Method for crystallizing .alpha.-L-aspartyl-L-phenylalanine methyl ester

A method for crystallizing .alpha.-L-aspartyl-L-phenylalanine methyl ester from a hot aqueous solution containing .alpha.-L-aspartyl-L-phenylalanine methyl ester by cooling, which comprises the steps of:

• (i) continuously supplying a hot aqueous solution of .alpha.-L-aspartyl-L-phenylalanine methyl ester to a crystallization vessel;
• (ii) monitoring the content of L-aspartyl-L-aspartyl-L-phenylalanine methyl esters in said solution and adjusting the concentration of said solution so that (a) the content of L-aspartyl-L-aspartyl-L-phenylalanine methyl esters is less than 0.6% by weight based on the weight of .alpha.-L-aspartyl-L-phenylalanine methyl ester in said solution, or (b) if the content of L-aspartyl-L-aspartyl-L-phenylalanine methyl esters is 0.6% by weight or more based on the weight of .alpha.-L-aspartyl-L-phenylalanine methyl ester in said solution, the concentration of L-aspartyl-L-aspartyl-L-phenylalanine methyl esters in said solution is adjusted to be 150 ppm by weight or less;
• (ii) lowering the temperature of said solution to at least a solubility temperature of .alpha.-L-aspartyl-L-phenylalanine methyl ester to thereby form a slurry;
• and
• (iii) continuously discharging the formed slurry from the crystallization vessel.