Abstract: This study sought to analyze the reason why the viscosity of the fermentation broth containing poly-γ-glutamic acid (PGA) can be significantly reduced by adding KCl to the culture medium from the perspective of PGA’s molecular structure. The medium without KCl was used as a control. The molecular structure of PGA obtained from the medium with addition of 15 g/L KCl was characterized systematically by gel permeation chromatography (GPC), chiral chromatography (CC), static light scattering (SLS), circular dichroism (CD) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, thermal analysis and shear viscosity measurement. The results showed that the molecular mass of PGA decreased by 13.9%, the proportion of D-glutamic acid in PGA increased from 47.66% to 53.77%, the PGA was more likely to form an aggregation state in aqueous solution, the conformation changed from single β-sheet to the co-existence of β-sheet and α-helix, and the shear viscosity of PGA aqueous solution decreased by 32.08% when 15 g/L KCl was added to the medium. The decrease of the fermentation broth’s viscosity by KCl may be related to the molecular structural change of PGA. This study provides some new information useful for understanding the mechanism of KCl for reducing the fermentation broth’s viscosity from the perspective of PGA’s molecular structure.

Key words: Bacillus subtilis; poly-γ-glutamic acid; fermentation broth viscosity; potassium chloride; molecular structure