Method for purifying aspartic acid
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for purifying aspartic acid, and more specifically, it relates to a method for obtaining high purity aspartic acid from which Cl -   has been reduced or removed. 
     2. Description of the Related Art 
     Aspartic acid has heretofore been produced by a fermentation process, an enzymatic process and a chemical synthetic process, and its purification is carried out by using various purification techniques in combination. 
     In any production process, since the obtained crude aspartic acid crystals contain impurities such as other amino acids, colored substances, inorganic salts, etc., their purification requires a lot of labor and cost. Especially in the case where high purity aspartic acid crystals are to be obtained, it is usual practice to dissolve the entire quantity of the crude crystals once and to carry out such techniques as ion exchange resin treatment, decoloring treatment, crystallization, etc. 
     The present invention aims to solve the problem that the crude aspartic acid crystals obtained by the above-described prior art processes contain impurities, and to obtain high purity aspartic acid. 
     SUMMARY OF THE INVENTION 
     The present inventors have intensively sought a method for easily removing impurities contained in crude aspartic acid crystals and, as a result, have surprisingly discovered that the impurities may be removed by suspending the crude aspartic acid crystals in an aqueous solution and stirring at a high temperature. Although the impurities cannot be removed when the crystals are suspended at a low temperature, such as 10°-40° C., the impurities are suddenly reduced at a temperature of 50° C. or higher. This is a characteristic property of aspartic acid crystals. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to the present invention, other amino acids, colored substances and inorganic salts, especially Cl - , contained in the crude crystals in very slight amounts may be greatly reduced. Generally, the amount of chloride ion contained in the crude crystals before purification will range from about 200 to about 5,000 ppm, based on the aspartic acid molecules. Preferably, the starting concentration of the chloride ion will be from about 300 to about 5,000 ppm. After purification, the aspartic acid crystals will generally have a concentration of not more than 100, preferably not more than 50 ppm chloride. The lower limit could be as low as 0-5 ppm. 
     Crude aspartic acid crystals crystallized from a solution containing amino acids other than aspartic acid and sodium chloride tend to contain other amino acids and sodium chloride, and upon purification of such crude aspartic acid crystals, the present invention is extremely effective. Some examples of other impurities which can be reduced by the present process are, for example, alanine, proline, glutamic acid, SO 4   2- , etc. The amounts of these materials before and after purification can be the same as the amounts given for chloride above. 
     The temperature of the suspension is satisfactorily 50° C. or higher, and an easily operable temperature range is suitably 55°-90° C. The suspension time depends on the temperature, but 5 minutes or longer will usually suffice and, in general, a range of 10-120 minutes is satisfactory. The concentration of the crude aspartic acid crystals may be the concentration suspended and is not particularly restricted, and in general the operation is conducted at 5-40 g/dl. 
     The operational procedures of the present invention may comprise either adding the crude aspartic acid crystals first, then adding water and raising the temperature, or adding the crude aspartic acid crystals to heated water. The purifying effect may be enhanced by stirring so that the crystals flow appropriately. 
     After practicing the present invention, the crystals may be separated directly, but in order to increase the yield, it is generally preferred to separate after cooling to 5°-25° C. 
     According to the present invention, the effect of purification of other amino acids, colored substances and inorganic salts such as sodium chloride from the crude aspartic acid crystals is extremely great, and high purity aspartic acid crystals may be easily obtained without carrying out other purification procedures. 
     Therefore, since the present method does not require conventional purification such as ion exchange resin treatment, crystallization etc. and the operation is quite easy without reducing the yield, this is an extremely advantageous method from an industrial point of view. 
     The invention now being generally described, the same will be better understood by reference to certain specific examples which are included herein for purposes of illustration only and are not intended to be limiting of the invention or any embodiment thereof, unless specified. 
