Metabolism of Lactic Acid Bacteria in Wheat Sourdough and Bread Quality

Study on metabolism of lactic acid bacteria in wheat sourdough can improve bread quality such as bread flavour, texture and shelf life. This dissertation focused on several metabolic pathways in Lactobacillus strains on both biochemical and genetic level to fulfill three objectives.;The first objective was to produce bread preservative propionate from lactate by cometabolism of Lactobacillus buchneri and Lactobacillus diolivorans to extend bread shelf life. The results showed that propionate was formed in cofermentation. Bread containing 20% sourdough from coferementation effectively inhibited growth of three of the four selected moulds for more than 12 days compared to traditional sourdough. The use of 10% experimental sourdough deferred growth of two molds by one day.;The second objective was to characterize the metabolism of alpha-ketoglutarate (alpha- KG) in sourdough strains. Alpha-KG is an important amino group acceptor for flavour production during sourdough fermentation, however, knowledge on metabolism of alpha-KG in sourdough strains is lacking. Study of metabolism of alpha-KG in Lactobacillus sanfranciscensis and Lactobacillus reuteri showed that L. sanfranciscensis and L. reuteri utilized alpha-KG as electron acceptor, which was converted to 2-hydroxyglutarate in both media and wheat sourdough. The presence of phenylalanine and citrate in addition to alpha-KG partially redirected the use of alpha-KG from electron acceptor to amino group acceptor.