a process for selectively esterifying α - l - aspartyl - l - phenylalanine to the corresponding alkyl esters using a proteolytic enzyme with specific esterase activity is described . the process is carried out in an aqueous - alcohol medium in which the alcohol concentration is sufficient to reverse the esterase activity . the α - l - aspartyl - l - phenylalanine alkyl esters so produced are useful as artificial sweeteners .