    
    
     EXAMPLE 1 
     50 g of crude L-aspartic acid crystals containing 360 ppm of Cl -   and 250 ml of water were mixed. Three such mixtures were heated to temperatures of 60° C., 70° C. and 80° C. respectively, and stirred for 40 minutes, respectively. Thereafter, they were cooled to room temperature (25° C.), and stirred for 2 hours. The crystals were filtered through filter paper and the crystals were washed with 20 ml of cold water (5° C.), respectively. The Cl -   content in the L-aspartic acid crystals obtained after drying was measured by silver nitrate titration analysis. The Cl -   content of the L-aspartic acid crystals as a result of treatment at the respective temperatures is shown in Table 1. 
     
                       TABLE 1                                                     
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             After     After      After                                   
Before       Treatment Treatment  Treatment                               
Treatment    at 60° C.                                             
                       at 70° C.                                   
                                  at 80° C.                        
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Cl.sup.-                                                                  
       360       80        Not Detected                                   
                                    Not Detected                          
Content                    (not more                                      
                                    (not more                             
(ppm)                      than 5 ppm)                                    
                                    than 5 ppm)                           
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     COMPARATIVE EXAMPLE 
     Similar procedures were carried out except that the temperature in Example 1 was replaced by 40° C. As a result, the Cl -   content in the L-aspartic acid crystals was 250 ppm. 
     EXAMPLE 2 
     50 g of L-aspartic acid crystals containing 1000 ppm of Cl -   and 0.1% of L-glutamic acid and which were colored yellowish brown was added to 250 ml of water heated to 50° C., then further heated to 80° C. with stirring, and stirred for 30 minutes. Thereafter, the solution was cooled to 25° C. and centrifugally separated 2 hours later. The obtained L-aspartic acid crystals were colorless, and when measured for Cl -   by silver nitrate titration analysis, Cl -   was not detected. Further, when measured by an amino acid analyzer, L-glutamic acid was not detected either. The yield of L-aspartic acid was 43 g. 
     APPLICATION EXAMPLE 
     Ten ml of an aqueous solution (adjusted to pH 5.4) containing 5 g of the L-aspartic acid crystals obtained in Example 1 and 10 g of L-phenylalanine methyl ester obtained by the procedures described below was aseptically added to a culture liquor of Brevibacterium linens ATCC 8377 which had been cultured by using the medium described below at 30° C. for 12 hours, then the pH of the culture liquor was aseptically adjusted to 5.4, and culture was further carried out for 10 hours. During culture, the pH was aseptically controlled to 5.4 at intervals of 2 hours. 
     Measurement of the product in this culture liquor by an amino acid analyzer showed that 315 mg/dl of APM had been produced. 
     Preparation of L-Phenylalanine Methyl Ester (PM) 
     279.0 g of wet crystals of L-Phe 1/2 sulfate hemihydrate (20% of adsorbed water based on the total weight, 1.00 mole as L-Phe) was suspended in 250 ml of MeOH and, after adding 60 ml of 98% sulfuric acid, reacted at 85° C. for 4 hours. The reaction was carried out while distilling off the water and MeOH and replenishing the MeOH. 
     The PM in the obtained reaction mixture was quantitatively analyzed by high performance liquid chromatography, to obtain a yield of 99%. 
     MEDIUM 
     Fifty ml of a medium (pH 7.0) containing 2.0 g/dl of glucose, 0.5 g/dl of (NH 4 ) 2  SO 4 , 0.1 g/dl of KH 2  PO 4 , 0.05 g/dl of MgSO 4 .7H 2  O, 1 mg/dl of FeSO 4 .7H 2  O, 1 mg/dl of MnSO 4 .4H 2  O, 1.0 g/dl of yeast extract, 0.5 g/dl of malt extract, and 4.0 g/dl of calcium carbonate (separately sterilized) was added to a flask having a capacity of 500 ml, and sterilized at 120° C. for 15 minutes. 
     The invention now being fully described, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention as set forth herein.