This study presents an engineered approach for sustainable biosynthesis of curdlan by Alcaligenes faecalis using orange peels. To confirm the substrate suitability a four step study was organized. Firstly, drying of substrate was carried within temperature range of 60–120 °C, along with the application of moisture diffusion control model. Secondly, fermentation medium was obtained via saccharification and detoxification, releasing highest sugar at 72.34 g/L with phenolics removal of 95–98%. Thirdly, curdlan fermentation was conducted in detoxified orange peel hydrolysate followed by optimization of batch culture fermentation via kinetic modeling using Logistic and Luedeking-Piret equations. In 5 L bioreactor, highest specific growth rate ( μ m = 0.233/h), highest curdlan production ( P m = 23.24 g/L) and growth associated rate constant ( α = 3.403) were achieved. Moreover, the total sugar consumption and conversion rates were 83.27% and 53.20%. Lastly, characterization techniques such as FTIR, NMR, XRD, TGA, HPGPC and EDS were applied to biosynthesized curdlan for qualitative validation.
Curdlan is a microbial polysaccharide composed exclusively of β-(1,3)-linked glucose residues. Until now only bacteria belonging to the Alcaligenes and Agrobacterium species have been reported to produce Curdlan. In this study, a bacterium capable of producing extracellular Curdlan, identified as Pseudomonas sp. on the basis of 16S rDNA gene sequencing, was isolated from soil samples. From the HPLC, permethylation linkage analysis, 13C NMR, and FT-IR analytical data, the polysaccharide consisted exclusively of glucose; the most prominent sugar was 1,3-linked glucose, and most glycosidic bonds joining these sugar residues were of the β-type. This also supported that the exopolysaccharide produced by Pseudomonas sp. was actually Curdlan. In addition, the Pseudomonas sp. was studied for the production of Curdlan by conventional “one-factor-at-a-time technique” and response surface methodology (RSM). It was observed that glucose and yeast extract were the most suitable carbon source and nitrogen source for Curdlan production, respectively. By using RSM, Curdlan production was increased significantly by 188%, from 1.25 to 2.35 g/L, when the strain was cultivated in the optimal condition developed by RSM, and the highest Curdlan production rate of 0.81 g/(L h) was obtained. To the best of the authors' knowledge, this is the first report on Curdlan production by Pseudomonas sp.
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