Glutamate dehydrogenase in non-starter lactic acid bacteria

The in vitro screening of several non starter lactic acid bacteria (NSLAB) strains from Lactobacillus plantarum, Lactobacillus casei ssp. casei, Lactobacillus curvatus, Lactobacillus paracasei, Lactobacillus rhamnosus, Lactobacillus parabuchnerii, Lactobacillus alimentarius and Lactobacillus paraplantarum revealed the presence of NADP-dependent glutamate dehydrogenase (GDH). NADP-GDH activity was markedly strain dependent and varied according to the interactions between temperature, pH and NaCl. The role played by GDH in Lb. plantarum was investigated by preparing a GDH-deficient mutant strain of Lb. plantarum UC1001 in which the gdh gene was disrupted by plasmid integration. The growth rate and acidification displayed by this mutant strain were lower than those of the wild-type strain, especially when cultivated in reconstituted skim milk powder (SMP). When both strains were used as adjunct starter for cheese-making, the highest cell survival was found for wild-type. In comparison with the GDH-deficient mutant, cheeses made with the adjunct of wild-type showed a lower level of free amino acids (FAA) (7181 ± 52 mg/kg for mutant vs. 4790 ± 40 mg/kg for wild-type). Accordingly, the highest level of volatile organic compounds (VOC) (alcohols, aldehydes, miscellaneous and carboxylic acids) was found in cheeses made by wild-type strain. The findings of this study provide evidence of the important role of NADP-GDH in cheese ripening and flavour development