A High-Accuracy Method For Quantifying Poly-Gamma-Glutamic Acid Content In Natto

Poly-gamma-glutamic acid (gamma-PGA) is a major constituent of the fermented soybean product "natto". We developed a high-accuracy method for quantifying gamma-PGA content in natto rich in gamma-PGA by lyophilizing and powdering the natto and optimizing protein and peptide removal, ethanol precipitation, degradation by hydrochloric acid, and quantification as glutamic acid using high-performance liquid chromatography. In this method, protein and peptide contaminants were removed from natto samples by trichloroacetic acid precipitation. The gamma-PGA content of both gamma-PGA-rich and gamma-PGA-poor natto was measurable using this method. A single-laboratory validation test showed a repeatability relative standard deviation of 1.9 %-4.3 % and intermediate relative standard deviation of 1.8 %-3.9 %. The recovery of gamma-PGA from natto samples spiked with three different amounts of gamma-PGA was 98.5 %-101.0 %. These results show that our developed method has high accuracy and repeatability. Therefore, this method serves as a practical quantification protocol for determining the gamma-PGA content of natto.