Characterization of levan produced from acetic acid bacteria strains

Fructans are biopolymers synthetized from sucrose by plants and a wide range of microorganisms. Levan and inulin are the two structural forms of fructans, with mainly β-(2→6) and β-(2→1)-linked fructofuranosyl residues, respectively. Recently, studies related to levan synthesis by acetic acid bacteria (AAB) have been heavily exploited, since this exopolysaccharide (EPS) shows important functional properties in food and biomedicine. In this context, the aim of this study was test two AAB strains (BM11 and MA3) for their ability to produce fructose-EPS. Firstly, inoculum of each strain was cultivated in medium containing sucrose and incubated on agitation. After, cells were removed by centrifugation and the supernatant was precipitated with cold ethanol. The formed precipitates were dissolved, dialyzed and lyophilized. For the determination of monosaccharide composition, EPS’s were hydrolyzed and subjected to chromatographic analysis. The structure of the EPS's, such as the type of linkages, was further characterized by Fourier Transform Infrared (FTIR) spectroscopy and 1H and 13C nuclear magnetic resonance (NMR) spectra. Chromatographic analysis indicated that more than 90% of the biopolymers consisted of fructose monomers. In region corresponding to the fingerprint of the molecules, FTIR spectra showed bands close to 1125, 1055 and 1015 cm-1, that were attributed to glycosidic linkage stretching vibrations. Characteristic absorptions at 925 and 808 cm-1 were also observed, indicating the presence of the furanoid ring of the sugar units. 1H NMR spectra presented chemical shifts corresponding to fructofuranosyl residues of levan (3.45 to 4.08 ppm). The 13C NMR spectral showed signal at the chemical shift of 104.2 ppm was attributed to the β-configuration of fructose (C-2) and the chemical shift in 63.3 ppm (C-6) corresponded to the presence of the β-(2→6)-linkage at C-6 of fructose. The physicochemical characterizations suggested that EPS’s would constitute a repeating unit of fructose, such as levan.