Structure and macromolecular properties of Weissella confusa and Leuconostoc citreum dextrans with a potential application in sourdough

Over the past few years, interest in dextrans produced by lactic acid bacteria (LAB) has experienced a renaissance because of their prospective application as natural hydrocolloids in fermented products. Though the benefits of dextrans as hydrocolloids in sourdough bread have been the subject of several studies, only in a few of these studies have the structural features of the potential dextrans been elucidated. In this thesis the structure and macromolecular properties of W. confusa E392 and L. citreum E497 dextrans were studied to understand their functionality in sourdough. Since functionality also depends on concentration, an enzyme-assisted assay was developed to estimate the amount of dextrans produced in sourdough. The experimental part included several other dextrans for comparison and method development. Structural analysis revealed that W. confusa E392 dextran contains few α-(1→3)-linked branches (3%), while L. citreum E497 dextran contains α-(1→2)and α-(1→3)-linked branches (11% and 4%, respectively). Further details on the nature of these branches from the analysis of structural segments indicated that the α-(1→3)-linked branches in both dextrans are either a single unit or elongated by two or more α-(1→6)-linked glucosyl residues. Macromolecular characterization in aqueous solutions showed them to be high molar mass dextrans (10 g/mol). In dimethylsulfoxide (DMSO), however, the molar mass of the dextrans was lower (1.5 and 1.9 × 10 g/mol). The lower values in DMSO were considered to originate from individual dextran chains, while the values obtained in aqueous solutions were skewed by the presence of compact aggregates. The enzymeassisted assay developed for dextran quantification was limited to dextrans with few branch linkages. L. citreum E497 dextran was therefore not quantifiable with this method. During 17-24 hours of fermentation, W. confusa E392 produced 1.1-1.6% dextran from an initial 10% sucrose. Preliminary studies indicate that the strain channeled the remaining glucose (the theoretical maximum glucose was 5%) to the production of oligosaccharides via dextransucrase acceptor reactions with maltose. In conclusion, the study revealed that despite their simple monosaccharide composition, dextrans have a complex ramified structure even in the case of W. confusa E392 that only has a few branch linkages. Aqueous solutions of high molar mass dextrans contain compact aggregates, which, in addition to the ramified structure of dextrans, complicate their macromolecular characterization. Consequently, deducing the functional properties of dextrans in sourdough or any other food application is not straightforward. When comparing the functional properties of dextrans, the size (hydrodynamic properties and intrinsic viscosity), which reflects the shape and conformation of the dextrans, should be considered in addition to molar mass and structural features. Since food applications are aqueous systems, the functionality of dextrans may result from a contribution of both the properties of individual chains and compact aggregates.