This study aimed to investigate the effects of soybean cake powder as nitrogen source and precipitation method on the monosaccharide composition, molecular parameter, and hot-water solubilization rate of the exopolysaccharide (EPS), gellan, from the mutant strain GL-1 of Sphingomonas paucimobilis ATCC 31461, so as to determine a proper precipitation treatment for preparing highly water-soluble gellan. S. paucimobilis was cultivated with the medium containing soybean cake powder (and sucrose), the EPS (EPS-Sw) yield was 29.1 g/L, with the total soluble carbohydrate content of 80% and acyl content of 4.0%, significantly higher than those cultivated with yeast extract and peptone, and collected by ethanol precipitation (EPS-Ye and EPS-Pe, respectively). EPS-Sw, EPS-Ye, EPS-Pe, and commercial gellan Gellan-F (reference) were commonly composed of four major monosaccharides: rhamnose (Rha), glucose (Glc), glucuronic acid (GlcA), and galacturonic acid (GalA). The molar ratio Rha∶Glc∶GlcA∶GalA of EPS-Sw was 1∶0.9∶2.3∶1.7, close to those of EPS-Ye and EPS-Pe. The gellan relative purity (total amount of the common sugars) was 62.8% for EPS-Sw. After microfiltration, above-mentioned ethanol-precipitated EPS and Gellan-F exhibited triple molecular distributions. For the supernatant of mash fermented by soybean cake powder, adding 5 mmol/L Ca2+ coupled with ethanol precipitation led to the EPS preparation (EPS-CaEt) of the increments in yield, total soluble carbohydrate content, acyl content, gellan relative purity, hot-water instant solubility, and solubilizing rate constant (ks1). Super large molecular fraction (F1) was a crucial factor and highly correlated positively with gellan-related sugar compositions and negatively with hot-water instant solubility and ks1. Conclusively, this study elucidates the key monosaccharide composition and molecular parameters of S. paucimobilis EPS produced by soybean cake powder as nitrogen source. A cost-effective preparation method for the EPS of improved gellan relative purity and high water-solubility was established. The results provide a theoretical basis for gellan-producing industry.
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