Recovery of low-molecular-weight γ-PGA by metal cation from the fermentation broth

Abstract

Abstract The recovery of low-molecular-weight γ-polyglutamic acid (γ-PGA) by metal ion precipitation from the fermentation broth was explored. The broth containing 20 g/L γ-PGA was firstly adjusted to a low pH (2 or 3) to remove the bacteria cells easily, and then Cu2+ was chosen to recover γ-PGA in the diluted cell-free broth, resulting in the precipitation of a Cu2+-γ-PGA complex. The optimum concentration of copper sulfate for maximum γ-PGA precipitation was 0.08 M, and 0.64 M EDTA solution could completely redissolve the Cu2+-γ-PGA complex pellet. The recovery of γ-PGA from the original culture broth by this method was more than 90%. The redissolved γ-PGA was dialyzed against deionized water to obtain the purified γ-PGA biopolymer. As detected by infrared spectroscopy and GPC, the purified γ-PGA recovered by low pH and Cu2+ was characterized as γ-PGA biopolymer with a low molecular weight of 50.5 kDa and narrow polydispersity. The total metal content and Cu2+ concentration of the purified γ-PGA were found to be below the US FDA permissible limits. Therefore, the method combining low pH with the metal cation-induced precipitation offered an option to recover γ-PGA with low molecular weight and narrow polydispersity from the fermentation broth.