Structural studies of exopolysaccharides produced by Lactobacillus delbrueckii ssp. bulgaricus

Exopolysaccharides (EPSs) produced by lactic acid bacteria are widely used in the food industry, especially for the preparation of dairy products, such as yoghurt and cheese. Their interesting physical and rheological properties make them suitable as gellifying or viscosifying agents in the development of new food products. In addition, the GRAS (generally recognized as safe) status carried by lactic acid bacteria, enables to use the EPS-producing bacteria in situ during processing of the products. Knowledge of the EPS structures is important to establish relationships between their structures and their physical or rheological properties, so that chemically, enzymatically or genetically modified EPSs can be developed and used, matching certain properties of interest for each particular case. In this framework, the present thesis deals with the determination of the primary and three-dimensional structure of the neutral EPSs produced by two strains of Lactobacillus delbrueckii ssp. bulgaricus, namely LBB.B26 and LBB.B332. The primary structures were determined by a combination of gas-chromatography-mass spectrometry and NMR spectroscopy techniques, together with partial depolymerizations of the native polysaccharides. After the elucidation of the primary structures, molecular modelling and 2D NMR spectroscopy experiments were carried out to determine the three-dimensional structures of the polysaccharides. The results described in this thesis are helpful to get a better insight into the viscosity properties of these polysaccharides.