Molecular weight distribution, rheological property and structural changes of sodium alginate induced by ultrasound

•To investigate the properties of sodium alginate (ALG) treated by ultrasound.•The molecular weight and distribution of ALG treated by ultrasound were studied.•The ratio of M/G was changed by the ultrasound treatment.•Ultrasound treatment of ALG increased its hydrophobic interaction and interfacial activity. In this study, the effects of ultrasound with different ultrasonic frequencies on the properties of sodium alginate (ALG) were investigated, which were characterized by the means of the multi-angle laser light scattering photometer analysis (GPC-MALLS), rheological analysis, circular dichroism (CD) spectrometer and scanning electron microscope (SEM). It showed that the molecular weight (Mw) and molecular number (Mn) of the untreated ALG was 1.927×105g/mol and 4.852×104g/mol, respectively. The Mw of the ultrasound treated ALG was gradually increased from 3.50×104g/mol to 7.34×104g/mol while the Mn of ALG was increased and then decreased with the increase of the ultrasonic frequency. The maximum value of Mn was 9.988×104g/mol when the ALG was treated by ultrasound at 40kHz. It indicated that ultrasound could induce ALG degradation and rearrangement. The number of the large molecules and small molecules of ALG was changed by ultrasound. The value of dn/dc suggested that the ultrasound could enhance the stability of ALG. Furthermore, it was found that ALG treated by ultrasound at 50kHz tended to be closer to a Newtonian behavior, while the untreated and treated ALG solutions exhibited pseudoplastic behaviours. Moreover, CD spectra demonstrated that ultrasound could be used to improve the strength of the gel by changing the ratio of M/G, which showed that the minimum ratio of M/G of ALG treated at 135kHz was 1.34. The gel-forming capacity of ALG was correlated with the content of G-blocks. It suggested that ALG treated by ultrasound at 135kHz was stiffer in the process of forming gels. The morphology results indicated that ultrasound treatment of ALG at 135kHz increased its hydrophobic interaction and interfacial activity. This study is important to explore the effect of ultrasound on ALG in improving the physical properties of ALG as food additives, enzyme and drug carriers.