2021 Volume 27 Issue 3 Pages 463-471
Poly-gamma-glutamic acid (γ-PGA) is a major constituent of the fermented soybean product “natto”. We developed a high-accuracy method for quantifying γ-PGA content in natto rich in γ-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 γ-PGA content of both γ-PGA-rich and γ-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 γ-PGA from natto samples spiked with three different amounts of γ-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 γ-PGA content of natto.
