Direct Utilization and Conversion of Raw Starch to Exopolysaccharides by a Newly Isolated Amylolytic Streptococcus sp.

Abstract

A newly isolated culture is identified as Streptococcus lutetiensis with significant starch saccharifying activity. Along with considerable amylolytic property (~ 2.71 U/mL), the culture exhibited significant production of exopolysaccharide (EPS) in starch medium. Interestingly, the glycosyl transferase activity which is essential in the biosynthesis of polysaccharide is also detected in the culture and after screening and process optimization, a maximum EPS titre of 19.92 ± 0.5 g/L was obtained from cassava starch. The crude EPS, after purification and characterization (monosaccharide analysis, FT-IR, TGA, GPC NMR, and SEM) was found to be of dextran nature with a Mw of 1275.36 kDa. Dextran type exopolysaccharide are synthesized by dextransucrase enzyme by the transfer of glucosyl residues from sucrose to dextran polymer. Interestingly, the glycosyl transferase enzyme activity which is essential in the biosynthesis of EPS is also detected in the culture. The particle size (447.8 dnm) and the zeta potential (􀀀 33.4) analysis of the purified EPS showed that the EPS produced is a stable molecule and has a random coil confirmation when exposed to alkaline condition with shear thinning property. One step conversion of sustainable low-cost starchy raw materials without adding external enzymes for hydrolysis, improved the economic viability of EPS production.