Biopolymer-Levan Characterization in Bacillus Species Isolated from Traditionally Fermented Soybeans (Thua Nao).

Bacterial levans are biopolymers composed of fructose units linked by β-2,6 glycosidic bonds that are degradable, nontoxic and flexible, representing a green technology with significant applications across various industries. Fermented soybeans are a common source of bacteria-producing polysaccharides. In this study, Bacillus siamensis KKSB4, Bacillus velezensis KKSB6 and Bacillus amyloliquefaciens KKSB7 isolated from traditionally fermented soybean (Thua-nao), along with B. velezensis strain 5.18 isolated from Jerusalem artichoke were investigated for their levansucrase activity and subsequent levan production. B. siamensis KKSB4, B. amyloliquefaciens KKSB7 and B. velezensis strain 5.18 exhibited the highest enzyme activity at 30% sucrose, whereas B. velezensis KKSB6 demonstrated optimal activity at 20% sucrose. Characterization of the levans using Fourier transform infrared spectroscopy (FTIR) revealed the peak at 3310.02, 1122.42, 1011.14, and 923.89 cm(-1), corresponding to the O-H group in fructose, C-O-H stretching, C-O-C stretching vibrations, and pyranose ring structures, respectively. (13)C NMR spectrum of levan aligned with the reference spectrum of B. siamensis, showing proton shifts at H3 (4.19 ppm), H4 (4.10 ppm), H5(3.96 ppm), H6a (3.89 ppm), H1a (3.76 ppm), H1b (3.67 ppm), and H6b (3.56 ppm). All strains produced high molecular weight levans (10(7)-10(8) Da) with thermal stability, exhibiting melting temperatures exceeding 200 °C. The levans also demonstrated high water and oil holding capacities of over 100%, attributed to their large molecular complexes. Additionally, they exhibited antioxidant activity by scavenging DPPH radicals and oxidizing ABTS, particularly B. siamensis KKSB4 and B. velezensis KKSB6, although low FRAP activity was observed. Furthermore, the levans were produced by B. siamensis KKSB4 and B. velezensis KKSB6 promoted the growth of Streptococcus thermophilus DKT-3, suggesting its potential prebiotic properties. This work highlights B. siamensis KKSB4 and B. velezensis KKSB6 is an outstanding candidate for levan production with diverse functional properties.