Eight grams of DL-phenylalanine, 8.1 g of N-carbobenzoxy-L-aspartic acid and 1.2 g of sulfuric acid were added to 100 ml of water and the resulting solution was heated to 60° C. This solution was cooled to 40° C. at a rate of 2° C. per hour while stirring, and was stirred for another two hours. The resulting crystal was separated, washed in 20 ml of water and dried to give 8.2 g of an L-phenylalanine/N-carbobenzoxy-L-aspartic acid salt. Analysis of this salt revealed that the optical purity of the L-phenylalanine was 97%.
