the present invention relates to a fermentation process for the preparation of l - amino acids in which the following steps are carried out : fermentation of the microorganisms of the family enterobacteriaceae producing the desired l - amino acid , in which microorganisms &# 39 ; open reading frames of yjfa and ytfp are individually or jointly inactivated by one or more methods of mutagenesis selected from the group consisting of deletion , insertional mutagenesis due to homologous recombination , and transition or tranversion mutagenesis with incorporation of a non - sense mutation in the yjfa and ytfp gene region ; concentration of the fermentation broth to eliminate water and increase the concentration of said l - amino acids ; and isolation of the l - amino acids .