Streptococcus mutans (MTCC 497) was grown anaerobically in acidic Brain heart infusion (BHI) medium with 15 % sucrose to produce cell-bound and extracellular water-insoluble polysaccharide mutan. Fourier transformed infrared (FTIR) and 13C NMR studies revealed a mixed linkage of α-1−3 and α-1–6 mutan with a production yield of 1.8 g/L. Mutan has a branched structure with a molecular weight (Mw) of 5654 Da. Water-insoluble mutan was carboxymethylated at 0.93 degrees of substitution. FTIR spectra with characteristic peaks at 1603 cm−1 and 1418 cm−1 due to symmetric and asymmetric vibrations of the COO- group confirmed carboxymethylation. Thermal gravimetric analysis showed that native mutan and carboxymethyl mutan exhibited higher thermal stability. Carboxymethylation enhanced solubility and antioxidative radical-scavenging activity. The in-vitro antioxidative radical scavenging analysis revealed 52 % and 47 % inhibition of DPPH and ABTS radicals. © 2020 Elsevier Ltd