available from Merck Co., Ltd.)-saturated vapor and 5 ml of acetic acid (commercially available from Merck Co., Ltd.)-saturated vapor were injected by means of respective syringes for gas into the vial and reacted with each other for 5 hours. Results were obtained from a quantitative analysis and are given in the following Table III. 
     
                       TABLE III______________________________________                         Concentra-Example                       tion of EthylNo.    Kind of Lipase         acetate (μM)______________________________________12     Lipase GL(sold by Amano Co., Ltd.)                         0.33113     Lipase AY(sold by Amano Co., Ltd.)                         22.54314     Lipase CES(sold by Amano Co., Ltd.)                         31.70315     Lipase CE(sold by Amano Co., Ltd.)                         1.67516     Lipase PS(sold by Amano Co., Ltd.)                         5.18217     Lipase 7023C(sold by R ohm Co., Ltd.)                         3.99718     Lipase 22-12E(sold by R ohm Co., Ltd.)                         3.41619     Lipase(sold by Novo Co., Ltd.)                         28.71620     Lypozyme IM(sold by Novo Co., Ltd.)                         0.66621     Lipase MY              40.01022     Lipase D*(sold by Amano Co., Ltd.)                         0.21923     Lipase EAP 15          0.30424     Lipase R(sold by Amano Co., Ltd.)                         1.77225     Lipase Jozo(sold by Toyo Co., Ltd.)                         17.457______________________________________ 
    
     EXAMPLE 26 
     Example 1 was repeated using propionic acid instead of acetic acid. The produced ethyl propionate was detected at 4.6 min. 
     EXAMPLE 27 
     Example 1 was repeated using butyric acid instead of acetic acid. The produced ethyl butyrate was detected at 5.6 min. 
     EXAMPLE 28 
     Example 1 was repeated using propanol instead of ethanol. The produced propyl acetate was detected at 5.2 min. 
     EXAMPLE 29 
     Example 1 was repeated using isopropanol instead of ethanol. The produced isopropyl acetate was detected at 3.6 min. 
     EXAMPLE 30 
     Example 1 was repeated using n-butanol instead of ethanol. The produced n-butyl acetate was detected at 5.9 min. 
     EXAMPLE 31 
     Example 1 was repeated using isobutanol instead of ethanol. The produced isobutyl acetate was detected at 5.4 min. 
     As described hereinbefore, the method according to the present invention is able to prepare ester compounds from vaporized alcohols and organic acids by use of lipase derived from pancreas of pig. 
     Other features, advantages and embodiments of the present invention disclosed herein will be readily apparent to those exercising ordinary skill after reading the foregoing disclosures. In this regard, while specific embodiments of the invention have been described in considerable detail, variations and modifications of these embodiments can be effected without departing from the spirit and scope of the invention as described and claimed.